🚀 go-pugleaf

FSC-Control: EID:d9a7 163896b5
From: Murray Thessman
Keywords: parallel port I/O
Organization: GenBOARD of Wellington New Zealand, INFN# (3:771/160)

In a msg from BonaFido (our UseNet gateway) 22-Jan-91, Winthrop Chan wrote ..

 >WDC:  Hi, I'm trying to find data on how to use the IBM-PC parallel port 
for 
 >WDC:  both output and INPUT. I know this can be done since quite a few
  >WDC:  transfer programs do this trick. I need port numbers and how they 
do  >WDC:  it.

I repeat a msg I wrote to David Ingram in Zone3_Tech on Fido on 5-Jan-91. I 
hope that this is what you were after....

In a message of <Jan 01 15:12>, David Ingram (3:770/110) writes: 
 >DI:  Could somebody please tell me how I can control the printer port for 
 >DI:  input and output (e.g. controlled a robot).

The standard parallel printer port found on most Herc, EGA or parallel 
printer 
 port is made up of 3 separate ports: an 8 bit output, a 5 bit input and a 4 
bit 
 quuasi bi-directional port.

The 8 bit is quite straight forward. The 8 bit data is sent to an Octal 
latch 
(74LS374) then directly to the D25 (bits 0-7 -> DB25 pins 2, 3, 4, 5, 6, 7, 
8, 
 9). There is also an octal tristate buffer (74LS244) to provide feedback 
which 
 allows some printer drivers to verify the integrity (ie no short circuits). 
Both these ICs have the same address.

The 5 bit input port is via means of inverting tristate buffers but before 
reaching the buffers SOME bits pass through additional inverters. These bits 
have been inverted twice and will therefore be correct! (bits 3-7 -> DB25 
pins 
 15, 13, 12, 10, 11). Only bit 7 needs inverting by software. Note that DB25 
pin 
 10 (Bit 6) is also connected to interrupt IRQ7 and can be used to generate 
interupts if these are enabled.

The 4 bit quasi bi-directional port use open collector output drivers with 
4K7 
 pull-up resistors on board. By switching the output drivers off (ie logic 1 
state) the port can be used as an input port. This means that to switch the 
output drivers off the bit pattern 0100 (04 decimal) must be output to this 
port. When inputting, the input buffer is itself an inverter so that the 
line 
that passes through a further inverter (bit b2) will be input in the correct 
sense. This means that the remaining bits b0, b1, and b3 will need inverting 
by 
 the software.

To save you working out the (almost complicated) logic <grin>

 Constant
       BasePort = $378                 { change to $3BC if on EGA or Herc }
                                       { see table below }

 Procedure InitInput;                  { init bi-direct port to input only}
   Port[BasePort + 2]:= 04;            { switch off drivers }
 end;

 Procedure InputIBM (var combined: byte);
 { Input from 4-bit and 5-bit ports, combine into a single 8-bit number
   and corrects inverted bits }
 var
   hi,lo : byte;
 begin
   hi:=Port[BasePort + 1] and $F0;      { input }
   lo:=Port[BasePort + 2] and $0F       { input }
   combined := hi or lo;                { combined nibbles }
   combined := combined xor $8B;        { invert bits 0, 1, 3, 7 }
 end;

 One last thing the port adresses

                         PARRALLEL      EGA or
  PORT                  PRINTER CARD   HERCULES CARD
 8 bit outport           378 Hex        3BC Hex
 5 bit input             379 Hex        3BD Hex
 4 bit bi-directional    37A Hex        3BE Hex

 [ The above information was obtained from NEWelectronicS/March 1988 
   'Getting the best from a standard printer port - Jeff Broard' Pg 33]

 >DI:  I'm sure it can be done somehow. 
Yes you where right! I plan to use the printer port it to switch the modem 
off 
 and back on if I get a 'Modem not okay' (exit error 2) from OPUS to reset 
everything. BTW LPT 2 & LPT 3 Will have different addresses.

[Just as a trailer I wouldn't mind a reply from you if you get this (always 
suspicious of gateways - sorry John)]

regards Murray
 

--- msged 2.05
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