View ds21q42 datasheet:

DS21Q42 Enhanced QUAD T1 FRAMER www.dalsemi.com FEATURES FUNCTIONAL DIAGRAM Four T1 DS1/ISDN–PRI/J1 framing transceivers Receive Elastic All four framers are fully independent Framer Store Each of the four framers contain dual two– Elastic Transmit Store Formatter frame elastic store slip buffers that can connect to asynchronous backplanes up to 8.192 MHz FRAMER #0 FRAMER #1 8–bit parallel control port that can be used FRAMER #2 directly on either multiplexed or non– FRAMER #3 multiplexed buses (Intel or Motorola) Control Port Programmable output clocks for Fractional T1 Fully independent transmit and receive functionality Integral HDLC controller with 64-byte buffers ACTUAL SIZE configurable for FDL or DS0 operation Generates and detects in–band loop