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