CMT 64K RAM strange behavior (HP-71B)


Ok, this is wierd. I haven't ever noticed this before. I have a few CMT 64K RAM modules that all behave as though they are battery backed up. These are the 64K RAMs that plug into the front ports and not the card reader.

When I first got these, I plugged them into a 71B but the 64K RAM didn't show up on my system when I executed MEM function. I traced this to the fact that these RAM modules were configured as independent memory or ROM. That is strange as the memory should have been cleared after being unplugged for so long. They appeared to be ROM.

I thought they might be mismarked as they came unpowered (not in 71B) but already had programs on them. They were used in a system as independent RAM. They have been unpowered for over a week (perhaps months) and the programs are still in the RAM.

I tried deleting one of the programs and it deleted ok. I then took another identical RAM and copied the deleted program from the 2nd RAM back to the first RAM. No problem; it copied fine. So, it is definitely RAM.

Then I created a test program and wrote it to the 1st 64K RAM. Again, no problem, it copied fine. I removed the RAM and let it sit for a day or so and plugged it back in. The program was still there.

I tried this same thing in some of my CMT 32K RAMS and programs were lost immediately. Programs are lost so quickly, on a CMT 32K RAMs, that I can't even quickly move one from port 1 to port 2 without losing data. But the CMT 64K RAMs are holding data days, weeks or longer.

Does the CMT 64K RAM have a battery inside? I know it is sealed so that seems unlikely. Besides how could one change it if it did?

Could this be modified? Or, is this just normal behavior for a CMT 64K RAM module?

Edited: 26 Dec 2003, 12:21 p.m. after one or more responses were posted


May be a capacitor/condensor inside?!
That should be an explanation...
Merry Christmas.


But it doesn't seem likely that a capacitor would retain memory for weeks.

I'm really interested in knowing how the CMT 64K (front port) RAM works for others.



I have some HP41CX machines that hold up their RAM for over a month without a battery installed. CV's that last well over two weeks. Most 41C's will last a week.


Those modules were indeed battery backed up. The battery is a very thin lithium cell. You can see the battery protruding from a slot on the end of the module that faces into the HP71. Just pull it out and there will no longer be battery backup. Make careful note of the orientation when you pull it out; the top side is positive. I marked a small + sign with a permanent magic marker on mine.
What I have just described is how my module looks. They may have come out with a non-removeable cell in later production, but the modules definitely were battery backed up.


I can't believe that all the time I've been using these, that I never knew that. I thought that was just some sort of shield .

I have taken a few photos to show what this battery looks like.

I guess that begs the questions:

  • How long the batteries last?
  • Can replacement batteries be found?
  • Where can they be obtained?
  • At what reduced voltage can the loss of RAM contents be expected?
  • When installed, is the drain of this battery blocked?

There seems to be quite a lot on paper thin batteries, via the internet, so I guess a substitute can be found, if these ever die.

The three that I currently have, measure 3.022V, 3.002V and 2.860V.

64K RAM with backup battery

Thanks for the info Rodger.

Edited: 27 Dec 2003, 10:00 a.m.

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