HP-41C: Module conflict

I have an HP-41C which shows some kind of failure when a Memory Module (MM) is plugged in (might well be that the same failure would show up with a Quad- or a completely different module, though I haven't checked it yet). The machines "operating mode" depends on the port that is occupied by the MM: When the machine is operated with a MM in port 1 it seems to work correctly (at least, the MM is recognized by means of SIZE). If a MM is plugged into ports 2 or 4, a very faint (actually barely perceptible) and high pitched tone is emitted by the machine and it doesn't respond to the keyboard anymore. In port 3 the MM is not recognized at all. Otherwise (without MM that is) the calc seems to work fine. Does anyone have an explanation for this behavior?


I once had a similar problem that was only solved by turning to another module of the same type. I still think it was a problem with the module itself and I was not able to faind a solution. I still have the "problematic" module because may be I can revive it.



check module's contacts. There are two contacts at the I/O assembly that are used to address the inserted modules. If there is a bad contact in any of them, you'll probably have problems (I am not sure if they will be like the ones you mentioned).

The only time I experienced this was when I tried plugging a MM in an HP41CV. You know, I rebuilt the babe believing it was a 41C, and it was a 41CV. After plugging the MM, the machine started a continuous, low-power tone. I removed the batteries and tried other doors. Only one accepted the module, and I am almost sure it was port 1. Without the module, I executed SIZE 000. Surprise: 00 REG 319. It was a 41CV!

This is not your case, I know, but I needed to tell the tale.

Cheers. And have a carefull look at the contacts.


That is normal. In a 41C the first MM has to be in port 1 the second one in port 2 and so on.

You can not put say, a PPC-Rom in port 1 and a MM in port 2 also MM can not skip ports.


port 1: MM

port 2: PPC

port 3: MM

This is explained in the directions for the modules.



Thanks a lot for the information, Christopher! I didn't know that the order in which MMs have to be plugged in do obey a strict rule! As for Jose and Viera: Thanks for your input anyway. Guess, we've just learned a new bit about our calculators, don't we? :-)

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