What is purpose of the “CH” button and “CH” switch position on this calculator?

What is purpose of the CH button and `CH` switch position on the Omron Elite 8002 Taschen-Rechner?

(For those who don't know German, “Taschen-Rechner” just means “calculator”.)

The switch has three positions: ON, OFF and CH. Only in the ON position is the calculator display lit up; in the CH position the calculator apparently does not do anything.

See the linked website for a photo of the calculator. I will provide a better photo later.

I've tried these things to get information about the CH button:

• Key in number 123, press CH. Nothing happens. Press again. The VFD shows `0.`.

• Key in 10, press CH (VFD still shows `10.`), key in 20, press =. Nothing happens.

• Calculate 200+100 (VFD shows `300.`), key in 1000, press CH. The result is `700.`.

According to the last try, it looks like it subtracts the previous value from the currently entered one. (More experiments confirm that it works in this way, as a reverse-subtract.)

So, the question is: What is the “CH” button for (maybe it is in German) and what is the “CH” position of the switch for?

I do not have any manual and I do not have any idea what these “CH” labels could mean. There is also a connector that looks like a 2-pin 3.5 mm jack on the bottom. What is its purpose? Could it be related to these buttons?

• It's not the power connector? Oct 20 '21 at 16:26
• On-Off-Ch sounds like "charge"? Oct 20 '21 at 16:27
• I agree with thomas about the "ch" switch position. The CH button likely is for "ch"ange as in money you have to return. The third example is: Customer purchases an item for 100 lira and another item for 200 lira. The payment is made using a 1000 lira bill, how much CHange to return? Oct 20 '21 at 17:05
• This calculator uses the "CH" switch position to charge batteries: vintage-technology.club/pages/calculators/decimo/… Oct 20 '21 at 17:21
• In the future, however, it would be helpful if you included the image inside your question itself instead of as an external link. Oct 21 '21 at 5:26