MCSI-0 and MCS-1 are in Bit Synchronous HDLC controller mode. MSCI-0 is used for B-Channel signaling while MCSI-1 is used for the FDL in T1 applications and Sa bits in E1 applications.
The DS2151, DS2153 is shown in a "loop timed" configuration. TCLK and TSYNC are derived from RCLK and RSYNC. The Elastic Stores are disabled. SYSCLK (not shown) should be connected to VSS. RCHBLK and TCHBLK are used to independently select portions (DSO's) of the transmit and receive PCM streams.