Abstract: Disclosed is a message oriented interface for communication between a bank controller unit and peripheral devices, such as channel units, in digital loop transmission systems. At least four registers are used for each peripheral device. Two registers are utilized to transmit control signals, and two registers are for data transmission. An additional data register can hold the last byte read by the bank controller unit in the event of an error so the byte can be re-read. The last byte of each message, which is utilized for error detection, is the exclusive OR function of the corresponding bits in all the previous bytes of the message. A service request can be asserted by a peripheral device to the Bank Controller Unit when a control signal is present in one of the registers. Means are also provided for indicating when a register is busy.

Abstract: Disclosed is a circuit for the automatic testing of channel units such as found in subscriber loop carrier (SLC.RTM.) systems. The circuit is included as part of the remote terminal of the system and performs type tests, power-up tests and routine verification tests. The circuit also monitors the subscriber channel, and connects the subscriber in the event of a ringing signal or an off-hook during a test. The results of the test are sent to a display unit which can be located at the remote terminal and/or the central office.