HEIDENHAIN SHB Data Interfaces for HEIDENHAIN - Manual (Page 27)

4 – 28 HEIDENHAIN Service Manual for Data Interfaces
Attention
This modification applies for hardware handshake with TNC 145 to TNC 155. In these
control models, the RTS pin is not connected but tied high internally.
The following modification is required for hardware handshake:
Cut the line on both sides between DRS and DTR and short-circuit DSR with DTR
(at the PC: pin 6 with pin 20).
Without this modification, data transfer using hardware handshake is not only stopped but
aborted immediately (like "power off).
Do not use this configuration for TNC 335 with new hardware.