RXT-300 SmartWireless™
RXT-300 Wireless IM
Rev. 2.0
Page 27 of 41
as Modbus™ address 4. So the HMI Panel will actually request sensor data from
Modbus™ address 4 in this instance.
Starting Address
Defines the starting address used for poll the Modbus™ device
Registers to Read
Defines the number of Modbus™ registers to read. This is limited to 22 registers.
Reading Register
Defines the register that contains the concentration (reading) value used for Alarm
processing and for displaying on the HMI Panel. It is the physical register number with 0
being the first register on a Modbus™ device.
Range
Generally set to 100 but defines the range of readings that will be read from a device. For
4-20mA devices the Detcon a reading of 400 to 2000 represents 4mA to 20mA. If a range
of 100 is entered internally the RXT-300 will translate Alarm thresholds entered later into
the appropriate 4-20mA thresholds internally. So if an Alarm threshold is set to a value of
10 this is 10% of the 4-20mA reading which equates to a reading of 560 internally.
Alarm Setpoints
There are three setpoints, Alarms 1, 2 and 3 that represent the three Alarm thresholds for
the sensor reading. A value is placed here and whether it is an ascending or descending
threshold. Every time the device is read the reading value is compared with this threshold
and will generate an Alarm event if the threshold is crossed.
Alarm Events
This is a submenu that allows the user to select which device events will generate alarms
and what Alarm Outputs these will be placed on. There are six events that are monitored:
Alarm 1, 2, 3 Thresholds, Fault, Communication Error and Modbus Exception. These can
be mapped to any of the four Alarm outputs. Alarm 1, 2, 3 Thresholds events occur when
the user defined setpoints are crossed. Fault is monitored if it is a Detcon device type and
the device has fault status that can be read. Communication Errors occur when there is no
response when a Modbus™ read is performed. Modbus™ exceptions occur when the
device returns Modbus™ exceptions during a read. The default settings can be taken
though.
5.0
Modbus Operation
The RXT-300 wireless transceiver incorporates a single RS-485 two-wire interface for communication with
various Modbus™ devices such as a control system or sensor. In a gas detection system, there can be up to 32
devices attached to one or more RXT-300 transceivers and up to 32 RXT-300. Per Modbus™ all slave devices
in the system will have a unique Modbus™ address assigned to them. In the RXT-300 based system every
RXT-300 and sensor is automatically assigned a unique Modbus™ address to allow each to be accessed
through Modbus™.
When a HMI Panel or controller is attached to a RXT-300, the RXT-300 Modbus™ interface is configured to
become a slave interface so the HMI / controller can poll that RXT-300. Additionally that HMI / controller
can use the Modbus™ address of any sensor in the system and perform a read of the sensor. The RXT-300
will return the sensor data that is updated from the network as if the HMI / controller were attached directly to
that sensor.
When one or more Modbus™ sensors are attached to a RXT-300, it will poll these sensors and store the data
locally. This data is updated in all the other RXT-300s when that RXT-300 it polled by the network Master
RXT-300.
