B&B Electronics USR604 - Manual - Manual (Page 30)

Section 6 - Advanced User Settings
Page 30 of 45 Document Number: USR60x-0812m
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Modbus Basics
Modbus is an industrial data communications protocol that emerged in
the mid-1970s and continues to be widely used in current industrial
systems. Originally designed to link terminals with Modicon PLCs, it is
simple, easy to learn and implement, and free to use. It quickly
became a defacto standard in the industry and has been widely
implemented with successful results.
Modbus is a message-based master/slave protocol (also sometimes
referred to as master/client) typically implemented across serial
communications links such as RS-232, RS-422 and RS-485. It supports
asynchronous point-to-point and multidrop communications.
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.
Although some newer equipment using Modbus incorporates USB
interfaces, most legacy equipment implements RS-232, RS-422 and
RS-485. Most new PCs have eliminated RS-232 ports as standard
features and RS-422/485 was never a standard feature. As a result
USB to serial converters such as the USR60x are often necessary to
communicate between PCs and legacy systems that implement Modbus
protocol.