The T1IB board ( Part # 3531-31 ) provides the T1 (1.544Mbps, 24 channel) interface circuit, control circuitry,  and synchronous clock control circuits. Up to two DTMF tone detection units (DTRU4) can optionally be installed on the T1IB.

The system can be equipped with one T1IB and it can be installed in slots 5, 6, or 7 of the BKSU or in slots 11, 12, or 13 in the EKSU. The Phase Lock Loop Unit (PLLU) must be installed in the BKSU for the T1 card to operate properly.

The T1IB has eight LEDs on the front edge of the PCB which indicate errors of T1 line, in-use status, and synchronous clock enable status.