Modbus Communication Protocol; Message Configuration - Omron MX2 Series User Manual

8-4

Modbus Communication Protocol

The inverter communicates with external controllers as follows.
External
controller
Inverter
(1) Frame (Query) that is sent from the external control device to the inverter
(2) After receiving a query frame, the inverter waits the total time of the silent Interval
and the Communication Wait Time (C078), before returning a response.
Silent interval
The wait time that is specified on Modbus communication; its data length is 3.5 characters
(3.5 bytes).
It depends on the Modbus communication speed setting.
(Example) 1 character:10 bits (1 start bit + 8 data bits + 1 stop bit)
(3) Frame (Response) that is sent from the inverter back to the external controller.
(4) After sending a response, the inverter monitors the time until it completes receiving
the query frame from the external control device. The inverter judges it as a commu-
nications error if it receives no response within the Communication Error Timeout
Time (C077).
Then, the inverter operates according the Operation Selection on Communication Error
(C076), while waiting for the reception of the first data again.
The monitoring of the Communication Error Timeout Time starts from the first send-
ing/receiving operation is established after the power supply is cycled or after the inverter is
reset.
The inverter does not recognize as a communications error timeout if the sending/receiving
operation is not established at all.
8-4-1

Message Configuration

The command message sent from the master to a slave (or slaves) is called "query" and the response
message returned from the slave(s) is called "response." The transmission format of a query/response
is as follows.
Query
Slave address
Function code
Query data
Error check (CRC-16)
Multi-function Compact Inverter 3G3MX2-V1 User's Manual (I585-E1)
(1) Query
(3)
Response
(2) Wait time
(Silent Interval + C078)
Time required per character: 1/9,600 (bps) × 10 (bit) × 1,000 = 1.04 ms
Time required for 3.5 characters: 1.04 ms × 3.5 = 3.64 ms
However, according to the Modbus specifications, this time is fixed to 1.75 ms
for communications speeds exceeding 19.2 kbps.
Confirmation slave address
Confirmation function code
Response data
Error check (CRC-16)
8 Communications Functions
(1) Query
(4) Communication Error
Timeout Time (C077)
Response
Time
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