TA Triumph-Adler LP 4024 - Manual (Page 210)

Serial Interface
C-6
DTR - Data Terminal Ready - (Pin 20)
This output is used as a buffer nearly-full handshake line. It is held high
(above 3 volts) when the buffer can accept more data.
RS-232C Interface Voltage Levels
The voltage levels of the interface signals conform to EIA RS-232C
specifications. SPACE is from 3 volts to 15 volts. MARK is from –3 volts to
–15 volts. Voltages between –3 volts and 3 volts are undefined.
SERIAL Connector
The connector marked IOIOI (RS-232C) on the rear panel is a DB-25S
connector. Use a DB-25P connector (or equivalent) for the connector on the
cable.