Mitsubishi Electric FR-A740-00023 Instruction Manual page 355

(9) Signal loss detection (Pr. 539 Modbus-RTU communication check time interval)
If a signal loss (communication stop) is detected between the inverter and master as a result of a signal loss
detection, a communication error (E.SER) occurs and the inverter output is shut off.
· When the setting is "9999", communication check (signal loss detection) is not made.
· When the setting value is "0", monitor, parameter read, etc. can be performed. However, a communication error
(E.SER) occurs as soon as the inverter is switched to the network operation mode.
· A signal loss detection is made when the setting is any of "0.1s to 999.8s". To make a signal loss detection, it is
necessary to send data from the master within the communication check time interval. (The inverter makes
communication check (clearing of communication check counter) regardless of the station number setting of the data
sent from the master.)
· Communication check is started from the first communication after switching to the network operation mode (use Pr.
551 PU mode operation command source selection to change).
· Communication check time of query communication includes data absence time (3.5 byte).
Since this data absence time differs according to the communication speed, make setting considering this absence
time.
Example: RS-485 terminal communication, Pr. 539 = "0.1 to 999.8s"
Query communication
Operation mode
PLC (master)
Inverter (slave)
Inverter (slave)
PLC (master)
Pr.539
Communication
check counter
Broadcast communication
Operation mode
PLC (master)
Inverter (slave)
Inverter (slave)
PLC (master)
Pr.539
Communication
check counter
External NET
Query Message1
Data absence time
(3.5 bytes or more)
Response Message1
Check start
External NET
Query Message1 Query Message2
Data absence time
(3.5 bytes or more)
Check start
Communication operation and setting
Query Message2
Alarm
(E.SER)
Response Message2
Alarm
(E.SER)
Time
Time
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