For BSC suspension, both sides issue a read CCW that is allowed to time out followed by the I/O sequence of a PREPARE CCW (X'06') command chained to a read CCW.

Both systems then wait until either one of them has something to send. When either of the systems has data to transmit to the other system, it must then issue a halt device (HDV) or halt I/O (HIO) to terminate the PREPARE. Then, before transmitting the actual buffer, it should transmit a DLE ENQ sequence. When a system waiting on a PREPARE receives any data, it must acknowledge the data with a null buffer before going back to wait on the PREPARE. This allows the system with the pending data to transmit the data and resume normal processing once data has been received. This sequence is shown in BSC Communication in PREPARE Mode.

If both systems attempt to initiate transmission at the same time, one of the DLE ENQs may be lost by the HDV, or may be lost when a read-skip is used to recover from a unit exception on the write. In this case, a contention resolution protocol is used (see BSC Error Protocols) to force resynchronization.

Figure 1. BSC Communication in PREPARE Mode