HP-21 fast counter

[TKuenzi]

Hello,

My first calculator was the Hewlett Packard HP-21

The calculator is not programmable. But I was able to display a fast counter:
-direction up / down not under my control
-10 digits counting capacity
-the units are counted in binary and displayed in hexadecimal mode
-the characters are 0…9 – E H o P r
-the speed of counting was between 100 … 1000 units per second.

To start a counter:
1. Plug in the battery charger
2. Slide the On-Off switch *slowly* to on.
3. If 0.00 is displayed, switch off and repeat step 2.
4. If a counter is running, you are done!

Has anyone noticed this counter?

Kind regards,
Thomas

[BobVA]

That's certainly odd. It might indicate that the ACT chip is starting to malfunction.

[teenix]

This method of turning the calculator on is probably upsetting the ACT power on reset and causing it to malfunction thus stopping the program counter from incrementing. Because of the first processor instruction, this has the effect of displaying an incrementing counter as you are seeing.

cheers

Tony.

[lrdheat]

A Nixie tube Wang in our high school physics lab in 1970 did that!