User guide

Rev 2.0 C Aug.10 161
B
B: Configuring Modbus/BSAP
The AirLink device supports Modbus ASCII, Modbus RTU, BSAP, and
can also emulate other protocols like DF1 or others using its Modbus
Variable feature.
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 just a brief overview of Modbus. For more information,
refer to your Modbus equipment distributor or manufacturer or 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.