No. 10A Remote Switching System: Remote Terminal Firmware

The No. 10A RSS RT acts as an intelligent peripheral controller which follows call processing and maintenance orders sent over the data link from the host processor. In addition, the RT performs a large number of autonomous maintenance activities to ensure that all of its functions are being done correctly. In the event that data link communication with the host is lost, the RT is also designed to enter a stand-alone mode that provides simple intra-RT call service to its connected customers. As with other telephone switching systems, high availability is also a requirement in the RT. This is achieved in part by duplication of the data link and duplication of the microprocessor controller (processor, 565 memory, peripheral access). The normal state is for one data link to be active and the other standby and for one microprocessor to be active and the other standby. Normally, the standby microprocessor is in a halted state that allows the active microprocessor to write simultaneously into both the active and standby memories. This keeps the standby memory up-to-date in the event that it is necessary to switch microprocessors. A certain portion of the RT firmware is devoted to the management of these duplicated units to ensure that the RT is in a working configuration and is capable of recovering from a hardware fault in one of these units. In the development of the RT firmware, several program design techniques were used in an attempt to produce a more understandable, maintainable, and easily debugged code.