The DS21352/552 T1 single-chip transceiver contains all of the necessary functions for connection to T1 lines whether they are DS1 long haul or DSX1 short haul. The clock recovery circuitry automatically adjusts to T1 lines from 0 feet to over 6000 feet in length. The device can generate both DSX1 line build outs as well as CSU line build-outs of -7.5dB, -15dB, and -22.5dB. The onboard jitter attenuator (selectable to either 32 bits or 128 bits) can be placed in either the transmit or receive data paths. The framer locates the frame and multiframe boundaries and monitors the data stream for alarms. It is also used for extracting and inserting robbed-bit signaling data and FDL data. The device contains a set of internal registers which the user can access and control the operation of the unit. Quick access via the parallel control port allows a single controller to handle many T1 lines. The device fully meets all of the latest T1 specifications including ANSI T1.403-1995, ANSI T1.231-1993, AT&T TR 62411 (1290), AT&T TR54016, and ITU G.703, G.704, G.706, G.823, and I.431.