CCIR 342-2
(aka TDM 342 / ARQM-2, ARQM-4 / ARQ28 / ARQ56)

Synchronous Duplex ARQ, 7 bit ITA 3 alphabet to CCIR Recommendation 342-2. One, two or four channels in TDM (time division multiplex). Two stations on different frequencies, working as ISS (transmitting) and IRS (receiving) station. At start of cycle first character of all channels are inverted;

1 channel: A-channel erect.
2 channel: A-channel erect, B-channel inverted, character interleaved.
4 channel: A-channel erect, B-channel inverted, C-channel inverted, D-channel erect,

Channels A and B character interleaved, A/B and C/D bit interleaved.

Marked cycle of 4 or 8 characters.