These rules are illustrated in the following transactions. Case Ten: Master AC [110 7] = Fundamentals of distributed network protocol 109 Simultaneous Transactions Outstation Description AC [110 24] Both master and outstations transmit simultaneously = = [111 7] Response to master station [110 7] = Confirmation to outstation response [110 24] = Confirmation to unsolicited message [FIR FIN CON SEQ] In Case Ten both master and outstation transmit messages at the same time. The outstation responds immediately to the master station. Because it responded immediately, it may be deduced that this was an immediate mode request, that is, the master station had not made a read request. Note that the order of receipt of confirmations at the outstation is unimportant. Case Eleven: Master AC [111 2] = Unsolicited Response without Confirm Outstation Description AC [110 22] Both master and outstations transmit simultaneously = Master stores and processes unsolicited response. No confirm was requested. = [110 2] Confirmation to master station request = [111 2] Response to master station request [110 2] Confirm [FIR FIN CON SEQ] In Case Eleven a normal message sequence is shown in which an unsolicited message does not require a confirmation. A simultaneous master station request is serviced by the outstation. Note that sending unsolicited responses without requesting confirmation is not recommended, as this may lead to data loss. Considering this situation leads to understanding when the use of non-confirmed messages is appropriate. This is when timeouts will function to ensure that a missed transaction is re-attempted. For example, requests from the master station requiring responses will be repeated if a response is not received. In this case the message response timeout will cause re-sending of the message when a response is not obtained.
110 Practical Modern SCADA Protocols: DNP3, 60870.5 andRelatedSystems Case Twelve: Master AC [110 2] = Simultaneous Transactions Multi-fragment Outstation Description AC [111 18] Master and outstation send messages simultaneously = = [101 2] Fragment 1 of response [110 2] = Confirm for fragment 1 = [011 3] Fragment 2 of response [110 18] = Confirm for unsolicited response [110 3] = Confirm for fragment 2 [FIR FIN CON SEQ] In Case Twelve the unsolicited response is confirmed during the course of a multifragment transaction. Case Thirteen: Simultaneous Transactions Master AC Outstation Description AC [110 3] [111 28] = Master and outstation send messages simultaneously. Unsolicited response not received. = [111 3] Response to request from master [110 3] = Confirm Timeout Confirmation timeout = [111 28] Unsolicited message resent [110 28] = Confirm [FIR FIN CON SEQ] In Case Thirteen an unsolicited response from the outstation is lost or garbled. The outstation responds to a separate transaction from the master station, but re-sends the unsolicited response after its confirmation timeout expires. Case Fourteen: Process After Confirm Mode (1) Master AC Outstation Description AC [110 2] = [111 18] = Master and outstation send messages simultaneously. Request is for read data. Outstation waits for confirmation [110 18] = Confirm for unsolicited response =  Response [110 2] = Confirm [FIR FIN CON SEQ] RTU now processes master request Case Fourteen shows the process after confirm mode. The outstation receives a master station read request after it has commenced an unsolicited response. It waits until it has received a confirmation for its unsolicited response before proceeding.