Oki OKIPAGE 14i Service Manual page 268

When the Ready/Busy protocol is used for the buffer busy control method, the busy signal can be set to the Pin-20 (DTR) in the menu.
4) Signal Level
l
MARK polarity : -3V to -15V (LOGIC = 1)
l
SPACE polarity : +3V to +15V (LOGIC = 0)
5) Interface Circuit
a) Receiving Circuit
b) Sending Circuit