Detcon RXT-300 - Manual (Page 34)

RXT-300 SmartWireless™
RXT-300 Wireless IM Rev. 2.0 Page 28 of 41
5.1 Modbus Addressing – Special Cases
5.1.1 Address Translation
The RXT-300 has the capability to take a unique Modbus™ system address and translating it to a different
address on the local Modbus™ interface. In this way there can be local Modbus interfaces on multiple
RXT-300s that have sensors with the same address but are addressed with unique system wide Modbus™
addresses. Each RXT-300 would be configured to perform the proper translation.
This is specifically used with Detcon 100 series sensors which are factory set to be fixed at Modbus™ address
= 1. So if there were five 100 series sensors in a system there would need to be at least 5 RXT-300s where
each would translate the unique system address to the local address = 1. This feature can be used to field
RXT-300s with sensors that have each been set to the same local Modbus™ addresses but during
configuration would be re-assigned to unique system Modbus™ addresses.
5.1.2 Accessing RXT-300 4-20mA Sensors
The RXT-300s support up to two 4-20mA inputs for 4-20mA sensors. Since these sensors are not attached to
the RXT-300 local Modbus™ interface they do not have unique Modbus™ addresses assigned. They can be
accessed through Modbus™ by using the Modbus™ address of the RXT-300 they are attached. When
configuring a RXT-300 4-20mA sensor then, the user would override the unique system address assigned and
use the RXT-300 Modbus™ address instead. The reading register would correspond to the RXT-300
Modbus™ register for that 4-20mA input.
5.2 General Modbus™ Description
Modbus™ communication operates basically the same on a RXT-300, whether the Modbus™ interface is
configured as a slave for a HMI / controller or as a master to poll local sensors attached. The physical
interface for Modbus™ is a two-wire half duplex RS-485 interface and runs at 9600 baud, 8 data bits, 1 stop
bit, no parity. The RXT-300 performs Modbus™ communication using the RTU transmission mode per the
Modbus™ specification.
The basic frame format for Modbus™ is shown in Figure 17 and consists of a Modbus™ address, function
code, data and CRC. As stated earlier, the Address Field is the unique Modbus™ address of each device for
the whole system. The Function Code is the function to be performed. The Data contains read or write data
and is formatted according to the function being performed. The CRC (Cyclic Redundancy Code) is used to
detect errors in the frame. Frames with errors are considered invalid and ignored.
Address Field Function Code Data CRC
Figure 17 Modbus™ Frame Format
Modbus™ transactions consist of both a request by the controller and response from the device being
addressed so there are two frames transferred for every transaction. Every request is evaluated by the
RXT-300 to determine if it is addressed to it and whether it falls within the register address range. If these two
conditions are true, the RXT-300 will then check to see if it is a valid Function Code. Function Codes
supported by the RXT-300 are:
Function Code 03 (03h) – Read Holding Registers
Function Code 06 (06h) – Write Single Register
Function Code 16 (10h) – Write Multiple Registers
If an invalid function code is performed, then the RXT-300 will ignore the request by default and the
controller will timeout and continue with the next transaction.