From: rhg@cpsolv.CPS.COM (Richard H. Gumpertz)
Newsgroups: comp.sys.3b1
Subject: Re: Two combo cards in a 3B1: repost
Date: 21 Aug 92 01:57:25 GMT
Organization: Computer Problem Solving, Leawood, Kansas, USA

[From feedback I have received recently, it appears that my previous posting
may have never gotten out to the world (or, at least, not enough of it).
Therefore this is a complete reposting of the original article describing a
proposal for fitting two 1MB combo cards in the same machine.  Will it work?]

I have been looking at how to install two combo cards (with 2MB of RAM total)
in the same 3B1.  My goal is 4 expansion serial ports and 2MB of expansion
memory using just two slots.

The following is an idea I am thinking about.  DO NOT TRY THESE MODIFICATIONS
EXCEPT AT YOUR OWN RISK!  THEY ARE COMPLETELY UNVERIFIED AND UNTESTED!  At
this point I just want feed back from other hardware gurus as to whether I
got it right and whether there might be an easier way.  Note that I have been
looking at loaded boards and some of the traces are a bit hard to follow:
mistakes are quite possible!

	1) Set up the first combo card in the standard 1MB configuration.
	   This card will be installed without modification.
	2) Set up the second combo card similarly for 1MB.
	   Before installation, however, this card will be patched as follows.
	3) Cut the trace on the component side at 1K pin 9.
	   (Cutting on the solder side, just after the first thru hole, would
	   allow the first jumper below to connect to the thru hole instead
	   of 1K pin 9.)
	4) Cut the trace on the component side at 4E pin 14.
	5) Cut the trace on the component side at the thru-hole to the
	   left of C20 (which is to the left of 2B pin 14).  (This trace
	   goes around C20 and then past pins 8-14 of 2B and 3B.)
	   (There are several easier-to-make equivalent cuts on the solder
	   side but this is the easiest-to-describe location.)
	6) Add a jumper from 1K pin 9 to 4E pin 14
	   (or, if you made the cut described in step 3 on the solder side,
	   then from the thru hole by 1K pin 9 to 4E pin 14).
	7) Add a jumper from 2J pin 12 to 4E pin 13
	   (or from the thru hole described in step 5 above to 4E pin 13).
	8) Add a jumper from 4E pin 10 to 4E pin 12.
The net effect of these changes should be to invert the signal LMA20.
This will put the memory in rows 7, 8, 9, and 10 in the upper half
of the 2MB expansion memory address space (as opposed to the standard lower
half).

Rows 5 and 6 (and, more importantly, jumper W2) should be left empty on both
combo cards so they don't overlap!

If I got it all right, the changes can be reversed (to get the electrical
equivalent of a standard combo board) by swapping the jumpers that connect to
4E pin 12 and 4E pin 13 so the second jumper is 2J-12/4E-12 and the third is
4E-10/4E-13.

Comments?

By the way, when given a choice, do you prefer connecting jumpers to pins or
to thru holes?

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Newsgroups: comp.sys.3b1