Mitsubishi Electric FR-D720-008 Instruction Manual page 187

(2) Signal loss detection (Pr.122)
If a signal loss (communication stop) is detected between the inverter and computer as a result of a signal loss detection,
a communication fault (E.PUE) occurs and the inverter trips. (as set in Pr. 502).
When the setting is "9999", communication check (signal loss detection) is not made.
When the setting value is "0" (initial value), RS-485 communication can be made. However, a communication fault
(E.PUE) occurrs as soon as the inverter is switched to the operation mode (Network operation mode in the initial setting)
with the control.
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 (refer to Mitsubishi inverter protcol control code (page 189), Modbus-RTU comunciation protocol (page 199))
from the computer 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 made from the first communication in the operation mode with control source valid (Network
operation mode in the initial setting).
Example: PU connector communication, Pr. 122 = "0.1 to 999.8s"
Operation mode
Computer
Inverter
Inverter
Computer
Pr. 122
Communication
check counter
Always set the communication check time interval before starting operation to prevent hazardous conditions.
Data communication is not started automatically but is made only once when the computer provides a
communication request. If communication is disabled during operation due to signal cable breakage etc., the
inverter cannot be stopped. When the communication check time interval has elapsed, the inverter trips
(E.PUE).
The inverter can be coasted to a stop by turning on its RES signal or by switching power off.
If communication is broken due to signal cable breakage, computer fault, etc, the inverter does not detect
such a fault. This should be fully noted.
Communication operation and setting
External
PU
Check start
CAUTION
Fault (E.PUE)
Time
4
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