An ending condition of unit exception on the write CCW occurs when data has been transmitted by the other side and a read was not active to receive the data. In this case, a read CCW with a large count and the skip bit should be issued to clear any data that is pending in the adapter. Then, the write should be reissued.

An ending condition of unit check with sense data of either command reject or intervention required should cause the line to be terminated. If a command reject occurred, either the adapter was not enabled, or an invalid CCW was executed. In either case, a software error has probably occurred and retrying would most likely result in a loop.

An intervention required can occur for many reasons. The problems can range from the telephone hanging up on a switched line, to an intermittent clock within the local modem. In any case, retrying the write will usually result in the intervention required happening again. If a unit check with sense data of time out, data check, or lost data is received, the SOH ENQ should be retransmitted. If successful status is received, but the data is not SOH ENQ or DLE ACK0, the SOH ENQ should be retransmitted. These conditions are summarized in the table below.