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replacing 71b batteries without losing data
07-14-2023, 11:27 AM
Post: #1
replacing 71b batteries without losing data
How long do you have to swap batteries (eg 1 at a time) in a 71b without any risk of losing data?

Is it safe to plug in the AC adaptor and change the batteries while the 71b is off?

Thanks, dmh

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07-14-2023, 11:48 AM
Post: #2
RE: replacing 71b batteries without losing data
You definitely must connect the AC adapter when swapping batteries, else you will lose MAIN RAM. Sometimes, if you're quick, it is possible to swap batteries w/o adapter connected and not lose contents of IRAM modules, but I have never been able to swap batteries w/o losing MAIN RAM, even when new.

I have heard/read that simply swapping one battery at at a time, rather than removing all 4 at once, and installing new ones as you proceed to do all 4 can will work, but I have not been able to pull this off. But I don't try since I have an 82059B plugged-in and always ready on the desk.

--Bob Prosperi
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07-14-2023, 12:10 PM (This post was last modified: 07-14-2023 12:11 PM by KimH.)
Post: #3
RE: replacing 71b batteries without losing data
A few quick points in response to your question

I have NOT done a methodic approach/charts for how long the machine can hold its memory without batteries, but would suggest (assuming the batteries IN the machine are not dead already) that even several minutes should be fine.

Switching batteries - I have taken all 4 out and replaced them, no rush - never dropped the memory.

One by one is even more "safe"

You may know this - but maybe not. Be careful that you don't accidently press a key on the bottom-row keyboard when you work on the replacing of the batteries - it will wake the CPU and drain whatever is in the power supply CAP (470uF) quite quickly.

With the Power connected, you have "forever" to swap. I have 2 machines on my desk both of which had no batteries for several months, just the power supply. No loss of memory, even with a short 30 min mains power outage.


What I HAVE done, which could be relevant - is to experiment with putting an MLCC (Y5R-Type) into a few 4k and 32k modules.

Mainly to have a ready set of LEX-Files and Programs, which I use all the time - EPROM is better, but I lack access to a programmer... - still :-(

Even a tiny (!) 4.7uF (C1206C475K9) can hold memory in a module for up to 10-15 minutes (best was just under 20 mins). Goes to show how low the current draw/drain is when power is removed.
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07-14-2023, 01:16 PM (This post was last modified: 07-14-2023 01:17 PM by KimH.)
Post: #4
RE: replacing 71b batteries without losing data
(07-14-2023 11:27 AM)dmh Wrote:  How long do you have to swap batteries (eg 1 at a time) in a 71b without any risk of losing data?

Is it safe to plug in the AC adaptor and change the batteries while the 71b is off?

Thanks, dmh

I just noticed that you are Calculator Clique...

Thanks for your posting on FORTH on the 75 - i watched that a couple of times...

IF you are working with FORTH on the 71B and have the Forth in a MULTIMOD card-reader module, you need to take a specific precaution for that module.

Elsewhere on this site Multimod - Memory Lost you can see that when you turn the 71B OFF, it is not really OFF for some time - and swapping batteries needs some extra care.
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07-14-2023, 01:40 PM
Post: #5
RE: replacing 71b batteries without losing data
Hi KimH !

Thanks for your feedback, much appreciated :-)

Thanks for both your replies - I do have a MultiMod and totally missed that post and the mention in the documentation so this is a life saver!

I've dabbled a little with FORTH on the 71b but intend to spend some more time on it now that I've spent a lot of time on the 75C.

Sounds like your experience with 71b battery change is the total opposite of Bob's.

Regards, dmh

(07-14-2023 01:16 PM)KimH Wrote:  
(07-14-2023 11:27 AM)dmh Wrote:  How long do you have to swap batteries (eg 1 at a time) in a 71b without any risk of losing data?

Is it safe to plug in the AC adaptor and change the batteries while the 71b is off?

Thanks, dmh

I just noticed that you are Calculator Clique...

Thanks for your posting on FORTH on the 75 - i watched that a couple of times...

IF you are working with FORTH on the 71B and have the Forth in a MULTIMOD card-reader module, you need to take a specific precaution for that module.

Elsewhere on this site Multimod - Memory Lost you can see that when you turn the 71B OFF, it is not really OFF for some time - and swapping batteries needs some extra care.

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07-14-2023, 02:46 PM
Post: #6
RE: replacing 71b batteries without losing data
Glad that I coincidently was able to help - looking forward to your posting on the tube.

Maybe I was simply lucky, Bob is pretty much always spot on with his responses!

(07-14-2023 01:40 PM)dmh Wrote:  Hi KimH !

Thanks for your feedback, much appreciated :-)

Thanks for both your replies - I do have a MultiMod and totally missed that post and the mention in the documentation so this is a life saver!

I've dabbled a little with FORTH on the 71b but intend to spend some more time on it now that I've spent a lot of time on the 75C.

Sounds like your experience with 71b battery change is the total opposite of Bob's.

Regards, dmh

(07-14-2023 01:16 PM)KimH Wrote:  I just noticed that you are Calculator Clique...

Thanks for your posting on FORTH on the 75 - i watched that a couple of times...

IF you are working with FORTH on the 71B and have the Forth in a MULTIMOD card-reader module, you need to take a specific precaution for that module.

Elsewhere on this site Multimod - Memory Lost you can see that when you turn the 71B OFF, it is not really OFF for some time - and swapping batteries needs some extra care.
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07-14-2023, 02:50 PM
Post: #7
RE: replacing 71b batteries without losing data
I have never had an issue losing memory during a leisurely battery change, either.
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07-14-2023, 05:54 PM
Post: #8
RE: replacing 71b batteries without losing data
Without a power supply plugged in, I almost get a ML all the time here.
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07-14-2023, 06:42 PM
Post: #9
RE: replacing 71b batteries without losing data
For reference, according to the HP-71B Owners Manual, you should have at least 30 seconds to replace the batteries without memory loss. From the comments here it sounds like this may not always be accurate. It does recommend connecting the AC adapter while changing batteries as a sure way to prevent memory loss.


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07-14-2023, 06:57 PM
Post: #10
RE: replacing 71b batteries without losing data
Maybe the capacitor holding the charge degraded more or less. Same with several HP-41s here. Some no hurry others one second and I got ML
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07-14-2023, 09:14 PM
Post: #11
RE: replacing 71b batteries without losing data
I've never had any memory loss from changing the 71's batteries.  Same with the 41.  I've heard of 41's going as much as 12 or 24 hours without batteries installed and all the memory was kept, but I still always do it as fast as I can without making mistakes.  I had a situation in about 1986 or '7 where a 10uF capacitor kept an 8Kx8 SRAM alive overnight, from closing time at work one day 'til I got back to work the next day.  I did not experiment to find out how much longer it would go.

http://WilsonMinesCo.com  (Lots of HP-41 links at the bottom of the links page, at http://wilsonminesco.com/links.html#hp41 )
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07-15-2023, 02:36 AM (This post was last modified: 07-15-2023 02:39 AM by rprosperi.)
Post: #12
RE: replacing 71b batteries without losing data
(07-14-2023 02:46 PM)KimH Wrote:  Glad that I coincidently was able to help - looking forward to your posting on the tube.

Maybe I was simply lucky, Bob is pretty much always spot on with his responses!

Very gracious of you, but sadly not really accurate... Wink

I am quite surprised by your comment that you can remove batteries for several minutes without losing MAIN RAM (though IRAM/Port RAM indeed is rarely lost) as this is not my experience, certainly for the last 20 years or so; my memory of such details in the 80s are a lot less clear, but I do recall learning the lesson back then to always connect power first.

But a comment you made in your reply notes an important distinction I should have made. My comments about hardly ever succeeding in not getting an ML when changing batteries w/o adapter power refer to units when they will no longer turn on, or when the BAT annunciator has been on for a while. There seems to be a short period when the unit won't power-on, yet RAM is maintained; it is essential to have power connected when in this state.

When replacing still-good batteries, it is possible to replace them quickly, one-at-time, and not get ML, probably about one time in 4-5.

Regarding EPROM modules, contact me off-list, I'd be happy to burn one for you.

@Garth - Have you tried this recently? Next time you have to change batteries, please give it a try and report back here. I suspect 71B units have reduced their capacity to maintain RAM over the decades, and it's far less likely to be preserved today.

As for the 41C, that's an entirely different situation, indeed they will sometimes last many, many hours w/o a battery pack installed, though they will often lose the time. But if you change battery packs quickly enough, you can swap w/o losing time. I keep a stack of 41C battery packs loaded with new batteries so I can swap one within a few seconds. With nearly 20 41C/CV/CX machines it seems at least once a week one of them needs batteries. It also took me a long time to learn that when swapping batteries with a 41CL, you must [XEQ] "MMUEN" after powering back up to restore the MMU configuration.

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07-15-2023, 02:45 AM
Post: #13
RE: replacing 71b batteries without losing data
(07-15-2023 02:36 AM)rprosperi Wrote:  It also took me a long time to learn that when swapping batteries with a 41CL, you must [XEQ] "MMUEN" after powering back up to restore the MMU configuration.
and to remove the HP-IL module before doing the battery swap, otherwise you will have a page 7 lock-up. (both HP-IL and YFNZ share the same page)
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07-15-2023, 04:24 AM
Post: #14
RE: replacing 71b batteries without losing data
(07-15-2023 02:36 AM)rprosperi Wrote:  @Garth - Have you tried this recently? Next time you have to change batteries, please give it a try and report back here. I suspect 71B units have reduced their capacity to maintain RAM over the decades, and it's far less likely to be preserved today.

The last time was a couple of months ago, ie, this year.  No problem.

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07-15-2023, 08:43 AM (This post was last modified: 07-15-2023 11:48 AM by KimH.)
Post: #15
RE: replacing 71b batteries without losing data
So I took 2 machines and tried it

All modules out except IL - did an OFFIO just in case

New plastic back 2CDCC/1B (2804) and early 1BBBB/1A (2506)

They behaved the same way

Charger Cable out - timed. Held for up to 10 minutes, ML at 11
Battery out - timed. Held for up to 5 minutes, ML at 6

So thereabouts would be the max/norm. For sure more than the promised 30 seconds

Considering the PSU circuit has two caps, this would even make sense

Battery only, has 330uF for power retention (for the exchange of batteries).

The charger circuit, has additional 470 uF for the voltage ripple out of the charger (AC) - with a “diode” drop on its way to the next cap

There, however, is a small but important “yes, but….”

The way the OS decides whether a Coldstart (ML) is required is to check a coldstart constant in memory (71 as 41) - the 71 has a value of 168F on address 2F438, the 41 has it in one of the upper 16 system registers (recall it as reg C with 169 hex??) If that value changes - then the coldstart will be done. Going to the limits as above you may have the coldstart value check out as OK, but something elsewhere could well be garbled, so the ML could come at a later point when/if the OS gets confused…

So the promised 30 seconds is the way to go, or - obviously - use the charger/adapter while swapping…
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07-15-2023, 12:39 PM
Post: #16
RE: replacing 71b batteries without losing data
(07-15-2023 08:43 AM)KimH Wrote:  So I took 2 machines and tried it

All modules out except IL - did an OFFIO just in case
[snip]

Aha, I wonder and now suspect this may be the difference. All of my 71B units have modules installed in 3 or 4 of the front ports, and of those most have RAM modules. In addition, most of them also have RAM or RAM/EPROM or FRAM modules in the Card Reader port.

Given that the 71B h/w seems to prioritize power to modules over MAIN RAM (since often the former is preserved while the latter is lost) it appears this could be making a big impact.

@Garth - thanks for confirming. Does your 71B have modules installed when you replace batteries?

--Bob Prosperi
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07-15-2023, 06:23 PM
Post: #17
RE: replacing 71b batteries without losing data
(07-15-2023 12:39 PM)rprosperi Wrote:  @Garth - thanks for confirming. Does your 71B have modules installed when you replace batteries?

I have the HP-IL module (and no, I never did OFFIO), the Math module, a CMT 64K RAM module (front port), and a 4K HP memory module.  I have a 96KB CMT card-reader-port RAM module, but there seemed to have been some sort of configuration problem with it, and I had taken it out and never put it back because I haven't done anything in recent years that needed so much memory.  I don't remember why my Forth/Assembler module isn't in it.  I still use my 41cx with Clonix-D, HP-IL, XIO, and double XMem nearly every day, but I almost never use either of my 71's.  (I have a backup 71 that I got, surplus but never used, for a song, back before there was eBay.)

http://WilsonMinesCo.com  (Lots of HP-41 links at the bottom of the links page, at http://wilsonminesco.com/links.html#hp41 )
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