Recently I had to create a T1 Crossover cable in order to connect two Juniper SRX240’s in the lab. A T1 crossover cable is different than an Ethernet crossover cable (but a regular ethernet cable can be used for T1 because both are straight-through).

Anyway, here are the pinouts for a T1 RJ-45 cable:

• Pin-1 = RX+
• Pin-2 = RX-
• Pin-4 = TX+
• Pin-5 = TX-

Therefore a crossover cable for T1 is:

• One End Other End
• Pin-1 === Pin-4
• Pin-2 === Pin-5
• Pin-4 === Pin-1
• Pin-5 === Pin-2

And a loopback connector is:

• (Same Connector)
• Pin-1 === Pin-4
• Pin-2 === Pin-5
• Pin-4 === Pin-1
• Pin-5 === Pin-2

See also fonality.com Troubleshooting T1/T1_Cross-over_Cable)