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Plotter ROM for the HP9810 - how to use it?
04-19-2014, 08:45 PM (This post was last modified: 04-19-2014 08:49 PM by Thomas Falk.)
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RE: Plotter ROM for the HP9810 - how to use it?
Hi Eric,

(04-19-2014 11:44 AM)waterlander2004 Wrote:  how did you fix the failed microcode ROM problem on your 9810?

This was easy: I have found that only one bit was wrong at a certain address. It was stuck at H. Therefore I decoded out this address and pulled the bit down. This was done using a 74HCT688 decoder and a AND-Gate made by diodes. The resulting fix is shown in the following picture.


Finding this was a little more difficult. When I started to repair this calculator I found a dead logic gatter and a dead transistor in the drivers for the DRAM ciruits. After repairing this it worked, but the display was wrong.

In the end I read the addresses and the data of the microcode ROMs and found the faulty bit. Actually after having done this it is quite easy: If you read the adresses and the data during running the calculator you get a partial dump of the mico code ROMs. You will get the data for the adresses the calulator is accessing only, but this is enough. This data you can compare with the listing of the ROMs in the patent US-3,859635 describing the calculator. My work place looked like this:


There are two logic analyzers in use. For comparing the read data and the data from the patent I wrote some small python scripts.

If there are more faulty bits in the ROM the fix above is not practicable and you would have to replace the ROM by some modern or older parallel EPROM. This should be possible.

When I repaired my HP9810, it was my only device. I got reference data from the HP9800e emulator and compared its program dumps with the data from my calculator to dig down to the micocode. To isolate a faulty card it is useful to have a second calculator for board swapping.

I have now a HP9810 and a HP9820, if somebody needs some material to swap for error search, I may be able to help. Just ask.

Best regards,


PS: I will show this calcuator and its plotter in use at the VCFE in Munich at the begin of May.
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Plotter ROM for the HP9810 - how to use it? - Thomas Falk - 12-12-2013, 12:09 PM
RE: Plotter ROM for the HP9810 - how to use it? - Thomas Falk - 04-19-2014 08:45 PM

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