When the last non-null buffer is acknowledged and no files are in the process of being received or sent, one side (side A) may decide to suspend I/O activity. It does this by writing a null buffer to the other side (side B) with the terminating sequence DLE ETX instead of the normal DLE ETB. This buffer is sent with the appropriate I/O CCW sequence for CTC adapters or BSC. Thus, the write is command chained either directly (BSC) or indirectly (through a control for CTC adapters) to a read. If side B wants to accept the I/O suspension, it responds with another null buffer also ended by DLE ETX instead of DLE ETB. Unlike the normal I/O sequence, this write CCW is not chained directly to a read (BSC) or to a control, read (CTC adapter). If it does not want to suspend I/O, side B sends either a non-null data buffer or a null buffer terminated with DLE ETB with its normal I/O sequence. In this case, normal I/O sequences continue to be used by both sides.

As soon as side B sends the acknowledging null buffer, it can suspend I/O. Side A must also suspend I/O if it receives an acknowledging null buffer (with DLE ETX) immediately following the buffer it used to request suspension.