BSC Error Protocols

The symbols used in the flows that follow are:

R = Read
P = PREPARE
W = Write
S = Sense
C = Control
TO = Time Out
cc = Command Chain

When a DLE ENQ for PREPARE resume receives a negative acknowledge, the condition is treated as a NAK to a NAK in normal processing. In this case, the last non-NAK is defined to be the last buffer (null or not) for which an acknowledgement has not been received. This buffer will be different depending on which side starts the PREPARE and which receives the NAK. The following example illustrates the different interpretations.
Note: In the example, an actual buffer is sent in all cases where just the BCB is shown.

Figure 1. BSC Error Protocols
In the case of a wake up contention condition on a BSC line, one DLE ENQ may be lost. This situation is resolved by using the primary/secondary relationship between the nodes (established at signon) to determine which side sends data first.
If one side receives a unit exception when it tries to write the DLE ENQ, that side uses the primary/secondary relationship in the same way as above after issuing a read skip to clear the contention.

If one side receives DLE ENQ instead of data in response to its DLE ENQ, then that side waits (issues a read CCW) for DLE STX data buffer DLE ETB to be transmitted if it is the secondary, or sends DLE STX data buffer DLE ETB if it is the primary.

The flow for contention is:
If one side goes down while I/O is suspended, the other side will detect it at resume time by a timeout as follows:

When the read times out in this case, normal time-out processing should be used. This includes sending a NAK.
If one side restarts while I/O is suspended, the following scenario will occur:

If the restart occurs at the same time as a resume, the following can happen:

Note: If both sides try to restart while I/O is suspended, the signon contention protocol will resolve contention problems.
A system that has not agreed to use PREPARE at signon time should never receive DLE ENQ. However, if such a sequence is ever received unexpectedly, the proper procedure is to respond to it with a NAK.
If the resuming side sends DLE ENQ and the sequence is lost (due to a hardware error), a timeout will occur and that side should send a NAK (following normal timeout recovery).
Note that the only valid sequences read by the READ chained to a PREPARE CCW are DLE ENQ, SOH ENQ, or NAK. All these cases have been covered in the preceding examples. The “resumer” never immediately sends a null or non-null data buffer, nor is there any way such a buffer can be received until normal sequences are used.
The following flow shows what happens when side A requests I/O suspension and side B does not want to enter this state because it has something to send.
Either side may continue to request suspension, even if the other side does not agree to it.