Hitachi L700 Series, Instruction Manual, Page: 176 / 263

Chapter 4 Explanation of Functions
4 - 114
4.3.2 Communication in Modbus-RTU mode
(1) Communication protocol
The communication between the inverter (slave) and external control system (master) is based on the
following protocol:
(1): Query frame that is sent from the external control system to the inverter
(2): Response frame that is sent from the inverter to the external control system
(3): Communication trip limit time (C077)
If the inverter cannot complete the reception of a query from the master system (external control
system) within the communication trip limit time, after having sent a response to the preceding query,
the inverter enters the status in which to receive the query from the beginning. Subsequently, the
inverter returns no response to the master system.
After reception timeout occurs, the inverter operates according to the setting of the selection of
operation after communication error (C076). For details, see the table below. Monitoring of reception
timeout begins when the first communication is performed after the inverter power has been turned on
or the inverter has been reset. Reception timeout is monitored only when the inverter communicates
with the master system.
Item Function code Range of data Description
Selection of operation
after communication error
C076
00: Tripping
The inverter trips (E41) after reception
timeout.
01: Tripping after
stopping the motor
The inverter decelerates and stops the
motor, and then trips (E41) after reception
timeout.
02: Ignoring the errors
The inverter ignores the error without tripping
and alarm output.
03: Stopping the
motor after
free-running (FRS)
The inverter stops the motor after
free-running without tripping and alarm
output after reception timeout.
04: Decelerating and
stopping the motor
The inverter decelerates and stops the motor
without tripping and alarm output after
reception timeout.
Communication trip limit
time setting
C077 0.00 to 99.99 (s) Limit time to determine the reception timeout
Communication wait time C078 0. to 1000. (ms)
Time to wait until the inverter starts sending a
response after reception of a query
(excluding the silent interval)
The inverter sends a response (frame (2)) always after receiving a query (frame (1)). The inverter does not
actively output any frame to the external control system.
(2)
(1) (1)
External control system
Inverter
Waiting time (silent in
communication wait time [C078]) (3) Communication trip limit time (C077)
(If reception timeout occurs, the inverter will
operate according to the setting of the
operation selection after communication
error [C076].)
Time