Chapter 6. Serial I/O, SNP & RTU Protocols
GFK-2222AD April 2018 255
Serial Link Timeout
The only cause for a RTU device to timeout is if an interruption to a data stream of 4 character times
occurs while a message is being received. If this occurs the message is considered to have terminated
and no response will be sent to the master. There are certain timing considerations due to the
characteristics of the slave that should be taken into account by the master. After sending a query
message, the master should wait an appropriate amount of time for slave turnaround before assuming
that the slave did not respond to the request. Slave turnaround time is affected by the Controller
Communications Window time and the CPU sweep time, as described in RTU Slave Turnaround Time.
Invalid Transactions
If an error occurs during transmission that does not fall into the category of an invalid query message or
a serial link time-out, it is known as an invalid transaction. Types of errors causing an invalid transaction
include:
â–ª Bad CRC.
â–ª The data length specified by the Memory Address field is longer than the data received.
â–ª Framing or overrun errors.
â–ª Parity errors.
If an error in this category occurs when a message is received by the slave serial port, the slave does not
return an error message; rather the slave ignores the incoming message, treating the message as though
it was not intended for it.
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