This 4ASTU PCB provides connections for only 4 analog extensions. The 4ASTU PCB has a Mode Switch, a port activity LED to indicate traffic on the PCB, and a status LED indicating proper PCB operation.
In Fixed Slot systems, you can only install a 4ASTU in the second universal slot. It provides connection for 4 analog extensions, but uses up 8 analog ports.
In Universal Slot systems, you can install 4ASTU PCBs in any universal slot except the first.
"B" Version required for Caller Identification to Single Line Telephone