TE 8500f Casio buttons don't work

Depends upon the model. Some are internal rechargeable, and some are AA batteries near the paper rolls. If you replace the batteries, always leave the power ON with screen lit up as you change them so you don't lose your program.

It uses a special Vanadium Pentoxide Lithium battery. It is available from Casio register dealers. If you are changing it yourself, make sure you have the power TURNED ON in reg mode while changing it. It has to have power to keep the memory while the battery is being changed or you lose the program.
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For those who asked, there is a Casio TE-8000f and TE-8500f programming manual here:
http://the-checkout-tech.com/manuals/cashregisters/Casio_TE-7000S_and_TE-8000F_and_TE-8500F_programming_manual.html

Has the machine been turned off for a while? When did it last have the memory battery replaced? -----------------------------------------------------
If anyone needs a Casio TE-8500f manual, C-tech collective have it here: http://the-checkout-tech.com/manuals/cashregisters/Casio5.php

Highlight "flag clear" and press YES button. If it still does not work, you have a flat or (usually) faulty memory battery and it will need to be RAM cleared then re-programmed.
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If anyone needs more informaton on the Casio TE-8000f or TE-8500f, the manuals are here:
Programming manual: http://the-checkout-tech.com/manuals/cashregisters/Casio_TE-7000S_and_TE-8000F_and_TE-8500F_programming_manual.html
Service (repair) manual: http://the-checkout-tech.com/manuals/cashregisters/Casio_TE-7000S_and_TE-8000F_and_TE-8500F_service_version_2_manual.html

This can be quite involved if you want to use a stock inventory system. Some models require a complete re-programming so that extra memory can be opened for stock files. Normally your dealer would set this up before you receive the machine from new. It would be best to get a dealer to set up the stock areas and memory allocation.
The machine will show you the amount of everything sold if you do a PLU report at the end of the day. This may be enough for what you need so give it a try.
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If anyone needs a Casio TE-8000f or TE-8500f manual, try here http://the-checkout-tech.com/manuals/cashregisters/Casio4.php

This indicates the actual processor in the machine has failed. Unfortunately the processor is the most important part of the whole register. If it is in fact a Casio register it can be repaired, but you will need to contact them. They are open right now actually, give them a call at 1-800-706-2534

Thanks
Tim

If you flag clear it (no loss of data) then most models will give you a temporary clerk in order to program with. While you are in program mode, set a clerk button before you return to REG mode. Consult your manual for flag clear instructions as this model works differently for flag clearing. If you need a manual, c-tech collective has one here: http://the-checkout-tech.com/manuals/TOC/cashregisters/Casio/TE-7000S%20and%20TE-8000F%20and%20TE-8500F%20programming.TOC.pdf

Wow. I haven't had that one before, and I thought I'd seen them all. Has this machine been unplugged for a while, or is it in use every day?

On some cash machines there is a fuse that can burn out. Normally this fuse is located underneath where the cord go in.

Most cash machines when nothing happens means it is dead.

With the power switch off, hold down the RECEIPT FEED button and turn the register on.

Continue to hold the button until white text shows up on the screen.

Release the button and press the SUB-TOTAL button.

When the register asks "1. Flag Clear / 2. Something Else", select "1. Flag Clear" and press yes.

This is the Casio ECR equivalent of CTRL-ALT-DEL; it should fix the printer-cycling issue.

Cheers!