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3.1.3
Message frames
As shown below, a transmission frame consists of four blocks, which are called fields. Details
depend on FC (RTU function codes). To make a clear distinction between RTU function codes
and the inverter's function codes, the former will be hereinafter referred to as 'FC'.
1 byte
Station address
Station address
The station address field is one byte long, in which a station address between 0 and 247 can be
selected.
Selecting address 0 means the selection of all slave stations and a broadcast message.
'FC' (RTU function code)
The 'FC' field is one byte long, in which a function code is defined with a number from 0 to 255.
The 'FCs' in the shaded rows are available. Do not use any unavailable (unused) 'FC'. Failure to
observe this rule results in error response.
0
1
2
3
4
5
6
7
8
9 to 14
15
16
17 to 127
128 to 255
Information
The information field contains all information (function code, byte count, number of data, data,
etc.). For further information about the information field for each message type (broadcast,
query, normal response, error response), see Section 3.1.4 "Message categories."
Error check
The error check field is a CRC-16 check system and two bytes long. Since the length of the
information field is variable, the frame length required for calculating the CRC-16 code is
calculated based on the 'FC' and the byte count data.
For further information about CRC-16 calculations and algorithm, see Section 3.4 "CRC-16."
For byte counts, see Section 3.1.4 "Message categories."
1 byte
FC (RTU function code)
Table 3-1 List of 'FC'
'FC'
Unused
Read Coil Status (80 coils maximum)
Unused
Read Holding Registers (50 registers maximum)
Unused
Force Single Coil
Preset Single Register
Unused
Diagnostics
Unused
Force Multiple Coils (16 coils maximum)
Preset Multiple Registers (50 registers maximum*
Unused
Reserved for exception response
Up to 105 bytes
Information
Description
3-2
2 bytes
Error check
)
1
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