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MODBUS Protocol
TCP Communication Parameters
Communication
Parameters
Application Data
Unit Definition
Slave Address
MBAP Header
C-2
Model 43i Trace Level-Enhanced Instruction Manual
iSeries Instruments support the MODBUS/TCP protocol. The register
TCP
definition is the same as for the serial interface.
TCP connection port for MODBUS: 502
Here are the MODBUS ADU (Application Data Unit) formats over serial
and TCP/IP:
Serial:
TCP/IP:
The MODBUS slave address is a single byte in length. This is the same as
the instrument ID used for C-Link commands and can be between 1 and
127 decimal (i.e. 0x01 hex to 0x7F hex). This address is only used for
MODBUS RTU over serial connections.
Note Device ID '0' used for broadcast MODBUS commands, is not
supported. Device IDs 128 through 247 (i.e. 0x80 hex to 0xF7 hex) are not
supported because of limitations imposed by C-Link.
In MODBUS over TCP/IP, a MODBUS Application Protocol Header
(MBAP) is used to identify the message. This header consists of the
following components:
Transaction Identifier
Protocol Identifier
Length
Unit Identifier
A Slave address is not required in MODBUS over TCP/IP because the
higher-level protocols include device addressing. The unit identifier is not
used by the instrument.
Slave Address
Function Code
MBAP Header
Function Code
2 Bytes
0x0000 to 0xFFFF (Passed back in response)
2 Bytes
0x00 (MODBUS protocol)
2 Bytes
0x0000 to 0xFFFF (Number of following bytes)
1 Byte
0x00 to 0xFF (Passed back in response)
Data
Error Check
Data
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Thermo Fisher Scientific
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