PACSystems™ RX3i and RSTi-EP CPU Reference Manual Section 6
GFK-2222AK October 2019
Serial I/O, SNP & RTU Protocols 160
CPE400 and CPL410 do not support this protocol.
6.3.1 Message Format
The general formats for RTU message transfers are shown below:
6.3.1.1 RTU Message Transfers
Figure 20: 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.
6.3.1.2 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 Section 4:.
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