Original article at http://p6ers.net/mm/pc-6001/cas/p6tech_11.pdf (thank you)
Basic control of CMT is performed by the SUB CPU. From the Z80 side, through the SUB CPU You can only write 1Byte to tape or read 1Byte from tape. Z80 is first First, tell the SUB CPU whether the operation you are about to perform is to start writing a tape or You need to tell it whether to start reading the file or not. The way to convey this is to use the Z80 explained in the previous chapter. By sending 1 byte or 2 bytes of data consecutively, The SUB CPU goes into CMT control mode.
| command code | processing content |