YOKOGAWA IR202, User Manual

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<3. MODBUS PROTOCOL>
3-6
IM 11G02Q02-01EN 2nd Edition : Sep. 04, 2020-00
3.5 Transmission control procedure
(1)Transmission procedure of master station
The master station must proceed to a communication upon conforming to the following items.
• Before sending a command message, provide 48 bits time or more vacant status. (a)
• For sending, the interval between bytes of a command message is below 24 bits time. (b)
• Within 24 bits time after sending a command message, the receiving status is posted. (c)
• Provide 48 bits time or more vacant status between the end of response message reception
and beginning of next command message sending [same as in (a)].
• For ensuring the safety, make a confirmation of the response message and make an
arrangement so as to provide 3 times or more retries in case of no response, error
occurrence, etc.
NOTE
The above definition is for most unfavorable value. For ensuring the safety, it’s recommended
the program of the master to work with safety factors of 2 to 3. Concretely, it is advised to
arrange the program for 38400 bps with 5 ms or more for vacant status (a), and within 1 ms for
byte interval (b) and changeover from sending to receiving (c).
(2) Description
(1) Detection of the message frame
The status on the line of the communication system is one of the 2 below.
(a) Vacant status (no data on line)
(b) Communication status (data is existing)
Instruments connected on the line are initially at a receiving status and monitoring the line.
When 24 bits time or more vacant status has appeared on the line, the end of preceding frame is
assumed and, within following 24 bits time, a receiving status is posted. When data appears on
the line, instruments receive it while 24 bits time or more vacant status is detected again, and the
end of that frame is assumed. I.e., data which appeared on the line from the first 24 bits time or
more vacant status to the next 24 bits time or more vacant status is fetched as one frame.
Therefore, one frame (command message) must be sent upon confirming the following.
• 48 bits time or more vacant status precedes before the command message sending.
• Interval between bytes of 1 command message is smaller than 24 bits time.
(2) Response of IR202
After a frame detection (24 bits time or more vacant status), IR202 carries out processing
with that frame as a command message. If the command message is destined to the
own station, a response message is returned. Its processing time is 1 to 30 ms (depends
on contents of command message). After sending a command message, therefore, the
master station must observe the following.
• Receiving status is posted within 24 bits time after sending a command message