Modbus/Tcp Command Set; Introduction Of Modbus/Tcp Protocol - Korenix JetI/O 6510 User Manual

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
01 Read Coils
02 Read Input Status
03 Read Holding Register
04 Read Input Registers
05 Write Coil
06 Write Single Register
15 Force Multiple 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 actors 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.
Name
Read the state of a digital output
Read the state of a digital input
Read holding register in 16-bits register format
Read data in 16-bits register format
Write data to force a digital output ON/OFF
Write data in 16-bits register format
Write data to force multiple consecutive coils
Usage
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