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It has really bothered me for a while that the hp15c-le could
just go blank without warning and loose all data.

A found now a solution to this problem! The jtag connector
provides a GND pin (minus pin of the battery) and the visible
side of the batteries is plus. Thus one can just use a voltmeter
and test the batteries without taking them out.

I have documented it here:


I am using the hp15c-le now with much more confidence.

thanks for a useful tip.

The jtag connector
provides a GND pin (minus pin of the battery) and the visible
side of the batteries is plus.

This should read "The
serial connector provides..."

Only the 20b/30b calculators come with a JTAG connector. It is hidden behind the back case (where a name plate can be put). The JTAG connector is only useful for low level development on the device. I used to have one on my very first 34S to debug the interrupt and other hardware related stuff.

The six pin connector visible in the battery compartment of the newer ARM equipped HP calculators has serial I/O, reset, erase and power pins which is quite different form a JTAG connector.

Edited: 3 Jan 2013, 11:22 a.m.

Thanks for the tip!

Unfortunately, during my 15c LE battery capacity experiment, I found that the memory often got cleared during battery changeover, even when changed one at a time. I also found that at 2.7 Volts, there was a lot of life left in the batteries. They hit 2.7 Volts very quickly, then held fairly steady for a long time. But when the unit finally quit during a run of the test program, the batteries read 2.6 Volts. So picking the optimum time to change may be tricky, and the act of changing may clear memory anyway.

Of course my voltage test procedure of moving the batteries to a 34s and using the BATT function was not the best. I wish I had known your trick when I did my test. I may do some experimentation using your technique to see how that compares to the wp34s method.