Chapter 6. Serial I/O, SNP & RTU Protocols
GFK-2222AD April 2018 231
Message Length
Message length varies with the type of message and amount of data to be sent. Information for
determining message length for individual messages is found in RTU Message Descriptions.
Character Format
A message is sent as a series of characters. Each byte in a message is transmitted as a character. The
illustration below shows the character format. A character consists of a start bit (0), eight data bits, an
optional parity bit, and one stop bit (1). Between characters the line is held in the 1 state.
Message Termination
Each station monitors the time between characters. When a period of three character times elapses
without the reception of a character, the end of a message is assumed. The reception of the next
character is assumed to be the beginning of a new message. The end of a frame occurs when the first of
the following two events occurs:
1) The number of characters received for the frame is equal to the calculated length of the frame.
2) A length of 4 character times elapses without the reception of a character.
Timeout Usage
Timeouts are used on the serial link for error detection, error recovery, and to prevent the missing of the
end of messages and message sequences. Note that although the module allows up to three character
transmission times between each character in a message that it receives, there is no more than half a
character time between each character in a message that the module transmits. 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.
End-of-Frame Timeout
The End-of-frame timeout is a feature that compensates for message gaps that can occur due to the use
of radio modems. The timeout is added to the amount of time allowed for receiving a message from the
master. The timeout should be sized according to the maximum gap time that could be introduced by
the master’s transmitting equipment. Starting with Release 6.70 for the RX3i, the end-of-frame timeout
can be configured with the Serial Port Setup COMMREQ function 65520. The timeout is specified in units
of 100 µs. If the specified time is less than 3.5 character times, then the RTU driver sets the timeout to
3.5 character times.
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