View ds21q41b datasheet:

DS21Q41B Quad T1 Framer www.dalsemi.com FEATURES FUNCTIONAL DIAGRAM Four T1 DS1/ISDN-PRI framing transceivers All four framers are fully independent Frames to D4, ESF, and SLC-96 formats RECEIVE ELASTIC 8-bit parallel control port that can be FRAMER STORE connected to either multiplexed or non- multiplexed buses Each of the four framers contains dual two- TRANSMIT ELASTIC FORMATTER STORE frame elastic stores that can connect to asynchronous or synchronous backplanes up to 8.192 MHz FRAMER #0 Extracts and inserts robbed bit signaling FRAMER #1 Framer and payload loopbacks FRAMER #2 Large counters for BPVs, LCVs, EXZs, CRC6, PCVs, F-bit errors and the number of FRAMER #3 multiframes out of sync CONTROL PORT Contains ANSI 1s density monitor and enforcer C