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I have an old Zenith “laptop” that runs on an 80c88 processor, and apparently was manufactured around 1985. It’s quite a beast of a laptop - size and weight, rather than computing power!.

Unfortunately it fails at boot with a Timer Interrupt Error. enter image description here

I’ve started digging for any info that will help with a fix, with not much to find on the error or generally on the computer itself. The manuals have some basic hardware descriptions, but I’ve not found any schematics or technical fault finding manuals. If anyone has any hints or tips, your advice would be much appreciated. Also, if there are any other online forums that are better for this kind of question, appreciate the info.

I’ve already posted on a retro computer forum on Facebook, with some good advice for fault finding, however the fix remains elusive, so looking to the experts on this forum for any further advice. Discussion on FB forum

Appreciate any help getting a fix for this. Thanks Rob

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  • My first suspicion on anything really old like this is a CMOS backup battery. These often were used to keep the BIOS settings in CMOS RAM and also to run an RTC. Dead battery means no RTC which ultimately means no timer interrupt which the startup may be expecting. Try opening the case and see if you can find a battery that can be replaced. – jwh20 Sep 21 '20 at 17:06
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    @jwh20, The (18.2Hz) Timer Interrupt in an 8088 PC compatible is not generated by the RTC but by the 8253 Programmable Interval Timer (or a derivative), counter 0, which might be a good place to start looking for clues as to what’s wrong. – StarCat Sep 21 '20 at 19:43
  • thanks guys. Yes, I've not found any batteries for RTC on the board. There are 8259 and 8254 chips that are part of this circuit. – RobC Sep 21 '20 at 20:20
  • I've done some basic checks for an output on the IR0 line of the 8259 (there was), but I don't have an Oscilloscope (or skills) to get in to the detail of interrupt signal timing. Keen to learn though. – RobC Sep 21 '20 at 20:22
  • The 8254 is equivalent to an 8253. Lacking a scope you could try one of the cheap 8-channel logic analyzers (and learn a lot in the process). – StarCat Sep 22 '20 at 17:04

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