Aquisitions

From: A.R. Duell <ard12_at_eng.cam.ac.uk>
Date: Fri Apr 11 09:59:33 1997

>
> Well I went out yet again today and found a really nice IBM 5155 Portable
> PC at the local Salvation Army thrift store. I picked it up and brought it

In case you've not spotted it, most of that machine is built from standard
IBM parts.

The motherboard in that machine is a standard XT one. The video card is a
standard IBM CGA card (although the monitor is monochrome, and plugs into
the modulator connector), and the floppy controller is the standard PC/XT
one

What is different is the PSU (different output cables and box, otherwise
pretty standard), the drives (half-height, or as IBM call them 'slimline
drives'), and the monitor. The latter seems to be a Zenith composite video
monitor.

Schematics for everything but the PSU are in the appropriate TechRefs
(PC/XT and Portable Techref for the motherboard and keyboard, Options and
Adapters TechRef for everything else).

> good price. BTW, the IBM has a whopping 256k of RAM.

If you want 640K without wasting a slot, here's what to do :

Remove the motherboard (Yes, it's a pain to do so - you have to remove the
drives first to get at one of the screws!)
Remove banks 0 and 1 of RAM chips and replace them with 41256 (or
equivalent) 256Kbit chips. Leave the 64K chips in banks 2 and 3. All the
rams face the same way.
Put a 74F158 (or 157) in the empty 16 pin socket at the front edge of the
board. A slower chip (like a 74LS158) may also work - I will have to
check. This chip faces the same way as the surrounding chips.
Solder a jumper between pads E1 and E2 at the right edge of the board
Reassemble and enjoy 640K :-)

This is an official IBM mod IMHO. My TechRef shows schematics for both
versions of the board.

>
> Jeff jeffh_at_eleventh.com


--
-tony
ard12_at_eng.cam.ac.uk
The gates in my computer are AND,OR and NOT, not Bill
Received on Fri Apr 11 1997 - 09:59:33 BST

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