Perle Systems 1700 - Manual (Page 100)

V.11/X.21 Null-Modem Cable Configuration
Figure D-13 V.11/X.21 Null-Modem Cable
The connecting cable must be a shielded cable.
Circuits which are paired (contain an (A) and (B) reference) should be connected to
twisted pairs within the connecting cable.
This cable is needed when it is necessary to connect two units back-to-back and a set of
modems is not available. Note that this cable specifies DB15 connectors on each end to
allow direct connection to the link interface connector on each unit. The link speed must be
defined for each of the two units.
When using this cable to connect two units back-to-back, a jumper must be installed on
pinheaders W8 and W9 on one of the V.11/X.27 interface modules. This allows that
particular module to generate the required timing signals.