Black Box LR1530A-EU-R3 - Manual (Page 88)

Servicing Information
82
Circuits which are paired (contain an (A) and (B)
reference) should be connected to twisted pairs within
the connecting cable.
NOTE For U.K. Approval:
The connecting cable may be any length between 0 and 5M.
One end must be terminated in a male 34 pin X.21 bis
connector as defined in ISO-2593 1984. The other end must
be terminated in a male 25 pin X.21 bis connector as defined in
ISO-2110 1989