PACSystems™ RX3i and RSTi-EP CPU Reference Manual Section 6
GFK-2222AK October 2019
Serial I/O, SNP & RTU Protocols 161
6.3.1.3 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.
6.3.1.4 Message Types
The RTU protocol has four message types: query, normal response, error response, and
broadcast.
6.3.1.4.1 Query
The master sends a message addressed to a single slave.
6.3.1.4.2 Normal Response
After the slave performs the function requested by the query, it sends back a normal
response for that function. This indicates that the request was successful.
6.3.1.4.3 Error Response
The slave receives the query, but cannot perform the requested function. The slave sends
back an error response that indicates the reason the request could not be processed. (No
error message will be sent for certain types of errors. For more information, refer to
Communication Errors below.)
6.3.1.4.4 Broadcast
The master sends a message addressed to all the slaves by using address 0. All slaves that
receive the broadcast message perform the requested function. This transaction is ended
by a time-out within the master.
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