HP 35 ROM fault?

[Harald]

I have replaced the faulty anode driver and now have a mostly working early HP 35 with split battery compartment and 2.02 bug.
Mostly working because sin, cos, arc sin and arc cos don't work. Everything else, even tan, works. Excecuting the non working functions results in a blank display and a crashed calculator, so I suspect a bad ROM. Would anybody know which instructions are only use by those four functions and which of the three ROMs that would reside in?

Cheers,
Harald

[teenix]

(11-21-2016 10:52 PM)Harald Wrote:  I have replaced the faulty anode driver and now have a mostly working early HP 35 with split battery compartment and 2.02 bug.
Mostly working because sin, cos, arc sin and arc cos don't work. Everything else, even tan, works. Excecuting the non working functions results in a blank display and a crashed calculator, so I suspect a bad ROM. Would anybody know which instructions are only use by those four functions and which of the three ROMs that would reside in?

Cheers,
Harald

Looking at the 35 microcode execute in my emulator, the code for these functions appears to be split over ROM 1 and 2.

cheers

Tony

[PANAMATIK]

(11-21-2016 10:52 PM)Harald Wrote:  ... Would anybody know which instructions are only use by those four functions and which of the three ROMs that would reside in?

Cheers,
Harald

One of my HP-25 calculated wrong sin, cos and tan results. As it turned out it was a single bit ROM error.

Recently I read the classic ROM code from an HP-70 by interrupting one single PCB path and attaching four wires to a PIC working as a address scanner. You could read the ROM code of your HP-35 and compare it with the original.

More interesting question is how to repair the ROM chips. An idea could be to add a tiny processor, which listens to the addresses and corrects the false bit by pulling down the serial data line at the right moment.

Bernhard