UM-0085-B09
DT80 Range User Manual
Page 361
RG
Modbus Channel
Not available on DT81/82E
About Modbus
Modbus
is a simple communications protocol which is widely used in
SCADA
(supervisory control and data acquisition)
systems. Modbus provides an efficient and standardised way to transport digital states and data values between a
remote terminal unit (
RTU
) or programmable logic controller (
PLC
) and a supervisory computer.
Servers and Clients
In a Modbus-based SCADA system, each RTU/PLC acts as a Modbus server, or slave. These servers/slaves listen for
and reply to requests from a Modbus client, or master system.
As described in
, the DT80 is capable of operating as a Modbus server; that is, it can act like an
RTU or PLC device, and make data available when polled by a client system.
The DT80 can also operate as a Modbus client, where it can read data from Modbus sensors in the same way that it
reads data from SDI-12 or serial sensors.
The remainder of this section describes the operation of the DT80 as a Modbus client.
The MODBUS Channel Type
To read data from a Modbus device (or write control information to it), the
MODBUS
channel type is used. Each time a
MODBUS
channel is evaluated, the DT80 will send a Modbus request to the device, retrieve the response, extract the
required data and return it so that it can be displayed or logged in the usual way.
Modbus can operate using a broad range of communications media. These fall into two main categories:
•
a serial connection, typically RS232, RS422 or RS485
•
a TCP/IP network, which can use a variety of physical link types e.g. Ethernet, wireless, fibre-optic, serial (PPP)
As with the
SERIAL
and
SDI12
channel types, for Modbus channels the channel number refers to the physical DT80
port to which the sensor is wired. That is:
•
1MODBUS
is used to read sensors connected to the serial sensor port
•
2MODBUS
is used to read sensors connected to the host RS232 port (not DT8xM)
•
3MODBUS
is used to read sensors connected to the USB port (not DT82I)
•
4MODBUS
is used to read sensors connected to a TCP/IP network.
Connecting Serial Modbus Sensors
A serial Modbus network has one client (master) system – in this case, the DT80 – connected to one or more server
(slave) devices. Serial networks using the RS485 or RS422 standards support multi-drop, i.e. multiple slaves connected
to one master. RS232 or USB can also be used for point-to-point connections (single master and single slave).
Slave devices on a serial Modbus network are identified by an 8-bit
slave address
(1-247). Every slave device on a
particular serial network must have a unique address. (Slave addresses are not required on a TCP/IP Modbus network,
because the slaves are identified by their IP address.)
The DT80 can be connected to a serial Modbus network using either the serial sensor port, the host RS232 port, or the
USB port.
Note that if a particular serial port is set up to read Modbus sensors then that port must be dedicated to that function. It
cannot also be used for controlling generic serial devices or sending commands to the logger's command interface.
Serial Sensor Port
The serial sensor port is the one most commonly used for connecting serial Modbus sensors. It supports RS232, RS422
and RS485 connections. Note however that the serial sensor port is not available on the DT81/ 82E.
To configure the serial sensor port for Modbus master operation, set the following profile:
PROFILE SERSEN_PORT FUNCTION=MODBUS_MASTER
If you try to define a
1MODBUS
channel without setting this profile then an error message will be returned.
For more details on setting up the port and the possible wiring configurations, see
.