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5 Modbus/TCP Command Set
This chapter introduces the Modbus/TCP command set JetI/O provided. When you
creating application for your SCADA/HMI or coding your own programs. The command
set is helpful for you to find the value of each registers.
Following topics are covered in this chapter:
5.1 Introduction of Modbus/TCP Protocol
5.2 JetI/O 6510 Modbus/TCP Address Mapping
5.1 Introduction of Modbus/TCP Protocol
5.1.1 Modbus/TCP
Protocol
The Modbus protocol, developed by Gould-Modicon, is widely used in industrial
communications to integrate PLC’s, computer, terminals and other various I/O devices.
Intelligent JetI/O Server equipped with communication interface provides an Ethernet
communication links with Modbus/TCP protocol support.
Modbus/TCP is a variant of the Modbus family of communication protocol. Modbus/TCP
is a Master/Slave communication protocol. A master (a host PC) initiates queries, a slave
(one of the JetI/O servers) then responds by supplying the requested data to the master by
using Modbus/TCP commands.
5.1.2
Function Code (FC)
The Intelligent JetI/O Server uses a subset of the standard Modbus/TCP function code to
access device-dependent information. Modbus/TCP function code is defined as below.
FC Name
Usage
01
Read Coils
Read the state of a digital output
02
Read Input Status
Read the state of a digital input
03
Read Holding Register
Read holding register in 16-bits register format
04
Read Input Registers
Read data in 16-bits register format
05
Write Coil
Write data to force a digital output ON/OFF
06
Write Single Register
Write data in 16-bits register format
15
Force Multiple Coils
Write data to force multiple consecutive coils
5.1.3
Error
Checking
The utilization of the error checking will help eliminate errors caused by noise in the
communication link. In Modbus/TCP mode, messages include an error-checking field that
is based on a Cyclical Redundancy Check (CRC) method. The CRC filed checks the
contents of the entire message. It applied regardless of any parity check method used for
the individual BYTE acters of the message. The CRC value is calculated by the
transmitting device, which appends the CRC to the message. The receiving device
recalculates a CRC during receipt of the message, and compares the calculated value to
the actual value it received in the CRC filed.
