User guide

Rev 1 Oct.13 289

B: Modbus/BSAP Configuration

The AirLink device supports Modbus ASCII, Modbus RTU, and BSAP,

and can also emulate other protocols (like DF1) using the Modbus

Variable feature.

Modbus Overview

The Modbus Protocol provides for client-server (i.e., 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

Modbus variations are often used in conjunction with telemetry

devices.

Tip: This section is just a brief overview of Modbus. For more information,

refer to your Modbus equipment distributor or manufacturer or

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

satellites.

Remote Terminal Unit (RTU)

Modbus was originally designed to be used in a radio environment

where packets were broadcast from a central station (i.e., master or

host) to a group of remote units. Each remote unit, or Remote

Terminal Unit (RTU), has a hexadecimal 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 only sends to the unit with the matching ID; the

RTU then replies back to the central station.

The RTU connects to such physical equipment 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.