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5.1. MODBUS-RTU transmission format
MODBUS-RTU sends and receives binary data without a frame-synchronizing start code and de-
fines the blank time to recognize the start of a frame. MODBUS-RTU decides the data that is first
received subsequently as the first byte of a frame after a blank time for 3.5 bytes at the on-going
communication speed.
5.1.1. Read command (03)
Computer → Inverter *The text size is 8 bytes fixed.
Inverter
No.
Command
Commu-
nication
No.
(high)
Commu-
nication
No.
(low)
Number
of Data
Groups
(high)
Number
of Data
Groups
(low)
CRC
(low)
CRC
(high)
(3.5bytes
Blank)
03 00 01
(3.5bytes
Blank)
1) Inverter No.. (1 byte) : Specify an inverter number between 0 and 247 (00H to F7H).
Command processing will be executed only broadcast communication “0” and with
those inverters that match set inverter numbers. Data will not be returned if “0”
(broadcast communication) and inverter numbers do not match.
2) Command (1 byte) : Set the read command (03H fixed).
3) Communication No.. (2 bytes) : Set in the order of high to low numbers.
4) Number of data groups (2 bytes) : Set the number of data words 0001 (fixed) in the order of high to low numbers.
5) CRC (2 bytes) : Set generation results of CRC in the order of low to high numbers.. For the
method to generate CRC, see “5.2 CRC Generation.” Note that the setting se-
quence is reversal to that of others.
Inverter Computer (Normal return) *The text size is 7 bytes fixed.
Inverter
No.
Command
Number of
Data
Read data
(high)
Read data
(low)
CRC
(low)
CRC
(high)
(3.5bytes
Blank)
03 02
(3.5bytes
Blank)
1) Command (1 byte) : Read command (03H fixed) will be returned.
2) Number of data : A number of data bytes (02H fixed) will be returned. The number of data groups for
transmission to the inverters is 2 bytes and 01H fixed. Note that the number of data re-
turned by the inverters is 1 byte and 02H fixed.
3) Read data (2 bytes) : Returned in the order of read data (high) and (low).
Inverter Computer (Abnormal return) *The text size is 5 bytes fixed.
Inverter No. Command Error Code
CRC
(low)
CRC
(high)
(3.5bytes
Blank)
83
(3.5bytes
Blank)
1) Command (1 byte) : 83H fixed (Read command error) (Command + 80H)
2) Error code (1 byte) : See “4.3 Transmission errors.”
Example: Reading output frequency (During 60Hz operation)
(Computer → inverter) 01 03 FD 00 00 01 B5 A6
(Inverter → computer) 01 03 02 17 70 B6 50
Example: Data specification error
(Computer → inverter) 01 03 FD 00 00 02 F5 A7
(Inverter → computer) 01 83 03 01 31
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