TANDBERG 8000 MXP - Manual (Page 211)

Administrator Settings
197
Network clocking
The network clock setting specifies the number of physical external clock signals.
RS449/V35
Compatible
Use RS449/V35 Compatible when the external equipment provides two
clock signals, one for transmit and one for receive. The difference
between RS449 and V35 is only the cable.
X21
Compatible
Use X21 Compatible when the external equipment provides a common
clock signal for both transmit and receive.