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MAN-147-0018-A
5.16.
S
ERIAL
I
NPUT
(RS485
/
MODBUS)
The logger may be constructed with a 4-pin connector (see Figure 7, on page 12) for
connection of a sensor which employs the RS-485/MODBUS method of signalling; this is
a serial data interface.
Note: Ensure the attached sensor has the RS485/MODBUS protocol selected, otherwise
communications will fail.
Two types of MODBUS interface are available:
•
Passive.
•
Active.
For a Passive interface, the logger does not provide power to the sensor; it must have its
own source of power.
For an Active interface, the logger provides temporary power to the sensor, just before
(and during) the measurement cycle.
The port type (active or passive) can be determined by inspection as to whether (or not)
there is a voltage output control shown within IDT. In addition, the connector label will
indicate ‘MODBUS’ or ‘POWERED MODBUS’.
A wide variety of sensors are available with this interface.
When ordered from HWM, the sensor will have the correct connector fitted for the
Multilog2 logger. In addition, the sensor type will have been tested with the logger to
confirm compatibilty for use to obtain certain measurements. However, this may
require selecting a specific driver for the sensor within IDT.
The Multilog2 operates as the master device when using the Modbus protocol. It sends
setup instructions and other information to the attached sensor equipment (which
operates in slave mode). The protocol includes the ability to address each register in
order to read and (depending on the attached unit) write to the registers. Measurement
results are made available to the logger by reading them from specific registers in the
sensor equipment over the Modbus link.
The sensor equipment will have an address that the logger must use to identify it when
communicating. The setup of the logger should therefore include the sensor address as
well as the register access details (function code, start register address).
The quantity of registers to be read will depend on the format of the data within the
sensor registers. The logger can handle multiple formats of numeric data (e.g., 16-bit
signed, 16-bit unsigned, float, double); however, the expected data format must be
specified in the logger setup; this will ensure that the required number of registers are
read and that the data is correctly interpreted by the logger. The read data can then be
used to obtain the channel datapoints.
When setting the logger for use with your sensor, usually the “generic” settings are
suitable. However, some modification of the logger operation is required for certain
types of sensor equipment in order to get the best out of them. IDT provides a control
to select specific sensors from a list. Once chosen, the logger will handle any
peculiarities of the behaviour of the sensor, its protocol, or additional needs for the
