controller power fail condition or when the manual override switch is activated during maintenance the pcm bus isolator contains the logic circuitry required to isolate the pcm bidirectional data bus from the system bus it is controlled by the system power rail of vcc o p control latch and the local power fail logic the station trunk address register is an octal latch required to select one of twenty four line interfaces station trunk control is an 8 bit parallel to serial shift registers required for station trunk control functions i e line card identity or an alarm condition the register is clocked at a frequency of 1 54 mhz the codec control is an 8 bit parallel to serial shift register required to direct the codecs on the line cards to transmit and receive pcm data on the time slot specified the station trunk status circuit is a serial to parallel 8 bit shift register with tri state outputs it is used to monitor the line card information such as hookswitch condition ground recall detection or ring detection the msb register is a bidirection 8 bit parallel to serial and serial to parallel shift register for the most significant bits of a data word the group controller cpu loads an 8 bit word relating to the.