View ds21q50 datasheet:

DS21Q50 Quad E1 Transceiver www.maxim-ic.com FEATURES Four complete E1 (CEPT) PCM-30/ISDN- Configurable parallel and serial port PRI transceivers operation Long-haul and short-haul line interfaces Detects and generates remote and AIS alarms 32-bit or 128-bit crystal-less jitter attenuator Fully independent transmit and receive Frames to FAS, CAS, CCS, and CRC4 functionality formats Four separate loopback functions 4MHz/8MHz/16MHz clock synthesizer PRBS generation/detection/error counting Flexible system clock with automatic source 3.3V low-power CMOS switching on loss of clock source Large counters for bipolar and code Two-frame elastic-store slip buffer on the violations, CRC4 code word errors, FAS receive side word errors, and E bits Inte