The integration of Modbus RTU / ASCII into a Emerson RX3i controller has become easier and more convenient thing to do in recent times. The RX3i PACs system has two options for connection to serial networks:
- Four port modules (IC695CMM004)
- Two port modules (IC695CMM002) c/w a RJ45 port
The CMM module supports many other serial protocols such as DNP3 master/slave, CMM, SPL, Serial IO, and Modbus master/slave with all ports being individually configurable. Further information is available in GE’s manual ‘GFK-2460’. The wiring of the module is covered in great depth within the aforementioned manual however one notable feature is the internal termination resistor. Below is a table showing PIN assignments.
To add a module in Proficy Machine Edition Pro™ select the desired slot on the back pane, right click and select “Add Module”, select the tab “Communications” and then the desired module. For this tech tip we have opted to go with the 4-port unit and will focus solely on the Modbus Master setup. The below screenshot is showing the slot selection of the module from within Proficy machine Edition™.
The below screen shot shows a IC695CMM002 module base setting which allows for a memory-based address (tag based is also supported) system for status and control for code driven functionality.
Once the module is set, you can opt for TAG based addressing like all GE RX3i modules. Simply change the variable mode in the Inspector window to “TRUE”. For this tech tip, we will remain with memory-based addressing which is required for Modbus mapping. To configure mapping for a particular channel (our case at hand is for port 1 and a Modbus RTU master), select the desired channel and after setting the Protocol another tab will become available where you configure the actual memory to bus location/address and polling.
Select the “PortData_ModbusMaster X” tab and from here you can configure up to 64 exchanges per port for all standard Modbus queries/commands. Note:
- ‘Operation’ defines the command (e.g. Read Continuous Bit Control is function code 01)
- ‘Station Address’ equals the node number of the slave device on the RS845 bus
- ‘Target Type’ defines the Modbus RTU address nature (e.g. 1X,100000) for ‘Discrete In’ ‘Target Address’ is the starting register for the defined address type
- ‘Ref Address’ defines the memory address in the GE AutomationRX3i CPU
- ‘Ref Length’ defines the number of registers taken/written to starting at the defined starting address in ‘Ref Address’
From the above screenshot, Data Exchange 1 is reading continuously from Node 1 on the RS485 and is reading using function code 01, 16 consecutive addresses form starting address 100001 and is placing the returned value into memory address %I00065 (100001 is into %I00065, 100002 is into %I00066 and so on). Once set via the hardware these values can be simply acted on via internal logic.
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