User`s manual

6-13

6.5 MODBUS Communication Protocol

INP-TN5A0202-E

Transmission control procedure

Master communication method

Master should start communication following the rules below.

1. Provide more than 48-bit time blank state before sending the command message.

2. The interval between the sent messages should be less than 24 bit times.

3. The master will be in the receiving standby mode within 24 bit time after sending the command message.

4. Provide blank state more than 48 bit time prior to sending the next message after receiving the response message. (the

same as the 1st rule)

5. For safety, create the framework where the master checks the response message, and if there is no response or errors

occur, retry at least 3 times.

Note

The deﬁ nition above is the minimum required value. For safety, creating the master side program with providing margins 2 to 3

times as large is recommended. For a concrete example, creating the blank state (the 1

rule above) more than 10ms, the interval

between bytes (the 2

rule above) and switching from sending to receiving (the 3

rule) within 1ms for 19.2kbps is recommended.

Frame detection

There are 2 states with the circuit as below.

Blank state (no data on the circuit)

Communication state (data running on the circuit)

The equipment connected on the circuit is in the receiving state ﬁ rst and monitors the circuit. When the blank state more than 24

bit time appears, it detects the end of the previous frame and will be in the receiving standby mode within the next 24 bit time.

When data appears on the circuit, it begins receiving and when the blank state more than 24 bit appears again, it detects the

end of the frame. In other words, the data appears on the circuit during appearance of the ﬁ rst blank state more than 24 bit time

to the next blank state more than 24 bit time is loaded as one frame (a bundle of data). Therefore, one frame should be sent

following the rules below.

Provide the blank state more than 48 bit time before sending the command message.

The interval between bytes of the message sent should be less than 24 bit time.

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