Casio fx-991ES (Natural Display)
The Casio fx-991ES is a scientific calculator with 8 digits precision and formula input logic. It has 403 functions, 50 keys, and an LCD (liquid crystal) display. The power source is Solar. The calculator was manufactured in China. Note: Functionally the same as fx-570ES with solar power
Facts at a glance:
You can download the Casio fx-991ES manual from this external link.
Note that diagnostic tests generally delete any data on the calculator. Be sure to back up any important information held in the calculator before running these tests.
While holding SHIFT and 7 keys (or just SHIFT key for early models) press the ON key. Pressing the SHIFT key shows 5 display test patterns in turn, followed by the ROM version. Press AC to get the contrast test screen, and press AC again to get to the keypad test - the numbers then increase as each key is pressed in the correct order (omitting the ON key which exits test mode).
If you have a Casio fx-991ES that is no longer working, you can consider replacing the calculator with a modern equivalent. We suggest the following current model as a replacement: