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- PROBLEM COMMUNICATING WITH MODBUS POLL SLAVE HOW TO
- PROBLEM COMMUNICATING WITH MODBUS POLL SLAVE UPDATE
- PROBLEM COMMUNICATING WITH MODBUS POLL SLAVE CODE
In our example, I have created a variable to act as a coil. If a 0 is entered for this parameter, the discrete commands are mapped to the data logger control ports C1 to C8, and a compile warning is generated.
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In our example, we use 502, because we are using an Ethernet connection and 502 is the default Modbus TCP port.
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ModbusSlave (COMPort,BaudRate,ModbusAddr,ModbusVariable,BooleanVar,ModbusOption)įor reference, in our program code, our instruction with the parameters looks like this: The ModbusSlave() instruction contains six parameters as shown below: The ModbusSlave() instruction is placed between these two statements because it only needs to execute once at compile time rather than being executed during each scan. The instruction is added between the BeginProg and Scan statements. Let’s take a closer look at the ModbusSlave() instruction we added to the program code. 'Populate Modbus Registers ModbusRegisters(1) = PTemp ModbusRegisters(2) = batt_volt ModbusRegisters(3) = analog_meas 'Configure the datalogger as a Modbus Slave ModbusSlave (502,0,1,ModbusRegisters(),ModbusCoil,2)
PROBLEM COMMUNICATING WITH MODBUS POLL SLAVE CODE
This is accomplished using the code below:
PROBLEM COMMUNICATING WITH MODBUS POLL SLAVE UPDATE
To do this, we need to use the ModbusSlave() instruction, declare a variable array to hold our Modbus data, and then update that array with our measurements. Therefore, we need to program our data logger to listen to Modbus polls on the appropriate communications port and to respond with our most current measured data. The CR1000 datalogger in our example is connected to a SCADA system using an NL121 Ethernet Interface. 'Measure the datalogger panel temperature In our example, the CR1000 is programmed to measure battery voltage, panel temperature, and an analog measurement in a simple program as shown below: You may remember our previous discussion about SCADA systems and Modbus from the “Why Modbus Matters: An Introduction” blog article. We’ll use a CR1000 datalogger connected to a SCADA system.
PROBLEM COMMUNICATING WITH MODBUS POLL SLAVE HOW TO
To better understand how this works, we’ll go through an example exercise to show the basic concepts of how to implement the Modbus protocol on a CR1000 datalogger. Turning your data logger into a Modbus TCP/IP server is a great way to allow systems that use the Modbus protocol to have access to your live measurement data. This means that the data logger is configured to implement the Modbus communications protocol and listen for Modbus polls from a Modbus client, such as a SCADA (supervisory control and data acquisition) system. Did you know that you can turn your Campbell Scientific data logger into a Modbus TCP/IP server? Do you know why that’s a good idea? Campbell Scientific data loggers are commonly used as Modbus server devices.