Holding memory during battery change



#2

Last year the batteries in my HP-28S were low. No problem, I thought. I had heard about the potential difficulty with Rube Goldberg battery cover and had practiced changing batteries without experiencing any sizeable delay. This time I wasn't so lucky. I struggled with the battery cover for about a minute and when I finally got it in place the memory had cleared. I was not a happy camper. I had lost about fifty matrices and several programs. Back in 1984 I had done some testing which showed that the memory of a TI-58C would hold for as long as seven days with it's battery pack removed. I wondered what HP machines other than the HP-28S could do.

I started with my HP-33C. Page 90 of the HP-33E/33C Owner's Handbook anf Programming Guide states "...Continuous memory requires that batteries be replaced as quickly as possible. Normally you have a minimum of 5 seconds to change the batteries. Leaving the batteries out of the calculator for extended periods or allowing the batteries to discharge completely will result in loss of information in Continuous Memory. ..." I stored a short program and stored a constant in several data registers. I started with the battery pack removed for a minute and progressed through five minutes, ten minutes and twenty minutes and the memory held. After an hour with the battery pack removed the memory was lost.

I moved to my HP-38C. I don't have a manual so I can't quote the battery pack change limitations. I started with the battery pack removed for five minutes and progressed through ten minutes, twenty minutes, thirty minutes and one hour and the memory held. I did not test my HP-38C with the battery pack removed for more than one hour.

What about the HP-41's. Page 241 of the HP-41C manual says "The continuous memory of your HP-41C will normally be preserved for about 30 to 60 seconds while the batteries are out of the calculator." Page 241 of the HP-41C/CV manual says the same thing. I don't have a manual for an HP-41CX. I have two HP-41C's, an HP-41CV and an HP-41CX. I didn't want to test one of my HP-41C's or my HP-41CV because I have programs on them that I don't want to re-enter.

I started testing with my HP-41CX. I started with five minutes with the battery pack removed and worked my way to four hours and thirty minutes and the memory still held.

I switched to the HP-41C which didn't have any programs on it. Again, I started with five minutes and memory held. But when I reached thirty minutes I lost memory. I wondered why my HP-41CX held memory so much longer. Then I saw that my HP-41C had two 82106A memory modules installed. I removed the modules and did the test again. The memory held with the battery pack removed for six hours. The memory was lost when the battery pack was removed for ten hours. I installed two different memory modules. Memory held when the battery pack was removed for as much as two and one-half hours. I did not test further with those memory modules installed.

I did a lot of additional testing with the HP-41C with various combinations of two memory modules installed. The end result was that I identified a particular memory module which was responsible for shortening the memory retention time substantially. All four of my memory modules were made in Singapore. I verified that data could be stored to either memory module. I did not test with more than two memory modules installed.

These tests all suggest that battery pack replacement can be a leisurely process with the HP-33C, with the HP-38C and with the HP-41, but maybe not if I had the bad fortune to have four of the memory modules which shorten memory retention time stored in the same HP-41C. Why is my HP-28S so unforgiving? Does my experience with the memory modules in my HP-41C suggest that I might have a "weak" HP-28S ?


#3

Could it be that the 28S is actually running somehow when turned off and not in a deep sleep status? The older machines are fully static and just stop their processors.

#4

I read somewhere that the 41C/CV/CX would hold a charge in its capacitor for 3 to 5 days if you did not press the ON key while the batteries were out. Some years ago I had a battery pack that died on my 41CX. I removed the battery pack to replace it with the standard N cell module that came with the 41CX. I was not able to find any N cell batteries for about 3 days. When I placed the batteries in and turned the calc on everything was still there. The clock had reset to default but all programs and data was still there.

#5

Hi, Palmer --

Quote:
I started with my HP-33C. Page 90 of the HP-33E/33C Owner's Handbook anf Programming Guide states "...Continuous memory requires that batteries be replaced as quickly as possible. Normally you have a minimum of 5 seconds to change the batteries. Leaving the batteries out of the calculator for extended periods or allowing the batteries to discharge completely will result in loss of information in Continuous Memory. ..." I stored a short program and stored a constant in several data registers. I started with the battery pack removed for a minute and progressed through five minutes, ten minutes and twenty minutes and the memory held. After an hour with the battery pack removed the memory was lost.

I moved to my HP-38C. I don't have a manual so I can't quote the battery pack change limitations. I started with the battery pack removed for five minutes and progressed through ten minutes, twenty minutes, thirty minutes and one hour and the memory held. I did not test my HP-38C with the battery pack removed for more than one hour.


If the memory on a Spice-series holds long enough to allow a battery swap, consider it very modest good fortune. The main advantage of CM was that programs need not be re-entered when the calc is turned off. I still remember the non-CM TI-59(?) that Dad brought home from work once in the 1970's. The card reader was cool, but we had to run the cards through to re-load the program each time the calc was turned on. What a nuisance!

Spice-series program and data storage space was limited anyway -- even on the HP-34C -- and the programming capabilities weren't well-suited for permanent-residence "keeper" routines.

-- KS


Edited: 20 May 2008, 3:00 a.m.

#6

The electronics of the 28S is packed into a significantly smaller volume than that of any of the other models you tested. When the clamshell hardware was being designed (1984-1986), the highest capacitance that could be fit into the available space was very limited. Since that time, capacitor technology has advanced significantly, with super caps and ultra caps.

When I first learned about capacitors, I was told that a farad was such a large unit that I would never be able to buy a one farad capacitor. Today ten farad capacitors are readily available and not even terribly expensive.


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