In-Situ Aqua TROLL 400 Modbus and SDI-12 Reference Guide - Manual (Page 55)

8. Appendix—Modbus Additional Information
The Multi-PRO 400 Instrument supports Modbus as its primary communication protocol.
This appendix contains a brief Modbus tutorial intended to accelerate learning for a person who
is not familiar with the protocol. This document is not an official protocol document. More
information about Modbus, including protocol specifications, can be downloaded from
www.modbus.org .
The remaining sections of this document provide the information required to program a
PLC/DCS and/or SCADA system to poll data. The user is expected to have a working
knowledge of Modbus.
8.1 Modbus Tutorial
Modbus is a lightweight communication protocol developed in the late 1970’s by Modicon as a
digital communication protocol for its PLC’s. The protocol requires very little code space and
processing power to implement and has become a worldwide favorite for embedded devices.
Modbus is royalty free and the specifications and standards can be downloaded from the web.
Modbus is a transport protocol. What this means is that Modbus does not have any protocol
features that indicate what types of data are being transported in a message packet. This is
similar to TCP/IP, the internet protocol standard. If we use TCP/IP as an example, consider that
when a person connects to a web site, the primary data content being moved via TCP/IP is
HTML. When a person downloads an instrument manual from an FTP site, typically the data
content being moved is an Adobe PDF file. In both cases, TCP/IP is used as the protocol. The
TCP/IP protocol simply ensures all the bytes (whatever they represent) are transferred from
point A to point B correctly. Modbus is similar in concept. It provides a transport mechanism to
move data safely over a communication link from a device to a computer.
Modbus can be used on a wide variety of communication links. In most applications, the
protocol is used over an RS485 or RS232 link. This is because these types of communication
links are inexpensive and efficient, perfectly suited for communication with embedded devices.
Modbus can also be used over wireless radios, satellites, TCP/IP (Ethernet, token ring, etc.) and
any other protocol-independent communication link.
Modbus is designed to be a Master/Slave protocol. This means that the protocol assumes that a
single Master computer will initiate control and commands to the slave devices. The slave
devices do not send any data on the communication link unless specifically asked for that data
by the Master.
8.2 Modbus Modes
Modbus message packets can be formatted in three ways, RTU, ASCII and IP.
• RTU is the format of choice for hard connected serial connections such as RS485 or
RS232 because it is the most compact and therefore the most efficient.
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