331 lines
12 KiB
Plaintext
331 lines
12 KiB
Plaintext
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Using a Printer Port for Simple GPIB/IEEE-488 Operation
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All programs and documentation, Copyright 1990 by
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Sydex
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P.O. Box 5700
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Eugene, OR 97405
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Voice: (503) 683-6033
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FAX: (503) 683-1622
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Data: (503) 683-1385
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All Rights Reserved.
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INTRODUCTION
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A while back (quite a while, actually) we had a need for the use
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of a color plotter for some 1-2-3 spreadsheet graphs that we were
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doing. A friend offered us the use of his HP 7470--but there was
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a catch. It seems that the thing had the HPIB/GPIB/IEEE-488
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interface, not a straight serial or parallel connection.
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This posed a problem. A GPIB card for a PC would set us back
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about $350--and there was no guarantee that 1-2-3 could even talk
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to the thing.
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I recalled that the old Commodore Pet, as well as the Victor 9000
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and the Osborne I could do a limited amount of GPIB interfacing
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with their parallel ports--so why not a PC? I even had an extra
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port that I wasn't using.
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After looking at the IBM Tech Reference manual, I discovered that
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there's a problem with this. Although the IBM Parallel Printer
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adapter bills itself as being bi-directional (input and output),
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it's hard-wired for output mode only! Now, I probably could
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drive the plotter with an output-only port, but this was
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aesthetically unsatisfying. How could the parallel port be wired
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to be REALLY bi-directional?
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The offending item appeared to be the 74LS374 IC used as the data
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output latch--the Output Enable (OE*) pin was grounded (wired
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active). However, there was a spare bit (bit 5) available on the
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74LS174 6-bit latch used to control the printer handshaking
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lines. The solution was obvious--wire the unused bit to Output
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Enable on the LS374--viola!
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Incidentally, if you're using an IBM PS/2, ignore the next
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section, IBM finally came to its senses and implemented the same
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change when it brought the PS/2 out--you can use the software
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that accompanies this documentation will operate with no changes.
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ALTERING A PARALLEL ADAPTER
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Once again, note that this applies to PC XT- and AT- type
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computers (ISA bus) only. If you've got a PS/2, your changed
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parallel port is standard equipment.
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I'll discuss what has to be done to the simple $15.00 born-in-
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Taiwan parallel adapter, but note that these changes can be made
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to most other parallel adapters that use generic LSTTL IC's.
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This category also includes a number of monochrome display
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adapters.
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You should know one end of a soldering iron from another; a short
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length of wire-wrap or other small-gauge wire and an X-acto or
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other hobbyist utility knife is useful. The change is simple and
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will take you about 15 minutes (assuming your iron is hot).
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Most inexpensive parallel-only adapters come from the same basic
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design--12 SSI IC's on a half-slot card.
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First, locate the 74LS374 IC adjacent to the printer connector on
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the rear of the board. Note that one end of the IC has a
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recessed notch on one end. Also locate the 74LS174 just above
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it. Note the pin numbering:
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74LS374 74LS174
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+-U-+ +-U-+
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Pin 1 [| |] Pin 20 Pin 1 [| |] Pin 16
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Pin 2 [| |] Pin 19 Pin 2 [| |] Pin 15
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Pin 3 [| |] Pin 18 Pin 3 [| |] Pin 14
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Pin 4 [| |] Pin 17 Pin 4 [| |] Pin 13
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Pin 5 [| |] Pin 16 Pin 5 [| |] Pin 12
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Pin 6 [| |] Pin 15 Pin 6 [| |] Pin 11
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Pin 7 [| |] Pin 14 Pin 7 [| |] Pin 10
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Pin 8 [| |] Pin 13 Pin 8 [| |] Pin 9
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Pin 9 [| |] Pin 12 +---+
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Pin 10 [| |] Pin 11
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+---+
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Remembering that the pin numbering becomes "mirrored", turn the
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board over and locate pin 1 of the 74LS374. Note that it is
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connected by an enlargement of the solder pad to the wide ground
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trace above it. Being careful not to sever the wide ground
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trace, take your knife and separate the pin 1 pad from the ground
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trace. Check your work with a continuity tester.
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Next, locate pin 7 on the 74LS174. Take a short piece of wire
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and connect pin 1 on the 74LS374 to this pin. Be neat and check
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your work.
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That's it! Your card will still function normally as a printer
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adapter, but now has a true bi-directional mode of operation.
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BUILDING A CABLE
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The next task is to build a cable from the 25-pin parallel
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printer connection to a 24-pin male GPIB connector.
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Unfortunately, some "criss-crossing" of connections is necessary
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between the two, so you can either solder up a multiconductor
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cable between a male solder-cup DB25P connector and a 24 pin
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"Blue Ribbon" connector, or you can do what I did.
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I took a length of 24-conductor ribbon cable and crimped a male
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DP25P IDC connector on one end and a male 24 pin "centronics"
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connector (Scotch No. 3548, for example) on the other end. I
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then took an inexpensive solder-type DB25 "breakout box" (cost:
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about $7.50) and performed my "wire weaving" in it.
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In any case, you'll have to make sure the wiring works out this
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way:
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GPIB Pin Signal Name DB-25
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======== =========== =====
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1 -DATA 1 2
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2 -DATA 2 3
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3 -DATA 3 4
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4 -DATA 4 5
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5 -EOI 13
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6 -DAV 1
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7 -NRFD 16
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8 -NDAC 17
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9 -IFC 10
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10 -SRQ 15
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11 -ATN 14
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12 GND 18
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13 -DATA 5 6
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14 -DATA 6 7
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15 -DATA 7 8
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16 -DATA 8 9
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17 -REN 12
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18 GND 19
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19 GND 20
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20 GND 21
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21 GND 22
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22 GND 23
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23 GND 24
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24 GND 25
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Check your work for accuracy!
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THE SOFTWARE
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We've supplied the source and object for a set of "C"-callable
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routines to manage the GPIB interface. We use Microsoft "C" and
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Microsoft MASM, though there should be no reason why this
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wouldn't also work with Borland's Turbo "C" and TASM. Note that
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we make use of the "small" memory model--you could alter this to
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use the huge, large or compact models by making sure that full
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segment-offset addresses are handled and the necessary segment
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register juggling is done.
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Similarly, there's no reason that the package couldn't be modifed
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to work with BASIC or FORTRAN with appropriate changes.
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The source file is the file "GPIB_C.ASM" and is written in
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8086 assembly language. These are the routines that are included
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in it:
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int GPIB_Init( int io_port, int our_address)
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Initializes the GPIB interface. "io_port" is the address
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of the printer adapter being used--usually 0x378 for the
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first and 0x278 for the second. If your adapter is part
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of a monochrome display adapter, its address is 0x3bc.
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"Our_address" is the GPIB talker/listener address that
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the PC is to consider to be its own.
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The interface is initialized; if no response can be had,
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a status of -1 is returned; a return of 0 signifies no
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error.
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int GPIB_Send( int listen, char what)
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This routine sends a single character to the GPIB device
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addressed by "listen". If this function returns -1,
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there's a problem, otherwise the return value is 0.
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int GPIB_PutStr( int listen, char *string, int count)
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This routine sends a string of bytes to the GPIB device
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addressed by "listen". "count" bytes are sent. Returns
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are -1 for failure, 0 for success.
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int GPIB_Stat(void);
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Simply returns the value of the GPIB status lines.
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These are encoded in the lower 8 bits of the return
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value as:
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IFC REN EOI SRQ NDAC NRFD ATN DAV
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int GPIB_Get( int listen)
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Reads a character from the GPIB device addressed by
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"listen". Returns the character or -1 if error.
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int GPIB_GetStr( int listen, char *buf)
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Reads a string of data from the device addressed by
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"listen". Returns the number of bytes read into "buf"
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or -1 if error.
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int GPIB_SerPoll( int listen)
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Executes a Serial Poll on the device at "listen".
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Returns the serial poll status in the lower 8 bits of
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the return value or -1 if error.
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int GPIB_PutAdd( char what)
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GPIB primitive. Puts the value "what" out as an address
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byte. Returns 0 if success or -1 if failure.
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int GPIB_PutData( char what)
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GPIB primitive. Puts the value "what" out as a data
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byte. Returns 0 if success or -1 if failure.
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int GPIB_GetData(void);
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GPIB primitive. Reads the value on the GPIB bus as a
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data value and returns it or -1 if failure.
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As an example of usage, a terminate-and-stay-resident program,
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LPPLOT, is included to talk to an HP 7470 plotter disguised as
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LPT3. It works--but note that the GPIB version of the 7470 lacks
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arc and circle-drawing HPGL extensions.
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MISCELLANY
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Clearly, this scheme represents a way to get by on the cheap.
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This method will not support all GPIB functions, nor is it likely
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to be able to drive more than one GPIB device at a time--the
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output current drive capability just isn't there.
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If I had it to do over again, I'd change the way I wired the
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cable and wire the ATN signal to pin 10 on the DB25 so that I
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could use the interrupt capability on the printer card to service
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asynchronous requests such as Parallel Poll.
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But the thing does work--and with a little work, 2 PC's could be
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coupled to do full 8-bit transfers in either direction. Current
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printer port data transfer schemes "nibble" the data, rather than
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use the full data bus width.
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USAGE AND LICENSING
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License is hereby granted by Sydex for single-use non-commercial
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application of this code. Contact Sydex for commercial use and
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system-integration licensing rights. Any other use of this code
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or documentation is hereby prohibited without explicit written
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permission from Sydex.
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Text and programs by Chuck Guzis
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Certain products and terms referred to in this document are
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property of the following firms:
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Hewlett-Packard Corporation, International Business Machines,
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Microsoft Corporation, Borland International, Centronics,
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ITT-Cannon, 3M Corporation.
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