PBX circuit cards are based on solid-state integrated circuitry mounted on printed wiring boards. A label, usually color coded to simplify installation and maintenance operations, identifies each circuit card. The circuit cards usually have fault, test, and busy multicolor LED indicators. A metal latch for electrostatic discharge protection is typically included. Circuit cards can be categorized into three basic types: Control, service, and port. Control circuit cards support PBX call processing and switching functions. Service circuit cards enable some call processing features and functions. Port circuit cards serve as connection gateways between perihperal equipment and the PBX call processing, switching network, and power distribution systems.
Each port circuit card supports a unique type of perihperal endpoint, but all share several common design elements. Each port circuit card has a TDM bus buffer, control channel interfaces, an onboard microprocessor controller with limited memory storage, and additional processing elements to perform functions such as voice quality control. Port circuit card bus buffers are the digital interface between the backplane TDM transmission line wires and the onboard integrated circuitry. The buffers have wire leads that transmit or receive electronical signals from that transmission line. The control channel interfaces access signaling information on the TDM bus. Control channel interface wire leads interface to the TDM bus, pickup common channel information intended for the port circuit pack, and transmit it to the onboard microprocessor controller. When the microprocessor controller transmits information to the TDM bus control channel, it is transported by the control channel interface. In addition, the control channel interface initiates poder-on start-up procedures, performs diagnostics tests on the onboard microprocessor controller, and follows Main system processor instructions to disable the port circuit pack in case of onboard problems. It also can perform several localized processing functions at the station user desktop, such as control of voice terminal LED/LCD status indicators.