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CHAPTER 6: MODBUS BASICS
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
