Information Technology Reference
In-Depth Information
18.5 Notes from the author
Modbus is an important protocol and has grown in its popularity because of its simplicity. It
has a very basic structure, and is easy extremely easy to implement as it is based on a mas-
ter-slave relationship where a master device sends commands and the addressed slave re-
sponses back with the required information. Its main advantages are its simplicity, its stan-
dardization and its robustness.
Modbus can be operated on a wide range of computers running any type of software,
from a simple terminal-type connection, where the user can enter the required commands
and views the responses, through to a graphical user interface, with the commands and re-
sponse messages hidden from the user. The basic protocol is, of course, limited in its basic
specification, such as the limited number of nodes (256, maximum) and the limited address-
ing range (0000h to FFFFh).
The basic communications link is also simple to implement (normally, RS-232), but newer
Modbus implementations use network connections, such as Ethernet. Another change is to
implement the Modbus protocol over a standard TCP/IP-based network. This will allow
Modbus to be used over an Internet connection.
RS-232 does not have strong error checking, and only provides for basic parity check.
Modbus using ASCII-based transmission of the Modbus protocol adds a simple checksum to
provide an improved error detection technique (LRC). For more powerful error detection the
data can be transmitted in RTU format, which uses the more powerful technique (CRC).
The Modbus Plus protocol now allows for devices to be either a master or a slave. This
allows for distributed instrumentation, where any device can request data from any other
device, at a given time.
