Black Box LR1530A-EU-R3 - Manual (Page 93)

Servicing Information
87
V.35 Null-Modem Cable Configuration
Figure C - 11 V.35 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 DB25 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.