GE PACSystems RX7i Reference Manual page 239

Chapter 6. Serial I/O, SNP & RTU Protocols
Message Types
The RTU protocol has four message types: query, normal response, error response, and broadcast.
Query
The master sends a message addressed to a single slave.
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.
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.)
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.
228 PACSystems* RX7i, RX3i and RSTi-EP CPU Reference Manual GFK-2222AD