GE PACSystems RX7i, Reference Manual

Page: 238 / 317

Chapter 6. Serial I/O, SNP & RTU Protocols
GFK-2222AD April 2018 227
6.3.1 Message Format
The general formats for RTU message transfers are shown below:
RTU Message Transfers
Query Message Master
Slave
Master
Slave
Broadcast Message
Response
(No Response)
Slave Turn-around Time
Query Transaction
Broadcast Transaction
Figure 49: RTU Message Transactions
The master device begins a data transfer by sending a query or broadcast request message. A slave
completes that data transfer by sending a response message if the master sent a query message
addressed to it. No response message is sent when the master sends a broadcast request.
RTU Slave Turnaround Time
The time between the end of a query and the beginning of the response to that query is called the slave
turnaround time. The turnaround time of a PACSystems slave depends on the Controller
Communications Window time and the sweep time of the PACSystems. RTU requests are processed only
in the Controller Communications Window. In Normal sweep mode, the Controller Communications
Window occurs once per sweep. Because the sweep time on PACSystems can be up to 2.5 seconds, the
time to process an RTU request could be up to 2.5 seconds. Another factor is the Controller
Communications Window time allowed in Hardware Configuration. If you configure a very small
Controller Communications Window, the RTU request may not be completed in one sweep, causing RTU
processing to require multiple sweeps. For details on CPU window modes, refer to
Window Modes in
Chapter 4.
Receive-to-Transmit Delay
Part of the RTU Slave Turnaround time is the receive-to-transmit delay. The RTU driver inserts this delay
after 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 Serial
Port Setup COMMREQ function 65520. The timeout is specified in units of 10 ms, with a range of 0
– 255
units (maximum delay is 2.55 seconds). If the specified time is less than 3.5 character times, then the
delay is set to 3.5 character times.