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IBM 5170/ AT clone P8042 replacement

UnicycleFunStuff

New Member
Joined
Jul 8, 2021
Messages
8
So several months ago I was able to get an IBM 5170 with 30mb st4038(and a model F AT keyboard that I'm typing on rn) for a decent price because some caps in the PSU blew up. After it arrived and I replaced some PSU caps it worked, until it didn't and would refuse to POST and didn't even give any video signal or beeps. After a lot of troubleshooting, thanks to minuszerodegrees.net and a post card I was able to determine that the P8042 chip was failing. It would buffer for POST code 0A but would hang at 0B (it wouldn't drop A20 low for the CPU to reset) leaving me with a blank screen.
At this point I was stressed out because it meant I had to get a blank 8742 chip and somehow tracking someone down who was able and willing to program it, which would cost me money and time that I was low on already (I quit my job to do school work). So doing what I usually do when I hit a barrier on a computer restoration, I did some research. I read just about every data sheet on the 8042 chip series available, and then when I was reading the sheet on the P8042AH chip I read:

"The current list of available 8242 versions include keyboard controller firmware from both Phoenix Technologies Ltd., IBM, and Award Software Inc. The 8242 is programmed with Phoenix Technologies Ltd. keyboard controller firmware for AT-compatible systems. This keyboard controller is fully compatible with all AT-compatible operating systems and applications. The 8242PC also contains Phoenix Technologies Ltd. firmware. This keyboard controller provides support for AT, PS/2 and most EISA platforms as well as PS/2-style mouse support for either AT or PS/2 platforms."

The datasheet later read that the 8242BB, 8242WA and 8242WB are also fully compatible with the IBM PC AT/ PC clones. I immediately went on eBay and found a bunch of P8242 chips for 5-10$! I bought one of those chips with the right firmware, a DIP 40 IC socket, (and a pack of 50 rifa PSU caps because this was the third machine I've had blowup due to them). When they arrived I unsoldered the original P8042 and put in the IC socket & P8242 chip. I flipped on the power switch and the machine actually booted!

Considering the old method of replacing the P8042 chip with a programmed P8742 chip, I figured to post my findings about the 8242 chip to possibly help other people save time and money.
This is my first post and I created this account just to talk about this, but I might also post about my Osborne 1a and Apple iie later if I feel like it.
Also I attached a few photos of my project.
 

modem7

Veteran Member
Joined
May 29, 2006
Messages
7,792
Location
Melbourne, Australia
This is my first post and I created this account just to talk about this, ...
Welcome to these forums.

... put in the IC socket & P8242 chip. I flipped on the power switch and the machine actually booted!
I added the failure of your keyboard controller to the list at [here].

The datasheet later read that the 8242BB, 8242WA and 8242WB are also fully compatible with the IBM PC AT/ PC clones.
Readers should be aware that that data sheet will not mean 'all' AT clones. For example, the Dell System 200 deviates from the IBM AT in the keyboard area, enough so as not to be able to use IBM's keyboard controller (IBM 1503033) in place of the Dell supplied controller.

Considering the old method of replacing the P8042 chip with a programmed P8742 chip, I figured to post my findings about the 8242 chip to possibly help other people save time and money.
Thank you for that information. I am sure that it will be useful for some.

Along the same lines, I thought that the 'Via VT82C42 Keyboard Controller' IC (includes firmware) was going to be suitable for the IBM AT. The data sheet did not indicate that it was IBM AT compatible, but I read enough to think that when fitted to the IBM AT, the VT82C42 would run in its 'AT mode' and work in the IBM AT. But none of the VT82C42's that I bought did. It is on my to-do-list to investigate more.
 

UnicycleFunStuff

New Member
Joined
Jul 8, 2021
Messages
8
Thank you for entering my failure on your site.
I should also note that my AT is currently using quantel BIOS, but my rev-1 01/10/84 BIOS worked just fine with it and I don't see why any other AT BIOS would cause issues.
 
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