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RE:Programming a ST78C34 Parallel Port Chip Solved!

Well I figured out the problem I was having getting the ST78C34 strobe signal to work on out parallel ports S-100 board.   Nothing to do with software. The chip has an active high reset pin. The S-100 bus has an active low reset signal.   So the chip was all the time held in the reset state. Patch is simple, bend out pin of U10 and jumper its pin 1 to pin 3. The chip works fine after that. You can use the exact IBM-AT BIOS routine to initialize, print etc.  Was interesting the chip in its reset state behaved exactly as it should in every respect (status lines, data etc.) just no strobe signal on its pin 25.  I have written all this up here:-



bottom of the page.


BTW, I hate boards with patches (however small),  I’m thinking of getting a few V2 boards with the two patches corrected. Anybody else interested? 







From: n8vem...@googlegroups.com [mailto:n8vem...@googlegroups.com] On Behalf Of monahanz
Sent: Tuesday, November 5, 2013 10:27 PM
To: n8vem...@googlegroups.com
Subject: [N8VEM-S100:2022] Programming a ST78C34 Parallel Port Chip


Has anybody had any luck programming the ST78C34 PC Parallel port chip using our Parallel Port S-100 board (described here):


I can get it to output bit values to the base port. I can read the status, command registers etc. but I cannot figure the order of things to get the chip to pulse the strobe line.

The chip is described here:-





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