Data Link Layer
A message begins with five normal 1 bits (A driven low for 500 ns, then B driven low for 500 ns) for bit synchronization, followed by a special frame sync pattern, three bit times long, that violates the usual Manchester encoding rules. A is driven low for 1500 ns, then B is driven low for 1500 ns. This is like a 1 bit sent at 1/3 normal speed (although the premphasis pulses remain 250 ns long).
This pattern is followed by up to 256 16-bit data frames. Each data frame consists of a start bit of 1, an 8-bit data field, a 3-bit station address, and an even parity bit (which includes the start bit, so it equivalent to odd parity over the data and address fields only). This is then followed by three or more fill bits of 0. Unusually for an IBM protocol, the bits within each frame are sent lsbit-first.
All messages are sent between the controller (master) and one slave device. The first frame in a message from the controller contains the device's address, from 0 to 6. The address field of following frames can be any value from 0 to 6, although is usually set to the device's address as well. The final frame in a message includes an address of 7 (all ones) as an end-of-message (EOM) indicator. A single-frame message does not have an EOM indicator.
When a command calls for a response, the device is expected to respond in 30 to 80 μs. A device's response also consists of up to 256 frames, and includes its address in all frames but the last. In this case, a single-frame response includes the EOM address, and the controller assumes it comes from the device it most recently addressed.
Generally, the first frame in a message is a command byte, and following frames are associated data.
Read more about this topic: Twinaxial Cabling, Legacy Applications, IBM
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