HP 8753E Programming And Command Reference Manual page 69

Using key codes allows remote control of the analyzer keys and can be
used as an alternative to using other HP-ES commands. This may be
useful, but it is a highly recommended programming practice to use the
HP-IB command mnemonic appropriate for the function desired.
When using key codes, the following notes must be taken into
consideration:
An "invalid key" is reported with a 63.
Note 1:
Note 2:
This command reports a knob turn as a -1.
Note 3:
is positive, it is a key code. If it is negative, then set bit 15
equal to bit 14, and the resulting two byte integer is the RPG
knob count. It can be either positive or negative. There are
about 120 counts per turn.
Figure l-6. Key Codes