Modbus Basics
Manual Documentation Number: MES1A/MES1B-2106m PN7138-rev001
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If you are reading this manual you are probably in the process of interfacing
legacy Modbus ASCII/RTU devices to a network. Chances are you already
have some knowledge and familiarity with Modbus ASCII/RTU but possibly
somewhat less knowledge of Modbus/TCP and/or networking in general.
Most likely your biggest questions relate to what is involved in bringing the
two together.
This section provides:
•
a basic introduction to Modbus
•
some information on Modbus/TCP
•
some tips and suggestions for ensuring success
Modbus ASCII/RTU
The Modbus protocol emerged in the mid-1970s as an early protocol for
linking terminals with Modicon PLCs using a master/slave (sometimes called
a master/client) relationship. A simple, open, message-based protocol, it
caught on quickly and became a defacto standard in the industry. It supports
asynchronous point-to-point and multidrop communications and can be used
with a variety of serial interfaces (RS-232, RS-422, RS-485, modems, etc).
The original Modbus specification included two possible transmission
modes: ASCII and RTU. Modbus RTU mode is the most common
implementation, using binary coding and CRC error-checking. Modbus
ASCII messages, though somewhat more readable because they use ASCII
characters, is less efficient and uses less effective LRC error checking. ASCII
mode uses ASCII characters to begin and end messages whereas RTU uses
time gaps (3.5 character times) of silence for framing. The two modes are
incompatible so a device configured for ASCII mode cannot communicate
with one using RTU.
All Modbus communications are initiated by Modbus masters using a
polling, query/response format. The master can send broadcast messages
(using a slave address of 0), which all slaves accept, but do not reply to.
More commonly the master polls individual slaves sequentially. In each poll
