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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 instrumentation.
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 networks, 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 provided. 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 address,
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.
6 Modbus protocol overview ç RVT communication
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