HEIDENHAIN MANUALplus 620 Technical Manual page 1767

Signal levels
January 2012
The levels of the individual signal lines differ:
Data lines: The data signals are defined as being logical zero (SPACE) over
the range +3 V to +15 V and logical one (MARK) over the range –3 V to –15 V.
Control and verification lines: These signals are defined as being ON (High)
over the range +3 V to +15 V and as OFF (Low) over the range –3 V to –15 V.
Note
For all signals: The voltage range from –3 V to +3 V cannot be evaluated.
9.5 The Serial Interface of the Control 1767