Polycom VSX3000 Integrator's Reference Manual page 67

The DB-9 male connector on the VSX 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 (tied to pin 8, CTS)
8 CTS (tied to pin 7, RTS)
9 Not used
Most devices which connect to the serial port to control the VSX 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.
Cables
2 - 35