This 8ATRU PCB supports 8 analog loop start CO trunks. It has a Mode Switch, a status LED indicating proper PCB operation, and a port activity LED that indicates traffic on the PCB. You can install 2 8ATRU PCBs in a Fixed Slot 4-Slot Cabinet, and up to 3 8ATRU PCBs in a Universal Slot 4-Slot Cabinet, and up to 6 8ATRU PCBs in an 8-Slot Cabinet. Each 8ATRU PCB also provides 2 power failure cut-through circuits. When commercial AC power fails, the PCB automatically links 2 trunk circuits into 2 power failure telephones you have set up. "B" Version required for built-in Caller Identification.