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Introduction
Background
MODBUS
is a serial communications protocol originally published in 1979 by Schneider Electric
(formerly known as Modicon) for use with its programmable logic controllers (PLC). MODBUS
has become a
de facto
standard communication protocol and is now a commonly available
means of connecting industrial electronic devices. The main reasons for the use of MODBUS in
the industrial environment are:
•
developed with industrial applications in mind
•
openly published and royalty-free
•
easy to deploy and maintain
•
moves raw bits or words without placing many restrictions on vendors
MODBUS enables communication among many devices connected to the same network, for
example, a system that measures temperature and humidity and communicates the results to
a computer. MODBUS is often used to connect a supervisory computer with a remote terminal
unit (RTU) in supervisory control and data acquisition (SCADA) systems. Many of the data types
are named from industry usage of Ladder logic and its use in driving relays: a single-bit physical
output is called a
coil
, and a single-bit physical input is called a
discrete input
or a
contact
.
Supervisory Controls and Data Acquisition (SCADA) protocols are communications protocols
designed for the exchange of control messages on industrial networks. Over the past three
decades, several hundred of these protocols have been developed for serial, LAN, and WAN-
based communications in a wide variety of industries including petrochemical, automotive,
transportation, and electrical generation/distribution.
SCADA MODBUS is the most widely used SCADA Protocol.
There are many variants of MODBUS protocols:
MODBUS RTU
Used in serial communication and makes use of a compact, binary
representation of the data for protocol communication. The RTU
format follows the commands/data with a cyclic redundancy
check
checksum as an error check mechanism to ensure the
reliability of data. MODBUS RTU is the most common
implementation available for MODBUS. A MODBUS RTU message
must be transmitted continuously without inter-character
hesitations. MODBUS messages are framed (separated) by idle
(silent) periods.
MODBUS ASCII
Used in serial communication and makes use of ASCII
characters
for protocol communication. The ASCII format uses a longitudinal
redundancy check checksum. MODBUS ASCII messages are framed
by leading colon (":") and trailing newline (CR/LF).
MODBUS TCP/IP
or MODBUS TCP
Used for communications over TCP/IP
networks, connecting over
port 502. It does not require a checksum calculation, as lower
layers already provide checksum protection.
