Modbus Overview. Sierra Wireless AirLink device

B: Modbus/BSAP Configuration

The AirLink device supports Modbus ASCII, Modbus RTU, and BSAP, and can also emulate other protocols like DF1 using its Modbus

Variable feature.

B

Modbus Overview

The Modbus Protocol, developed by Modicon in 1979, provides for client-server (also referred to as master-slave) communications between intelligent devices. As a de facto standard, it is the most widely used network protocol in the industrial manufacturing environment to transfer discrete/analog I/O and register data between control devices. Modbus, BSAP, and other Modbus variations are often used in conjunction with telemetry devices.

Tip:

This section is a brief overview of Modbus. For more information, refer to your Modbus equipment distributor or manufacturer or go to http://www.modbus.org.

Telemetry

Telemetry is an automated communications process by which data is collected from instruments located at remote or inaccessible points and transmitted to receiving equipment for measurement, monitoring, display, and recording. Transmission of the information may be over physical pairs of wires, telecommunication circuits, radios or satellite.

Remote Terminal Unit (RTU)

Modbus was originally designed to be used in a radio environment where packets are broadcast from a central station (also called master or host) to a group of remote units. Each remote unit, Remote

Terminal Unit (RTU), has a hexidecimal identification number (ID).

The first part of the broadcast packet contains an RTU ID which corresponds to the ID of one of the remote units. The Modbus host looks for the ID and sends to only the unit with the matching ID. The

RTU would then reply back to the central station.

The RTU connects to physical equipment such as switches, pumps, and other devices and monitors and controls these devices. The

RTU can be part of a network set up for Supervisory Control and

Data Acquisition.

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Supervisory Control and Data Acquisition (SCADA)

Supervisory Control and Data Acquisition (SCADA) describes solutions across a large variety of industries and is used in industrial and engineering applications to monitor and control distributed systems from a master location. SCADA encompasses multiple RTUs, a central control room with a host computer (or network), and some sort of communication infrastructure.

SCADA allows for “supervisory” control of remote devices as well as acquiring data from the remote locations. Programmable Logic Controllers allow for a higher degree of automated SCADA.

Programmable Logic Controller (PLC)

A Programmable Logic Controller (PLC) is a small industrial computer which generally monitors several connected sensor inputs and controls attached devices (motor starters, solenoids, pilot lights/displays, speed drives, valves, etc.) according to a user-created program stored in its memory. Containing inputs and outputs similar to an RTU, PLCs are frequently used for typical relay control, sophisticated motion control, process control, Distributed Control System and complex networking.

Modbus TCP/IP

Modbus TCP/IP simply takes the Modbus instruction set and wraps TCP/IP around it. Since TCP/IP is the communications standard for the Internet and most networked computers, this provides a simpler installation. Modbus TCP/IP uses standard Ethernet equipment.

Raven Modbus on UDP

When Sierra Wireless AirLink devices are used in place of radios, a AirLink device is connected to the central station (host) and aAirLink device is connected to each remote unit. When the AirLink device is configured for Modbus with UDP, the

AirLink device connected to the host can store a list of IP addresses or names with matching IDs. When the host at the central station sends serial data as a poll request, the AirLink device at the host matches the RTU ID to a corresponding IP of a AirLink device at a remote unit. A UDP packet is assembled encapsulating the RTU ID and serial data transmitted from the host. The UDP packet is then transmitted to the specific AirLink device at the remote unit matching the RTU ID.

The remote AirLink device then disassembles the packet before transmitting the

RTU ID and serial data to the remote unit. The remote units operate in normal

UDP mode and their data is sent to the host via the remote AirLink device and host AirLink device.

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