MODBUS Protocol
Serial Communication Parameters
C-2 48i Instruction Manual Thermo Electron Corporation
Serial Communication
Parameters
The following are the communication parameters that are used to configure
the serial port of the iSeries to support MODBUS RTU protocol.
Number of Data bits : 8
Number of Stop bits : 1
Parity : None
Data rate : from 1200-115200 Baud (9600 is default)
TCP Communication
Parameters
iSeries Instruments support the MODBUS/TCP protocol. The register
definition is the same as for the serial interface.
TCP connection port for MODBUS : 502
Application Data Unit
Definition
Here are the MODBUS ADU (Application Data Unit) formats over serial
and TCP/IP:
Slave Address 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.
MBAP Header In MODBUS over TCP/IP, a MODBUS Application Protocol Header
(MBAP) is used to identify the message. This header consists of the
following components:
Serial: Slave Address Function Code Data Error Check
TCP/IP: MBAP Header Function Code Data
Transaction Identifier 2 Bytes 0x0000 to 0xFFFF (Passed back in response)
Protocol Identifier 2 Bytes 0x00 (MODBUS protocol)
Length 2 Bytes 0x0000 to 0xFFFF (Number of following bytes)
Unit Identifier 1 Byte 0x00 to 0xFF (Passed back in response)
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