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68
Manual – MOVI-PLC® advanced DHR41B for EtherNet/IP, Modbus/TCP and PROFINET IO
8
Protocol structure
The Modbus Protocol (Modbus/TCP)
8.2
Protocol structure
The Modbus protocol consists of a header and function code data. The header is the
same for all request/response telegrams and error messages (exceptions). Depending
on the function code, a different number of data is attached to the header (see following
picture).
8.2.1
Header
The protocol bytes of the header are described in the following table:
• The slave simply copies the transaction identifier (byte 0 and 1). It can help the mas-
ter to identify related actions.
• The protocol identifier (byte 2 and 3) must always be '0'.
• The length bytes (byte 4 and 5) specify the number of bytes occurring in the length
field. As the maximum telegram length is 255 bytes, the 'upper byte' must be '0'.
• The unit identifier (byte 6) can be used for distinguishing between several connected
stations (e.g. bridges or gateways). It has the function of a subaddress that is only
used for parameter access in SEW units. The process data are always mapped to
the unit that is addressed via the unit identifier 0 or FF
hex
.
• The 7 bytes of the header are followed by the function code and the data.
64064AXX
MBAP Header
Transaction-ID
Protocol-ID
Length (1+1+N)
UI-D
Function Code-Data
FC
N Data
Byte
Designation
Meaning
0
Transaction identifier
Often 0, is simply copied by the server (slave)
1
2
Protocol identifier
0
3
4
Length field (upper byte)
0
5
Length field (lower byte)
Number of function codes data bytes + 1 (unit identifier)
6
Unit identifier (slave address) This is the slave address. In order to access the DHR41B process
data, it must be set to '0' (0x00) or 255 (0xFF).
The following address assignments apply to the parameter chan-
nel access (Offset 200 - 203
hex
):
•
0 or 254 for parameters of the DHR41B
•
1 253 for parameters of a lower-level unit connected to
DHR41B. The assignment of unit identifier to the units on the
system buses is determined via the routing table on the
DHR41B memory card (see section '"Appendix"').
7
Function code
Requested service
8 ...
Data
Data depending on requested service
0
0
I
Содержание DHR41B
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