HEIDENHAIN MANUALplus 620 Technical Manual page 1766

9.5 The Serial Interface of the Control
9.5.1 RS-232-C/V.24 interface
Hardware
1766
With RS-232-C/V.24, data transfer is executed asynchronously, with a start bit
before each character and one or two stop bits after each character.
Transmission distance: up to 20 m
The physical connection between two RS-232-C/V.24 interfaces is an
asymmetrical line, i.e. the common ground connection between transmitter
and receiver is used as a return wire.
Physical connections:
HEIDENHAIN Technical Manual MANUALplus 620