TRS-80 PC-2 (pocket computer): How does it remember without batteries?

[Joel_D]

Hi folks. I purchased a used, vintage TRS-80 PC-2 pocket computer a while back, and I find it really neat.

I have a technology question:

To my surprise, when I remove all of the batteries (4 AA), and then reinstall them within a few minutes, the RAM contents remain intact! The clock seems to keep running as well. So how does this work?

There must be some kind of battery backup, right? Or a big capacitor that holds a charge long enough to keep it going? (I admit to only trying it for about 5 minutes at a time.)

OK, I could understand that, but how is this still working after what has been (I assume) 27 years? (1982, right?)

Also, how reliable is this mechanism? I mean, should I worry about replacing the backup battery at some time? I don't know how much the computer was used before I got it or if that has any bearing on how soon it wears out.

Thanks for your expertise.

PS: Sorry if I should have posted to the pocket/handheld group instead, but it seems there are a few PC-2 owners in this forum, so I thought I'd try here first.

Regards,
Joel

 

[TandyMan100]

Much like it's larger cousin, the TRS-80 Model 100, the Tandy Pocket Computers also use NiCADs to battery-back their RAM. Thus, when changing batteries or when your batteries die, the set time, date, and your files are kept intact for 28 days or more!

 

[TandyMan100]

I forgot to mention, whenever you acquire a machine with battery-backed RAM, the first thing to do is open it up and check for battery leakage. It the NiCAD leaks over everything, it will totall your computer.

 

[Joel_D]

Thanks TandyMan!

Wow - sorry - I'm just shocked that it's still working. I guess I'm used to modern rechargables in cell phones, laptops, etc. that seem to crap out after a couple of years. (I know, different technology, but still.)

So how long can one expect the NiCADs to last in a little computer like this? Are they recharged by the main batteries? If so, does that mean it's best to store the computer with main batteries installed? Or is it better to drain the backup battery and store it uncharged?

I'll try to open it up tomorrow to see if there are any signs of leakage.

Thanks again,

Joel

 

[Joel_D]

By the way, I took a look inside the PC-2, and it looked... really nice! No signs of corrosion or damage. Like new, really. I also noticed two things...

1) Not only do I not see any corrosion, but I also do not see anything that looks like a reserve battery either. Looking at the 'exploded view' in the Service Manual (http://www.pc1500.com/docs/pc2ServiceManual.pdf.zip, 10MB), I don't see any parts that resemble a battery either, but maybe I missed it. Are you sure the PC-2 has one?

2) With the main batteries removed for about 22 minutes, the clock lost 12 minutes, from which I can only conclude that it must have run for a period of time (say 10 minutes) and then stopped. But still, that 'last chronological moment' and all the RAM contents remained intact when I turned it back on.

So I'm still a little puzzled.

Any other thoughts?

Joel

 

[poptones]

nA current

nA current

Way back when this stuff was still new part of my job was one time researching just how much current it would take for one of the shiny new 8KB JEDEC RAM chips to hold its contents.

It took almost nothing. Most I tested would remember their contents down to less than 1 volt and the current draw was nanoamps. A power supply cap connected to a couple of these small static rams won't even notice it - when the power is shut off they will discharge at a rate pretty much detemined by their internal leakage current.

If you want to clear the ram, when the batteries are removed short out the pads that connect to the first and last batteries in the chain. This will discharge the caps (although they will retain some charge when you remove the short) and kill the power completely to the ram.

Or, safer, just replace the batteries and press the reset button on the back.

EDIT: just looked at teh system schematic and the shorting business won't even work - there's a fairly sophisticated power supply in there (4 transistors, fets, etc) and a few diodes in series with each power input. That machine also uses only a 2KB JEDEC CMOS ram (a 6116) which means its current draw is pretty much nothing. That's why it "remembers" so well - when the power goes off it's still powered by the combined capacitance of the power supply traces connected to its power leads.

 

[geoffm3]

Awesome! I spent many an hour playing with the PC-2 as a kid. I wish we had had the memory expander for it, but we never did.