Chapter 6. Serial I/O, SNP & RTU ProtocolsGFK-2222AD April 2018 2276.3.1 Message FormatThe general formats for RTU message transfers are shown below:RTU Message TransfersQuery MessageMasterSlaveMasterSlaveBroadcast MessageResponse(No Response)Slave Turn-around TimeQuery TransactionBroadcast TransactionFigure 49: RTU Message TransactionsThe master device begins a data transfer by sending a query or broadcast request message. A slavecompletes that data transfer by sending a response message if the master sent a query messageaddressed to it. No response message is sent when the master sends a broadcast request.RTU Slave Turnaround TimeThe time between the end of a query and the beginning of the response to that query is called the slaveturnaround time. The turnaround time of a PACSystems slave depends on the ControllerCommunications Window time and the sweep time of the PACSystems. RTU requests are processed onlyin the Controller Communications Window. In Normal sweep mode, the Controller CommunicationsWindow occurs once per sweep. Because the sweep time on PACSystems can be up to 2.5 seconds, thetime to process an RTU request could be up to 2.5 seconds. Another factor is the ControllerCommunications Window time allowed in Hardware Configuration. If you configure a very smallController Communications Window, the RTU request may not be completed in one sweep, causing RTUprocessing to require multiple sweeps. For details on CPU window modes, refer to Window Modes inChapter 4.Receive-to-Transmit DelayPart of the RTU Slave Turnaround time is the receive-to-transmit delay. The RTU driver inserts this delayafter a request from the master has been received, and before the response to the master is sent.Starting with Release 6.70 for the RX3i, the receive-to-transmit delay can be configured with the SerialPort Setup COMMREQ function 65520. The timeout is specified in units of 10 ms, with a range of 0–255units (maximum delay is 2.55 seconds). If the specified time is less than 3.5 character times, then thedelay is set to 3.5 character times.
