How to downgrade CASIO Prizm fx-CG20?

[SuperXX]

Hi,

I´m trying to downgrade a CASIO Prizm calculator but I can´t seem to find an answer for it. I saw somewhere that by pressing the F2-4-AC/ON and Restart once the calculator was turned off, I would be prompt to a black screen so I then I would press 9 and install the OS. But nothing happens. Currently the calculator is in version 3.1.02

Thank you in advance

[critor]

Hi.

You just need to put the calculator in "OS error" mode using the F2-4-AC combo, and it'll then accept any OS version.

The combo is just very tricky :

• press and hold the restart button on the back
• without releasing it, press and hold the

  F2

  4

  AC

  keys
• without releasing these 3 keys, release the restart button
• now only release the

  F2

  4

  keys together
• wait a second
• now finally release the

  AC

  key
• press and hold the

  9

  key
• wait a second
• release the

  9

  key
• press and hold the

  ×

  key
  You should now get the OS ERROR screen.
• release the

  ×

  key

[SuperXX]

It worked. Thank You. I didn´t have the correct key combination