XLINK 500/100 Operations & Maintenance Manual page # 168
18.
Modbus
Modbus is a communications protocol commonly used in industrial applications. Please see the
The station can be configured as a Modbus slave or as a limited Modbus master.
In Modbus master mode, the station collects data from Modbus sensors.
In Modbus slave mode, the station provides sensor data to a Modbus master.
18.1.1.
The Station as a Modbus Slave
The station is able to function as a Modbus slave. In this role, the station will respond to
requests from a Modbus master, typically providing data that the station has collected from
sensors.
The example below illustrates performance as a Modbus slave:
The station is setup as a Modbus slave on RS-485. This involves setting
Modbus RS485
Slave Enable
to ON.
Measurement M1 is configured to collect data from an SDI-12 sensor. This means that
the station collects data from the SDI-12 sensor on a scheduled basis.
Periodically, the Modbus master will get the most recent data that was collected by the
station from the SDI-12 sensor. This effectively makes the station a bridge between the
Modbus master and the SDI-12 sensor.
The Modbus master gets that sensor data by reading
the station’s
input registers 1 and
2, combining the result into a 32bit floating point value. Registers 1 and 2 reflect the
last reading from measurement M1. Similarly, registers 2 and 3 relate to M2. Please
see the
section for details on other registers.
Please note that if the station is setup as a Modbus slave, it will no longer perform any other
function on that port.
This means that if the station is a Modbus slave on RS-232, the RS-232 port cannot be used for
command line communications. Similarly, if the station is a Modbus slave on RS-485, then the
station cannot collect sensor data over RS-485.
In slave mode, extended Modbus commands allow the downloading of the station log, reading
and writing station setup, accessing status information, and more.
Please note that using Modbus slave will increase power consumption of the station. When it is
a Modbus slave, the station is always ready to accept incoming data. RS-232 and RS-485 do not
have control lines that can be used to wake the station up, resulting in an power consumption
increase of about 8mA @ 12V. Modbus over cell requires that the cell modem be on and
listening all the time, which increases power consumption too.
18.1.2.
The Station as a Modbus Master
The station is capable of operating as a Modbus master. It can collect data from sensors using
the Modbus protocol. When setup to collect data from Modbus sensors, the station will
periodically issue a master read register message on the data bus. Sensors on the bus act as
slaves: they wil
l reply to the message, providing ‘register values’ which translate to sensor
readings.
