at each physical end of the bus to reduce distortion due to reflection the tracks and the signals they convey are as follows a pcmlisa is the 34 listen 34 connection for the a channel of an slc and also the 34 listen 34 connection for the speech channel of a tlc b pcmlisb is the 34 listen 34 connection for the b channel of an slc c pcmtlka is the 34 talk 34 connection for the a channel of an slc and of a tlc d pcmtlkb is the 34 talk 34 connection for the b channel of an slc e auxpcm is a spare pcm bus for possible use for example as a data channel for isdn integrated services digital network aperatus the pcm bus has 32 slots at 64k b s which forms a 2 048m bit sec system and is serial in operation with 8 bit codes as already mentioned two slots 0 and 1 are reserved for tones which leaves 15 full duplex channels available timing is controlled by the clock bus pcm clks the signalling bus s16 bus enables ccp to communicate with the sip 39 s and is 8 bit wide bidirectional the sip 39 s acting as slaves to ccp an sip assembles any serial signalling from its terminal into message packets ready to send them on the sig bus when requested by ccp any messages to be sent to a telephone from ccp are reformatted by its sip to allow serial signalling the s1g bus is eight bits wide i e eight parallel data line tracks on the backplane with tracks for another five signals as listed below a dso ds7 are the eight data lines which.