Putting the ImageWriter II Into Hex Mode
The only way to put the ImageWriter II into hex mode is to hold down the select button while powering it on. Sending an escape-c afterwards should restore (software reset) the printer to its standard instruction set. (Alternatively, you write a program to convert the ASCII to hex, then print.)
If the ImageWriter II Drops into Hex Mode
After a logic error in the printer, the ImageWriter II sometimes prints in hex -- usually random garbage. If a printer consistently "drops" into hex mode, consider exchanging its logic board.
Putting the ImageWriter LQ Into Hex Mode
The ImageWriter LQ can be placed in hex mode via the software command:
esc,6
Your data will then print in hex until the printer is powered off, or until you send a software reset, esc-c.