CCIR 476-4 / CCIR 625 Mode A

Data burst type synchronous simplex ARQ system using the CCIR 476 alphabet which consists of 7 bits per character. Two stations usually use the same frequency, one of them is called the 'ISS' (transmitting) station, the other the 'IRS' (receiving) station. The pulsing repetition cycle is 450 msecs. Groups of 3 characters are sent at a time, followed by a receiving period. At the default speed of 100 Baud a 7 bit character takes 70 msec. 3 x 70 msec = 210 msec transmitting time leaving 240 msec to receive the reply from the 'IRS' station.