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From: hase@netmbx.UUCP (Hartmut Semken)
Newsgroups: comp.sys.atari.st
Subject: Re: How to buffer the parallel port (was "deskjet")
Keywords: DESKJET DESKJET+ BUFFER CABLE FIX
Message-ID: <3002@netmbx.UUCP>
Date: 6 Jul 89 11:25:16 GMT
References: <12047@s.ms.uky.edu> <2032@jhunix.HCF.JHU.EDU>
Reply-To: hase@netmbx.UUCP (Hartmut Semken)
Organization: netmbx Public Access Unix, Berlin, West Germany
Lines: 52

In article <2032@jhunix.HCF.JHU.EDU> ins_adba@jhunix.UUCP (Dom Alvear) writes:
>In article <12047@s.ms.uky.edu> avogel@ms.uky.edu (Andrew Lee Vogel) writes:
>>
>>Could someone send me, or post, the cable fix for the deskjet (+) printer.
>>
>Yes, please do post the correct way to buffer the parallel port line.
>A long time ago, I had to buy a new motherboard because the repair
>center was too stupid to figure out how to fix it.  (I guess they just
>had to replace the sound chip?)  This was NOT with a Deskjet or
>Deskjet+, but it was a parallel port problem.

Well, here it goes:

The Sound Chip (General Instruments AY3-8910 compatible Yamaha chip) is
one developed for some video games (like the good old 2600 system).
In a video game there is always some joystick to read in or the like, so
the 8910 includes 2 8-bit parallel ports (port A and B); these are used
in the ST for the Centronics port (port B) and some control lines (drive
selects, side select to floppy, Centronics strobe, GPO and the like).

The port B drives the data lines of the Centronics port directly,
without buffer chips. The sound chip outputs can sink about ten
milliamperes.

The outputs of the ports are open collector drivers: the output is a
transistor wit hthe emitter connected to ground and the output connected
to the collector. This can pull the line to ground but cannot push it to
+5 Volts.
To pull the line to +5 V a "pullup" Resistor (connected to +5V and the
line) is connected inside the printer. If this one is less than 500
Ohms, a current of more than 10 goes through the output transistor of
the sound chip and destroys it (and the rest of the chip: no more floppy
and sound...).

To prevent this, connect a buffer chip (two 7407/74ls07) with the input
to the data lines of the port and the output of the buffer to the
printer.
The power for the buffer chips You get from some TTL chip inside the ST,
the Joystick port (pluggeble!) or the floppy power plug.
The 7407 cat drive 30 milliamperes (thats enough for any printer...).

This will protect the sound chip against odd printers (never had
problems with Epson) but it isn't bidirectional afterwards.


Please fogive me for my errors and "English"

hase
-- 
Hartmut Semken, Lupsteiner Weg 67, 1000 Berlin 37 hase@netmbx.UUCP
If there is something more important than my Ego, I want it caught and shot.
Now! (Zaphod Beeblebrox)