Modbus Protocol Overview; Overview; Rs485 Modbus Adapter - Hitachi Abb Rvt Installation Und Betriebsanleitung

2. Modbus protocol overview

2.1.

Overview

MODBUS RTU
MODBUS RTU is a non-proprietary serial communications protocol that is widely used in the process control industry.
The protocol was developed by Modicon for PLC communications and later released for public use.
This protocol is available in all major Human Machine Interface (HMI) software packages and terminals. Many of the
major controller and PLC manufacturers also offer MODBUS protocol as a standard or optional protocol in their instru-
mentation.
The hardware over which MODBUS RTU communications are performed is not defined by the protocol. MODBUS
RTU is supported on RS-232, RS-422, RS-485, Ethernet and other electrical standards. It should be noted that
MODBUS RTU, MODBUS ASCII and MODBUS Plus are unique communication formats, and are not compatible with
each other. This document will discuss MODBUS RTU only.
MODBUS TCP
MODBUS TCP is a MODBUS RTU message transmitted with a TCP/IP wrapper and sent over an Ethernet network
instead of serial lines. The Server does not have a SlaveID as in RTU since it uses an IP Address instead.
2.2.

RS485 Modbus Adapter

The Modbus protocol communicates with the instrumentation by means of an industry standard serial interface. This
interface may be RS-232, RS-422 or RS-485. Some systems may also support the protocol over other busses or net-
works, such as Ethernet. An RS-232 interface allows only two devices to be connected together. RS-422 supports 1
driver and up to 10 receivers on a single network. For bi-directional communications, special tri-state circuitry is pro-
vided. RS-485 supports up to 32 driver/receiver pairs. With special hardware, the RS-422 and RS-485 limits can be
expanded to allow as many as 248 devices on a single network. Each device on a network must have a unique ad-
dress, which may be soft configured. Address zero is reserved for broadcast messages from the host to all slaves.
The RS485 Modbus Adapter is an option to the RVT. It enables the connection of the RVT controller to an RS485
Modbus network.
STATUS
Approved
SECURITY LEVEL
Public
DOCUMENT ID
9AKK106713A9758
RVT CO MM UNI CATI O N
REV. LANG. PAGE
K en 5/71
© 2022 Hitachi Energy. All rights reserved.