Hanna Instruments HI 23 Series Instruction Manual page 43

As soon as one or more keys are pressed, the appropriate
segments out of 88:88 corresponding to the pressed keys,
will light up on the screen. The correspondence is given
below:
For example, if UP and CAL are pressed together the LCD will
look like this:
The colon is a useful indicator for the correct position
squares.
Note
A maximum of two keys may be pressed simultaneously to be
properly recognized.
To exit the keyboard test procedure press LCD, CAL and SETUP
simultaneously.
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