Thermo Scientific 5028i Instruction Manual page 306

MODBUS Protocol
TCP Communication Parameters
TCP Communication
Parameters
Application Data Unit
Definition
Slave Address
MBAP Header
C-2 Model 5028i Instruction Manual
iSeries instruments support the MODBUS/TCP protocol. The
register definition is the same as for the serial interface. Up to three
simultaneous connections are supported over Ethernet.
TCP connection port for MODBUS : 502
Here are the MODBUS ADU (Application Data Unit) formats over
serial and TCP/IP:
Serial: Slave Address
TCP/IP: MBAP Header
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.
Function Code Data
Function Code Data
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)
Error Check
s
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