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501_916_01B


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DlOl A
Digital Input/Output                                          Module

The DIOlA Digital Input Output Module provides 32                      module, and is functionally compatible with the older
digital input or output channels at TTL levels. These                  module. The older DIOl module has hardware switches
channels are grouped into four &bit ports which can be                 for port configuration, while this function is performed
individually programmed for input or output by soft-                   through software for the DIOlA.
ware commands. The inputs are read directly by the host
computer, and the outputs are latched. A power on reset
circuit automatically configures all ports as inputs when
the DIOlA module is powered up.                                        User Connections
                                                                       User connections can be made to either the quick discon-
                                                                       nect terminal blocks at the top of the module, or to the
I/O connections are made to on-card quick disconnect                   mass termination connector located at one end of the
screw terminal blocks, which accept 16-24 gauge wire                   board o(91                     BASIC's PEEK command, or the equivalent
         10          xxx92                  xxx93                      memory read function of other languages.
        xXx=1 ;t 3 digits of IEUN addn s,e.g. "CFF                     This WAS NOT a capability of the older DIOl
                                                                       module, and should not be attempted using
                                                                       the older module.
SELECT/CONFIGURE PORT operations
                                                                 DIGITAL INPUT/OUTPUT             operations
Location: Slot dependent CMDA write
                                                                 Location: Slot-dependent CMDB read or write
The CODA write command is used to configure the ports            A CMDB read is used to accessdata from a selected port
for input or output, and to select the port that data is to be   when that port is configured as an input port. A CMDB
written to or read from. Refer to Figure 4 for the follow-       write is used to write data to the selected port when that
ing discussion of the CMDA write data byte.                      port is configured as an output port. Note that each of the
                                                                 four ports can be individually programmed as either an
                                                                 input or output port, as discussed above.

Bits D3 through D7 are used to configure the ports for in-
put or output. Bit D7must be set to a logic 1 to enable bits     When a port is configured as an input port, the CMDB
D3 through D6 to set their appropriate port to an input or       command location is read to accessthe status of the chan-
an output port. When bit D7 is set to a logic 0, bits D3         nels associated with the port. Reading this location will
through D6 are ignored. Bits D3 through D6 are used for          return an 8 bit binary number, with the DO thru D7 lines
PORT A through PORT D, respectively. Setting the ap-             representing the input lines for the selected port as indi-
propriate bit to a logic 1 sets that port and all associated     cated in Table 3. Since the inputs are not latched, the data
channels, to a digital output. Setting the appropriate bit       will reflect the condition of the input channels at the in-
to a logic 0 sets that port, and all associated channels, to a   stant that the read operation is performed. Note that the
digital input.                                                   channels being read will depend on the port selected
                                                                 with the CMDA write command discussed above.


                           NOTE                                  If you need to know the status of a particular channel, a
      Upon power up, all ports are set to be digital             logical AND instruction can be used to "mask oft?' the un-
      inputs.                                                    necessary bit positions. The mask should contain zeros in
                                                                 the irrelevant bit positions and logic ones in the bit posi-
                                                                 tions representing the channels in question. The result of
                                                                 ANDingthemaskwiththeinputdatawillthenbean8bit
Bits DO and Dl are used to select a particular port for in-      binary number with each bit in an irrelevant position
put or output. A value from 0 to 3 will select port A            equal to zero. The relevant channels will contain a 1 or a 0
through port D respectively. Port selection will always          depending on the status of the corresponding bit in the
occur with a CMDA write to the DIOlA module, regard-             masked byte (a 1 if the object bit is high, a 0 if the object bit
less of the logic value of bit D7.                               is low).


                                                                                                                      DIOlA-5
DIOZA
Dig&z2 Input/Output Module



When reading a DIOlA port with the BASIC PEEK com-                   rectly from the port using the readback feature (and
mand, the returned value will be a decimal number be-                stored in a variable). When the current status of all the
tween 0 and 255, depending on the status of the input                channels on the particular port is known, the value of the
channels. The status of a particular channel can be deter-           variable can be changed to correspond to the desired
mined by ANDing the returned value with the decimal                  status of the selected port, and this new value written to
weighting for that particular channel. If the ANDed re-              the CMDB location.
sult is nonkero, the channel is high (logic 1).

                                                                     In assembly language, this modification can be per-
EXAMPLE The status of channel 6 on port A is to be de-               formed by using logical AND and logical OR instruc-
telmlin~                                                             tions. When turning a single channel on (high) the vari-
                                                                     able representing the current port status should should
100 POKE CMDA,O                                                      be ORed with a number equivalent to an 8 bit binary
110 STATUS6 = PEEK(CMDB) AND 64                                      value with zeros in the bit positions to remain unchanged
                                                                     and ones in the bit positions to be turned on. When turn-
                                                                     ing a channel off, the port status variable should be
If the value returned in variable STATUS6 is non-zero,               ANDed with an 8 bit binary value with ones in the bit po-
then channel 6 is high.                                              sitions to remain unchanged and zeros in the positions to
                                                                     be turned off
If a DIOl A port is configured as output, the value written
to CMDB should be the equivalent of an 8 bit binarynum-
ber, with the status of the 8 bits (1 or 0) representing the         From BASIC, the decimal weight of the bit to be turned on
status of the 8 channels of the selected port (high or low).         or off should be added to (off to on) or subtracted from
The channels affected will depend on the selected port, as           (on to off) the decimal number that represents the current
                    - -                                                    r+-k.r I`"I rwrm,rle
                                                                     er.d- 3LaLUJ. Cr." GvuAyAc, Lrr Lr.... #.a.,.......-1  A -CL :r-..#...lA
outied hi `I'abie 3. Data wr&en to a port will be iatched            y"Lr                        I." LULLI  uLcul.LLtzL    tt vu, IL W"l.uU

into the outputs and will remain unchanged until the                 be necessary to subtract 8 from the decimal value repre-
port is written to again, or the port is configured as an in-        senting the port status. To turn on channel 8, it would be
put port. From BASIC, the value written to the port using            necessary to add 128 to the status.
the POKE command will be a decimal value in the range
from 0 to 255.
                                                                     It should be noted that the DIOlA module is command
                                                                     compatible with the older DIOl module, with the added
To change the status of one output channel while leaving             programmability of the port configuration versus the
the others unchanged, you must know the status of all of             switch selectability of the older module. Programs writ-
the channels. To accomplish this, a variable in the con-             ten for the older module can be modified for use with the
troiiing program could be assigned to the current status             new module by simply issuing a CMDA WRlTE to con-
of the port in question (and updated every time the port is          figure the ports before any input or output is accom-
written to), or the output on the port could be read di-             plished.




                                   Table 3. Digital I/O Channels and Bit Values


                           Bit Position         1 D7 1 D6       D5      D4      D3     D2

                           Weighting              128 64        32      16      8      4




                                                              I
                           Port   A ChanNos.      7     6       5       4      3       2
                           Port   B Chan Nos.     15    14      13      12     11      10
                           Port   CChanNos.       23    22      21      20     19      18
                           Port   D Chan Nos.     31    30      29      28     27      26



DIOlA-6
                                                                                                                    DIOlA
                                                                                              Digital Input/Output Module



Theory of Operation                                             Input/Output selection is performed by triple 3 input
                                                                nor gate Ul(74LS27) and quad latch U3 (74LS175). When
                                                                a CMDA write is issued (bit D7 high) the data on lines D3
A schematic drawing of the DIOlA module is located on           to D6 is latched into the inverting outputs of U3. A low to
drawing number 501-206.                                         the output control of the output latches US, US, UlO, and
                                                                U12 (74LS374) allows the Read/Write line to clock the
                                                                data from the bus to the port. When D3 to D6 are low, the
                                                                output latches t&state and the select port registers of U4
The DIOlA circuitry may be divided into six sections:           enable the input buffer for the selected port.
data bus buffering circuitry, port selection circuitry, port
configuration circuitry, channel input buffers, channel
output latches, and power up reset circuitry.
                                                                The Self-ID resistor (R40) connected between pins 4 and
                                                                41 of the card edge connector allow the mainframe to
                                                                identify the module by reading the value of the resistor
                                                                with an A/D conversion circuit and generating a code
Data from the baseboard bus circuitry is applied to the         based on the value of the resistor. The ID resistor has a
octal bus transceiver U13 (74LS245). Bus transmission di-
                                                                value of 4.53 kilohms, with a tolerance of 0.1%.
rection of U13 is controlled by the baseboard Read/Write
line, while the CMDB line is used to enable the device
when reading or writing data to one of the 4 ports on the
module using the DIGITAL I/O command.                           The power up reset circuitry is used to ensure that all out-
                                                                puts power up in a high impedance state by holding the
                                                                clear line of U3 low until power up is complete, forcing all
                                                                inverting outputs of the quad latch high.
The two least significant bits of the data bus (DOand Dl)
are also applied to the port selection circuitry made up of
a dual D type flip flop U2 (74LS75) and a 3 to 8 line de-       Troubleshooting         Information
coder U4 (74LS138). When a CMDA write is issued in the
select port mode, the values on the DO, Dl, and Read/           Troubleshoot the DIOlA by using the procedure out-
Write lines are latched into U4. When the CMDB line             lined in Table 4. This process is fairly straightforward, as
next goes low, U4 decodes the input from U2 into an ena-        signal tracing is simply a matter of checking for the cor-
bling signal to go to only 1 of the 4 ports (either the input   rect digital logic level throughout the module.
buffer or the output latch, depending upon the state of
the Read/Write line). For example, if 11 (binary) were
present at DOand Dl, and the Read/ Write line was active        If a defective part is found, a replacement can be ordered
(low), executing a CMDA write would latch the data into         from Keithley Data Acquisition and Control. Refer to the
U4; then the Y7 output of U4 would go low with the next         Service Section of the hardware manual for ordering de-
CMDB write, latching the data present on the bus at'that        tails. Part numbers for the module are located on the
time to the output of the port.                                 component layout drawing (501-200).




                                                                                                                  DIOlA-7
DIOIA
Digital    Input/Output Module



                                  Table 4. DIOlA      Troubleshooting     Information


  Step Component               Required Condition                               Remarks

   1       DIOlA module        Install in slot 10                               turn power on
   2       channel O-31        apply logic high (+Rr)                           apply to inputs
   3       computer            program ah ports as inputs and select port A     use CMDA write DATA 11111X00
   4       computer            read back 255 from port A                        use CMDB read
   5       ports B-D           repeat steps 4 and 5                             to test ports ED
   6       channel O-31        apply logic low (WI                              apply to inputs
   7       computer            select port A                                    use CMDA write
   8       computer            read back 0 from port A                          use CMDB read
   9       ports ED            repeat steps 8 and 9                             to test ports ED
   10      computer            program all ports as outputs and select port A   use CMDA write
   11      computer            write 255 to port                                DATA 10000X00
   12      channelO-7          all outputs high                                 use CMDB write
   13      ports B-D           repeat steps 13 and 14                           use logic probe/DMM
   14      computer            select port A                                    to test ports B-D
   15      computer            write 0 to port                                  use CMDA write
           channelO-7          all outputs low                                  use CMDB write
   :;      ports ED            repeat steps 17 and 18                           use logic probe/DMM to test ports B-D

 1
 I\Tote:Slot 10 locations    CMDAXXX92
                             CMDBXXX93




DlOl A Specifications                                            General: AII inputs and outputs referenced to system
                                                                 ground
Channels: 32, software programmable in groups of 8 for
input or output
                                                                 AU inputs and outputs power up in high impedance state
                                                                 as inputs for safety
Signal connection: Quick disconnect screw terminal
blocks, 40 pin receptacle for ribbon cable assembly
                                                                 Accessories: Ribbon cable assembly, 6 foot, 40 conduc-
                                                                 tor; one end mates with 40 pin receptacle on DIOIA,
                                                                 other end is unterminated.    KeithIey part number
Input characteristics:                                           5OO-cA3.

        T&-compatible, high-true
        High-level Input Current: 2OpA                           Cable ribbon plug, only - 3M part number 3417.
        Low-level Input Current: -0.4mA

                                                                 Parts List
Output characteristics:
                                                                 The following components are used in the DIOlA mod-
        TI'L-compatible, high true                               ule. Spares can be ordered from the repair department of
        Drive capacity: 60 IS-TIT, loads or 15 standard `ITL     KeithIey Instruments, Inc., 28775 Aurora Road, Ueve-
        loads; 24mA sink at 0.5V                                 Iand, OH 44139. Please supply the part number, descrip-
        Self-ID: resistor value - 4.53kQ zt.1%. ID code 52       tion, and quantity when ordering.




DIOlA-8
                                                                                            DIOIA
                                                                         Digital Input/Output Module



Part No.      Quantity   Description                             Designation

C-365-.1          7      Cap, .l@?, 20%, 50V Ceramic             Cl-C7

Cs-368-40         1      Corm, 40-pin ribbon                     J6
(X-521-2          2      corm, strip, IO-pin                     JL J4
(X-521-4          2      Conn, Strip, 8-pin                      JZ J3
IC-157            1      IC, Quad D Flip-flop, 74LS175           u3
IC-173            1      IC, Comparator, LM311N                  u14
IC-182            1      IC, Decoder/DEMUX, 74LS138              u4
IC-230            4      IC, Octal Buffer/Line Driver, 74LS244   us, u7, u9, Ull
IC-233            1      IC, Triple 3-input NOR, 74LS27          Ul
IC-242            4      IC, Octal D-type Latch, 74LS374         US, U8, UlO, U12
IC-356            1      IC, Octal Bus Transceiver, 74LS245      u13
IC-366            1      IC, 4-bit B&able Latch, 74LS75          u2

R-176-4.53K       1      Res, 4.53K, .I%, 1/8W, Metal Film       R2
R-76-39K                 Res, 39K, 5%, 1/4W, Comp. or Film
R-76-4.7K         :      Res, 4.7K, 5%, 1/4W, Comp. or Film      E
R-76-47OK         1      Res, 47OK,5%, 1/4W, Comp. or Film       R4
R-76-37.4K        1      Res, 37.4K, I%, 1/8W, Metal Film        R2
R-76-4.99K        1      Res, 4.99K, I%, 1/SW, Metal Film        R3

W-28              2      Diode, Silicon, lN4148 0X-35)           Dl, D2

6-32xl/2PPH       2      Screw

500-323           1      Cable Clamp Assembly
500-321           1      Cable Clamp
500-322           1      Strip Rubber

501-916-OlA       1      DIOlA Manual




                                                                                           DIOlA-9
I-
-
                              Using the DIOlA and
                              KDAC500 Software

   The DIOlA digital I/O module          2-5 volts to drive a DIOlA input
can be used in a variety of control      high.
and monitoring applications to
source and sense digital logic levels.      A DIOlA output can drive up to
The module's 32 channels can be          60 IS-TTL inputs, or 15 standard
configured for input or output in        TTL inputs. This corresponds to a
groups of 8 channels. Each group of      current sink capability of 24mA at O-
8 channels is a called a "port". Port    0.5 volts. Drive current available
A consists of channels O-7,port B        from a high output is approximately
consists of channels 8-15, and so on.    1mA at 2-5 volts.
The DIOlA channels can be read or
written individually, or as ports.         Do not connect signals of less than
                                         OV or greater than 5V to the DIOlA
   The DIOlA channels are non-           or you may damage the module.
isolated, meaning that DIOlA digital
ground is connected to earth
ground. Therefore, the low side of
any circuitry connected to the
                                         Connection to Digital
DIOlA must also be at ground po-         Logic
tential or the circuitry, DIOlA, or         The DIOlA can interface many
data acquisition system may be           types of digital circuitry and logic
damaged.                                 families. However, it is important
                                         that any external voltages applied to
   DIOlA inputs and outputs are          the DIOlA inputs or outputs be
compatible with low-power                within the range of 0-5V. Any input
Schottky transistor-transistor logic     signal connected to the DIOlA must
(IS-`ITLJ. This defines the voltage      have sufficient sink and source capa-
and current levels associated with       bility to drive at least one IS-ITL
logical high and low. When con-          load. Any load connected to a
figured for input, each DIOlA chan-      DIOlA output must not cause the
nel presents one IS-TTL load to an       DIOlA output to sink more than
external circuit. The external circuit   24mA. Review the input and output
must be able to absorb (sink) at least   specifications for the type of logic
0.4mA at O-O.5   volts for the DIOlA     connected to the DIOlA to deter-
to recognize an input signal as a        mine what conditioning, if any, may
logic low. The same input must be        be required. This information is
capable of sourcing at least 20@ at      readily available in IC
                                         manufacturers' data books.
Using
    the DIOIA    and
KDAC500 Software


   The DIOlA can usually be                     typically being 3-15 volts according
connected to standard T-l-L logic               to the circuit supply voltage. These
without concern for voltage levels.             devices usually require level trans-                               , Current
However, the total current loading is           lation or buffering to interface                          Load 2S :; Limiting
important where one lTL output is               DIOlA channels. Integrated circuits                                  Resistor
connected to several DIOlA inputs,              such as the 4049 and 4050 buffer ICs
or where several external loads are             are available for CMOS-to-TTL
connected to one DIOlA output. A                buffering; Buffering circuits based                       lo       0     b
variety of TTL sub-families exist,                                                                         GND +!3V CH 0
                                                on discrete components can also be
including low- medium- and high-                used.
power types. Each sub-family has
different current sink and source
specifications associated with input
                                                    Note: For reliable operation, a
                                                DIOlA input must be actively
                                                                                           I L1
                                                                                           I
                                                                                                               I
                                                                                                                       DIOlA


and output. Make sure that a DIOlA
channel and the external logic can
each sink and source sufficient
                                                driven to 2-5 volts when the signal
                                                is high. Hardware switches, open-
                                                collector logic, and some other
                                                                                           I   Figure 2. Switching Small Loads with
                                                                                               DIOlA Output


current to control each other.                  types of signals may allow a
                                                DIOlA input to "float" then they           trolling higher-current DC or AC
   Open-collector digital circuitry is          switch high. A floating DIOlA              loads which are beyond the DIOlA's
another type of TTL logic which                 input may be susceptible to false          capacity. In this case, an AC or DC
requires special consideration. As              triggering. A 2k pull-up resistor          solid state relay can be used between
with standard TI'L, an open-                    should be connected between a              the load and a DIOlA output. Nor-
collector logic low sinks some                  DIOlA input and +5V if the signal          mally, these relays are energized by
current at O-O.5  volts. However, a             does not actively drive the input          switching a control terminal on the
logic high simply floats, and cannot            high.                                      relay to ground, although some may
actively drive an input. In contrast, a                                                    require application of a 3-5V control
standard TTL gate contains internal                                                        voltage. A typical control current is
circuitry to actively set a high output         Controlling External                       5-15 mA, which is well within the
to at least 2V. To assure that an                                                          sink capability of a DIOlA output.
open-collector output will reliably             Loads with the DIOlA                       The Keithley/DAC PROTO module
control a DIOlA input, connect a 2k                A DIOlA output can sink 24mA at         can be used for assembling one or
pull-up resistor between the DIOlA              05V when at logic low. The same            two auxiliary relay circuits for use
input and +5 volts.                             output can source about one mill&          with the DIOIA. If several relays are
                                                amp when at logic high. Thus, the          needed, it may be more cost-
  CMOS and other low-power logic                DIOlA is better used as a switchable       effective to use a PCM2 or PCM3
families operate at extremely low               ground point, rather than a power          module which can control up to 16
current levels, with logic high                 source for controlling external loads.     relays.
                                                For example, a DIOlA output chan-
                                                nel lacks sufficient current to light an      Caution: The DIOlA should not
                                                LED indicator. For sufficient bright-      be operated beyond the specified
                                                ness, the anode of the LED should be       ratings or the module will be
                                           2K   connected to an external source of         damaged.
                                       I
                                                voltage, and the cathode connected
                             06c$               to a DIOlA channel configured for             Where signals with relatively high
                                5V ]CHO         output. When the DIOlA output is           voltages are involved, or where
                                                switched low, current flows through        isolation is needed, an opto-isolator
  Open Collector                                the LED to ground, completing the          or other conditioning circuitry can
                                 DlOlA          circuit. A 5V supply terminal is in-
     Driver                                                                                be used between the DIOlA and the
                                                cluded on the DIOlA connector for          external circuit. The user must
                                                such applications.                         choose the conditioning method
 Figure 1. Pull-up Resistor for Open                                                       according to the application, taking
 Connector Input                                  Applications may require con-            care that the signal ultimately
                                                                                           Using the DIOlA  and
                                                                                           KDAC500 Software


                                                                                            from   a DIOlA input to ground.
                                                                                            However, this method will allow the
                               Current Limiting                                             DIOlA input to float when the
                                                                                            switch is opened. The channel may
                                                                                            be susceptible to noise or activity on
                                ~~$g-~Lk+v                                                  other channels. The solution is to
                                                                                            also connect a 2%pull-up resistor
                                                                                            between the DIOlA input channel
                                                                                            and +5V (Figure 4). When the switch
                                                                                            opens, the channel will be pulled to
                                                                                            +5V through the resistor, and be
                                                                                            immune to noise and false
                                                                                            triggering.
                                                                                               Second, mechanical switch
                                                                                            contacts usually exhibit `bounce", in
                                                                                            which electrical continuity rapidly
                                                                                            makes and breaks several times at
  Figure 3. Opto-Isolator for Controlling Loads                                             the instant the contacts close. In
                                                                                            some digital applications, this
                                                                                            bounce may be unacceptable. The
applied to the DIOlA is within                      Monitoring the status of .
                                                                                            solution is to use a "debouncing"
acceptable limits. Where many                     mechanical  switches presents two
                                                                                            circuit between the switch and
channels are involved, Keithley's                 problems. Where an application
                                                                                            DIOlA input. A flip-flop circuit
DIM1 and DOMl digital I/O                         must detect a contact closure, it is a
                                                                                            suitable for debouncing SPDT limit
modules offer 16 channels each, with              simple matter to connect the switch
                                                                                            switches is shown below. A second
optical isolation and the ability to
interface voltages up to 2SV and
loads up to 50&L Both modules
maintain isolation of up to 500V                                                     +5v
between channels, or from channel                                                                    Switoh at
                                                                                   ^., 1             position 2
to ground.


Connection to
Mechanical Switches




                   2K


                        I
              lo     6   0
                GND +5V CHO




                                                                                                              Release
 Figure 4. Mechanical SPST Switch with
 Pull-up Resistor                                  Figure 5. Debouncing Circuits
    Using
        the DIOAI    and
    KDAC500 Software


    remedy, ideal for debouncing               some   calculation method.               used. BGREAD will automatically
    momentary push-button switches,                                                     allocate and store the data in a
    uses a circuit which generates a                                                    KDAC5OOmemory array. If single
    single pulse when triggered by the         Software Considerations                  channels are being read, a KDAC500
    switch. The pulse must be longer                                                    bit array should be specified in the
                                                 The DIOlA powers up with all
    than the time it takes the contacts to                                              BGREAD command. If ports are
                                               ports configured for input. Any
    settle (typically a matter of milli-                                                being read, a KDACSOObyte array
                                               software used with the DIOlA must
    seconds). A 555 timer integrated cir-                                               should be specified in BGREAD.
                                               be able to configure the module's
    cuit or various types of one-shot ICs
                                               ports for output if the application
    can be used for this application. See                                                  The BGREAD command will
                                               requires digital output.
    IC manufacturers' data books for                                                    begin taking data when the
    more information.                                                                   KDAC5OOINTON command is
                                                 Keithley's KDAC5OO software
                                               provides complete control of all         executed. One data value will be
                                               DIOlA functions. The following           read when each interrupt fires. A
    Reading BCD Outputs                        suggestions cover programming in a       BGSTATUS command should be
       Before IEEE488 and RS232-C              BASIC or QuickBASIC environment,         used after the lNTON to determine
    serial interfaces became the pre-          but are equally useful in all            when all the data points have been
    ferred method, binary-coded                KDAC500 versions. Most appli-            collected. Afterward, the individual
    decimal (BCD) outputs were in-             cations of KDAC500 and the DIOlA         data values can be read from the
    cluded on some instruments to make         are simple in concept, and can be        array by using an ARGET command
    digitized data available to com-           summarized as follows:                   nested in a FOR-NEXT loop. If the
    puters, calculators, or other digital                                               array is a byte array, the values of
    equipment. In principle, BCD uses             1. Where one digital input reading    individual bits in each byte can be
    four lines to represent the numerals       is required, a foreground read           extracted by ANDing the byte
    O-9,but the actual encoding method         (FGREAD) command can be used.            values with bit weights as described
    and number of digits varied among          FGREAD will return the bit value         above.
    manufacturers and instruments. A           representing a single input channel,
    multi-digit BCD readout might use          or the byte value representing an           3. Where one digital value must be
    four separate lines for each digit,        input port (8 channels).                 programmed to a DIOlA output
    resulting in many lines but relatively                                              channel or port, a foreground write
    simple decoding. Alternately, an             Where a port is read, the values of    (FGWRITE) command is most con-
    instrument might multiplex several         an individual bit within the byte can    venient. FGWRITE can write a 0 or 1
    digits onto four lines plus a few          be extracted by "ANDing" the byte        to a single channel, or a byte value O-
    additional digit select lines, resulting   value with the weight of the bit's       255 to a port. The value of the byte
    in fewer lines but more complicated        position. For example, the weight of     can be calculated by adding up the
    decoding.                                  the most significant bit (MSB) in a      weights of all the high bits in the
                                               byte is 128. To test the value of the    byte. For example, a port is to be
                                               MSB in the byte value "BV", one          programmed with the bit values
       With attention to signal levels and
                                               would execute the BASIC statement:       11010011:
    the BCD encoding method, a DIOlA
    can be used to read BCD lines. The
    easiest method is to read the DIOlA                   BT=BVAND128
    inputs as ports. This will return
.   eight data bits at one time. The              If BT equals 0, the MSB value is 0,
    DIOlA's 32 inputs can thus deal            otherwise the MSB value is 1.              BYTE
                                                                                          WEIGHT
    with 6-8 BCD digits. If the digits are     Channel and bit values are covered         MSB                                  LSB
    multiplexed, the DIOlA can read an         in more detail in the DIOlA manual.                                   0     1         1
    unlimited number of digits, but the                                                    1:s    d4   2   1'6   :   4     2         1
    requisite program will be more com-           2. Where values must be read             128+64+0+16+0+0               +2+         1
    plex. The decoding of the signals          from a digital input channel at            TOTAL                                -211
    must be done in software, and may          regular intervals, a background read
    be based upon a look-up table or           (BGREAD) command should be
                                                                             Using      the DIOAl            and
                                                                             KDAC500Software


  The byte value 211 should be                The data values should be written   should be calculated as shown in the
written to the port.                      to the array using an AR??UTcom-        weighting example above, and then
                                          mand. A FOR-NEXT loop around            written to the array with ARMJT.
  4. Where several values must be         ARPTJT may be helpful in reducing
written to a digital output channel or    the number of program lines need to        In all casesfor BGWRlTE, output
port, a background write                  fill the array.                         will commence when interrupts are
(BGWRITE) command should be                                                       turned on with INTON. One data
used.                                       If the desired output is a simple     value will be written from the array
                                          digital square wave, a single 1 and 0   each time an interrupt fires. Note
   Before BGWRlTE, a KDACSOO              can be written with ARPUT to a          that one complete cycle through an
array must be allocated with              two-deep, KDAC500 bit array.            array may require many interrupts,
ARMAKE to hold the output data. If                                                so the period of square waves and
the data values are intended for a           If the output involves a more com-   other pulses will probably not equal
single channel, the array must be a       plex pattern of pulses, ARRJT can       the interrupt rate. BGWRITE's
bit array, and the data will consist ot   be used to fill a longer KDAC500 bit    cycling parameter can be set for a
l's and 0's. If the data values are       array with the necessary order of l's   specific number of repeats, or for
intended for a port, the array should     and 0's.                                continuous output of the array. If
be a byte array, and the data will                                                cycling is not continuous, a
consist of values O-255.The array           If the output involves a digital      BGSTATUS command should be
must be long enough to hold the           port, a KDAC500 byte array must be      used after INTON to determine
desired number of data points.            used. The values of individual bytes    when all the data points have been
                                                                                  written.
Data Acquisition and Control Division
Keithley Instrumegs, Inc. 28775 Aurora Road
                                l                          l   Cleveland, Ohio 44139 (216) 2484400
                                                                                          l                    l   Fax: 349-4569
WEST GERMANY:    Keithley   Instruments GmbH l Heiglhofstr. 5 l Miinchen 70 l 089-71002-0l Telex: 52-12160 l Fax: 089~7lOO25Q
GREAT BRITAIN:   Keithley   Instruments,Ltd. l 1 Boulton Road l Reading, E&shire RG 2 ONL l 0734-861287l Telex: 847 047 l Fax: 0734-863665
FRAXE            Keithley   Instruments SARL 03 All&z des Garays*B.P. 60 091124 Palaiseau/ZL l l-6-0115 155 l Telexz600 933.F~ 1-6-0117726
NETHERLANDS:     Keithley   Instruments BV* Avelingen West 49 04202MS GorincherwP.0. Box 559*42&l AN Gorinchan*OlS30-m*T&                    24 684*Fax: 01830-30821
SWITZERLAND:     Keithley   Instruments SA l Kriesbachstr. 4 l 86CODiibendorf l 01-8219444 l Telex: 828 472 l Faxz0222-315366
AUSTRIA;         Keithley   Instruments GesmbH l Rosenhiigelstrasse 12 0 A-1120 Vienna l (0222) 84 65 48 l Telex: 131677 l Fax: (0222) 84 35 97
ITALYz           Keithley   Instruments SRL l Viale S. Giignano 4/A l 20146 Milan0   l 024120360 or 02-4156540l Fax: 024121249



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