Polycom HDX series Reference Manual page 62

Integrator's Reference Manual for Polycom HDX Systems
2 - 40
The DB-9 male connector on the Polycom HDX system has the following
connections.
Pin Signal
1 Not used
2 Rx
3 Tx
4 DTR (tied to pin 6, DSR)
5 GND
6 DSR (tied to pin 4, DTR)
7 RTS
8 CTS
9 Not used
Most devices which connect to the serial port to control the Polycom HDX
system via the API only require pins 2, 3, and 5. For more information and to
verify the proper cabling, refer to the documentation for your control system.