Abstract: A serial optical link for transmission of payload data is utilised so as to permit transmission of information facility data unrelated to the payload data. A superframe is constructed at the transmission facility made up of a serial series of data frames. Each data frame consists of serially arranged words including a number of payload data words and a non-payload word. One non-payload word of the superframe has information facility data which provides a communication channel from the transmitting facility to the receiving facility for information unrelated to the communication of payload data. Another non-payload word has alarm and status facility data. The information facility data is stored in a FIFO buffer when received. If the FIFO buffer is within two words of being full, a full signal is incorporated in the next alarm and status facility data word which causes the receiving end to suspend sending information facility data words.

Abstract: A self-test circuit for a fibre optic transmitter is provided. The transmitter includes amplification circuitry which amplifies a first serial digital signal and introduces an unknown delay to produce a second serial digital signal. To detect a failure of the amplification circuitry, the first digital signal is passed through a shift register. When the shift register contains all ones or all zeros, a comparison of the shift register bit type (one or zero) is made with a most recent bit in the second digital signal. If a mismatch is detected, then an error signal is generated.