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Casio CLASSWIZ fx-991EX Diagnostics
03-30-2019, 10:27 PM
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Casio CLASSWIZ fx-991EX Diagnostics

Casio CLASSWIZ fx-991EX Diagnostics

Casio CLASSWIZ fx-570EX Diagnostics

Author: Guy Macon

Date: 30 March 2019

Released under the Creative Commons CC0 "No Rights Reserved" license.


The Casio CLASSWIZ fx-991EX and the Casio CLASSWIZ fx-570EX are almost identical, except the fx-991EX is powered by an LR44 battery and a solar cell, while the fx-570EX is powered by a AAA battery. The fx-570EX is 25% thicker and 10% heavier because of the different batteries. Both models have have the same display, electronics and software, and the diagnostics are the same for both.

If anyone reading this has an fx-82EX or fx-350EX, please see if these instructions work for those models and post the results with the title "Casio CLASSWIZ fx-82EX Diagnostics" or "Casio CLASSWIZ fx-350EX Diagnostics" so that a google search will find it.

Before you start testing, clear the calculator memory by pressing SHIFT, then 9 (it has RESET printed above it), 3 (initialize all), = (Yes), AC (exit).

The first test is the easiest. Turn the calculator on with the ON button. Every time you press that button (whether the calculator is tuned on or off doesn't matter) it stops whatever it was doing, resets the processor, and does a quick power on self test (POST).

Because these diagnostics have only been tested on good calculators, we don't know what the error message is if it fails. If you have a calculator that displays anything other than a flashing cursor when you power it on, please post the exact error message (with a screen shot if possible) with the title "fx-991EX POST failure" so a Google search will find it.

You might want to press the ON key ten times to make sure that it passes the POST test every time. You might also want to confirm that it turns off when you press and release SHIFT and then AC (which has RESET printed above it).

To perform the rest of the diagnostics Hold down the SHIFT key and the 7 key, press and release the ON key, then release the SHIFT key and the 7 key. As with anything else involving the ON key, it doesn't matter what the calculator is doing and it doesn't matter whether the calculator is on or off.

You should see:


Press AC

When you see the above, you have 15 seconds to press a key to start the diagnostics. If you are too slow it will go to the normal flashing cursor.

The only keys that do anything are ON (ON always works the same no matter what), AC, 6, 8, and 9. AC simply exits the diagnostics and brings up the flashing cursor. 6, 8, and 9 are the actual tests.

Test 6 (SHIFT+7, ON, 6):

Pressing 6 brings up the following menu:

3.Key test


As usual, AC exits the diagnostics and brings up the flashing cursor, and ON does the same. If you compare the two you should see that AC responds instantly while ON has a small delay. That's the amount of time the calculator spends doing the POST.

1 starts the display test.

The first test turns on everything on the screen. Look carefully; you should see no missing pixels and every icon on the top row should be on.

Now press SHIFT.

Everything on the screen should turn off. Make sure no pixels or icons are stuck on.

Press SHIFT again.

You should see all of the icons turn on and the screen should display a checkerboard pattern. Look carefully; sometimes two adjacent pixels can short together so that they are either both on or both off.

Press SHIFT again.

You should see the icons turn off and a reverse checkerboard (every pixel that was on is now off, every pixel that was off is now on). Look for adjacent pixels that turn on or turn off together.

Press SHIFT a final time, and you should be back at the menu.

2 starts the Version&Sum test.

This tells you the version of the software and displays a checksum of the firmware.

The unit I tested read:

CY-235 VerF
P00 Read OK
Press AC

AC should bring you back to the menu.

3 Starts the Key test.

The display starts with 00. Pressing SHIFT changes it to 01, ALPHA to 02, and so forth. Without pressing the ON key, just keep pressing keys row by row until you end at =.

The up arrow is 03, the right arrow is 04, the left arrow is 08, and the down arrow is 09.

When you press the last key the display should read:


Press AC

AC should bring you back to the menu.

4 starts the contrast test.

The display should read:

16h [see note]

Light Dark
[<] [>]

The left and right arrows should make the display lighter and darker.

AC should bring you back to the menu.

AC again should bring you back to the flashing cursor.

NOTE: The 16h may be any of 44 possible hexadecimal contrast values from 00h (as light as it will go) to 2Ah (as dark as it will go).


To get the display back if you do make the above mistake, do this:

First, try holding the calculator at different angles to see if there is an angle where you can read the display.

If that doesn't work, press the following keys (You may have to type blind):

ON, SHIFT, MENU, up arrow, 3, right arrow to make the display darker / left arrow to make the display lighter.

Test 8 (SHIFT+7, ON, 8):

Pressing 8 should display the following:

[1] KI1 KO1 [SHIFT] KI8 KO1
[5] KI2 KO2 [9] KI3 KO3
[)] KI4 KO4 [log] KI6 KO5
[logab] KI7 KO6 [0] KI5 KO7

"[1] KI1 KO1" means "pressing the 1 key will test the connection between key input #1 and key output #1 on the chip".

As you press each key, that line on the display should highlight/change to reverse (light text on a dark background).

When it asks you to press the "logab" key it means the x directly below the ON key.

This mislabeling may mean that the same diagnostic software can run on an fx-82EX or fx-350EX. both of which have a different key in that position.

After you highlight the last line, the solar model will do nothing for 50 seconds.

During this time you should put your thumb over the solar cell, which should instantly result in this display:

Battery Model OK!

And when you remove your thumb it should read:

Solar Model OK!

The non-solar model should simply display:

Battery Model OK!

AC should bring you back to the flashing cursor.

Test 9 (SHIFT+7, ON, 9):

This is the test that is used at the factory to test the calculator after it is manufactured.

Many of the tests are the same as the ones above, but this time you have to do all of them to finish.

Pressing 9 should display the following:



This appears to be a test to see if all of the segments in an eight segment display turn on, but this calculator uses a matrix display, so this is may be a sign that the same diagnostics are used on many different models.

Press SHIFT to enter the Display test.

Press SHIFT four more times to enter the Version&Sum test.

Press AC to enter the key test.

Finish pressing every key except ON to enter the Contrast test.

Press AC to display the final test screen, which should read:


Reset All

Press AC

Pressing AC again should bring you back to the flashing cursor.

The calculator should be reset at this point, but you can press SHIFT, then 9 (it has RESET printed above it), 3 (initialize all). = (Yes), then AC (exit) to reset everything except the contrast.


This document is released under the Creative Commons CC0 "No Rights Reserved" license.

See for details.

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To the extent possible under law, I, Guy Macon, waive all copyright and related or neighboring rights to this work.

You are free to use or modify this document for any purpose. I do NOT require you to give me credit.

I would prefer that if somebody specifically asks where you got it, you tell them, but that is up to you.

You are even free to pretend you wrote it, just as everyone else is free to mock you when they find out what you did.

Removing the CC0 License is a Bad Thing. Claiming that you have a copyright over this document is a Very bad Thing. If you find anyone doing that, please do whatever it takes to make them reconsider their life decisions.

Author: Guy Macon

Date: 30 March 2019

( www(dot)GuyMacon(dot)com/index(dot)html )

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