Detcon RXT-300 - Manual (Page 9)

RXT-300 SmartWireless™
RXT-300 Wireless IM Rev. 2.0 Page 3 of 41
The RXT-300 has been designed with multiple interfaces to support these three applications. There is a single
RS-485 interface that utilizes the Modbus™ RTU protocol and can be set up as a Modbus™ Master or
Modbus™ Slave. As a Modbus™ Master the interface will support up to four Modbus™ sensors and will poll
the sensors and process alarms for those sensors. As a Modbus™ Slave it supports a single HMI (Human
Machine Interface) Panel or Modbus™ controller to display status of the sensors and RXT-300s as well as
providing control over the network such as resetting of alarms.
There are two 4-20mA inputs that can be used for monitoring 2-Wire or 3-Wire 4-20mA devices. There are
four Alarm outputs that can be configured to be Energized or De-Energized, Latching or Non-Latching and
Silence-able or Non-Silence-able. The RXT-300 can also monitor a single Detcon Smart Battery pack for
remaining battery life allowing the user to replace the battery in a timely manner.
1.2 RXT-300 Wireless Radio
The RXT-300 transceivers utilize radios based upon the IEEE 802.15.4 standard that operate at 2.4 GHz using
DSSS encoding for robustness. DSSS was initially used by the military to resist jamming but later was widely
adopted for wireless implementations since it was robust in noisy environments. DSSS transmits data across a
wider frequency range than the actual frequency range required for the information. This operation minimizes
cross talk and interference from other transceivers and is less susceptible to noise from other sources.
The IEEE 802.15.4 defines 16 separate RF Channels that can be used in the 2.4 GHz range. The default
channel is 1 but can be change if there is RF interference or if there is an existing network using that channel.
Transceivers will only respond to other transceivers with the same RF Channel.
NOTE: If there are multiple Modbus™ networks in the same vicinity each system must reside
on a different RF Channel to keep data from one appearing on the other.
The 802.15.4 standard also implements a mesh network allowing any RXT-300 transceiver to relay or repeat
data between adjacent neighbors. This makes the network very robust and provides the following immediate
benefits:
• Allows re-routing of data in case of loss of a transceiver
• Allows re-routing around wireless obstacles
• Longer distances between transceivers because data can “hop” from one transceiver to the next
• Included in sensor, controller and alarm station transceivers
• RXT-300 transceivers can be deployed with less concern about physical location
Figure 3 Mesh Network Topology