Communication - Modbus in practice

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Peter Humaj

February 02 2021, 9 min read

In the latest blog series, we are bringing you freshly translated posts by Peter Humaj. This text was originally written in 2019. It focuses on establishing communication using the Modbus protocol. The information provided by the blog post is just as relevant, as it was in 2019.

Today we will try to establish communication using one of the most used communication protocols - Modbus.

In practice, we needed to communicate with the Honeywell UDC1700 controller. It is a compact PID controller on a DIN rail, which is optionally equipped with a module for Modbus communication via RS-485. It supports a binary variant of the Modbus RTU protocol as well as a Modbus ASCII text variant.

So - how to communicate with it using the D2000?

The documentation states that by default the UDC1700 communicates at 4800 baud, without parity, using the Modbus RTU protocol and its default Modbus address is 1.

According to the label directly on the UDC1700, the data connectors are no. 16 (+) and 17 (-). Power is connectors 13 (phase) and 14 (ground).

Figure 1: The terminal designation in the documentation is shown. Power supply and RS-485 bus are highlighted.

Since I don't have an RS-485 port on my computer, I used a proven RS-485 to Ethernet converter - Moxa NPort - more specifically the 5450I model with four universal ports that can be configured in RS-232, RS-422 or RS-485 mode.

According to the instructions for NPort, wires no. 4 and 5 are used for 2-wire RS-485 on the DB9 connector.

Figure 2: Connection pins on DB9 connector for Moxa NPort. RS-485 data wires are highlighted.

So I connected the power supply (220 V) to terminals 13 and 14 of the UDC1700 controller and connected terminals 16 and 17 to pins 3 and 4 of Moxa NPort. Subsequently, I connected NPort to the local network.

I then connected to NPort with the NPort Admin tool. Its IP address is shown on the built-in display. I will use port 4 to communicate with the UDC1700. Since it does not have the required parameters, we are going to modify them.

Figure 3: NPort Admin is displaying the tab with serial port parameters. Some information is missing here - whether the port is configured as RS-232, RS-422 or RS-485.

After checking the Modify checkbox, selecting port 4 and clicking on Settings, a configuration dialog will open in which I have set the relevant parameters:

Figure 4: We set it to 4800 baud, no parity, two-wire RS-485 interface.

On the Operating Mode tab, set the mode for each serial port. We can use the following modes when communicating with the D2000:

  • Real COM Mode, in which a virtual serial port is created on the computer (using Moxa drivers)
  • TCP Server Mode, in which NPort acts as a TCP server
  • UDP Mode, in which NPort receives and transmits UDP packets
Figure 5: The mode settings of individual RS-485 ports.

In practice, using the third mode works best for us. Why? The virtual serial port can get stuck (possible problems with NPort drivers) and in TCP Server Mode, timeouts caused by the TCP layer can occur during network outages. We have successfully used UDP mode on all platforms where D2000 has been ported (Windows, OpenVMS, HP-UX, Linux, Raspberry PI). Hence we'll use it here, too.

Figure 6: We will use UDP mode, ports 4004 on both sides. Data will be sent to the IP address, which belongs to a computer running a D2000 server.

This completes the configuration of both the controller and the Moxa, and now I can focus on the configuration of the D2000. Firstly, I created a new communication process UDC1700.KOM. I could use the standard SELF.KOM, but on a test application that is used for different purposes, I try to keep order and create different communications on different KOM processes. Then I run the process I need. In our case, I run it manually from the command line

kom.exe /WUDC1700

The /W parameter is used to specify the process name.

Figure 7: I will create the UDC1700.KOM process. The extension .KOM is mandatory and indicates that this is a communication process.

Then I will create a line. It will be called L.UDC1700 and as a parent, I will specify the UDC1700.KOM process. Within the line parameters, I select the line category - SerialOverUDP Device Redundant and turn on logging to the screen and disk.

Figure 8: These are the settings of the line parameters. In the title bar of the window, there is the name of the line, its status and the name of the parent KOM process.

On the tab of parameters specific for the SerialOverUDP Device Redundant line, I will set the IP address of the NPort and the ports used for UDP communication - I will set the local and remote port to 4004 (which are the values from Figure 6). By default, I use the same ports on the D2000 and NPort side so as not to be mistaken :-)

Figure 9: The setting of UDP communication parameters. Note that it is possible to enter the IP address and port of the backup device - this option is used in redundant systems. Ideally, redundant networks also exist and the backup device is on the backup network so that if one of the networks fails, one device is still available.

The creation of the station follows. Select the L.UDC1700 line as a parent and select the Modbus Client protocol in the Parameters tab:

Figure 10: Protocol selection in station parameters.

As next, enter the station address - according to the UDC1700 documentation, the default address is 1.

Figure 11: Station address selection.

In the protocol parameters on the last tab, I turned on complete debug messages and set the Max. Registers parameter to 64. Why? Because the UDC1700 documentation says that a maximum of 64 words (registers) can be read in one request: The maximum number of words that can be read is 64.

If it was necessary to optimize and speed up communication (e.g. with multiple devices connected to the same line), I would start by modifying the time parameters (Wait First Timeout, Wait Timeout).

Figure 12: The setting of protocol parameters.

The last point is to configure the measured points (in D2000, we call them the I/O tags). All measured points will have the B.UDC1700 station as a parent. The controller documentation contains a table with a list of bit variables:

Figure 13: The table of bit variables of UDC1700 - I will try to communicate the parameter with address 1.

Hence, I will create a measured point named M.UDC1700_Coils1_1 and the value type Di - Logical input.

Figure 14: General properties of the measured point of the logical input type. In addition to the value type, I also filled in the description. The order is important :-)

In the Address tab, I will set the address according to the documentation for the Modbus protocol. The address is in the functionRead.address format, where the functionRead is the Modbus read function. What function does the UDC1700 support? The answer is in the documentation:

Figure 15: Function 1 or 2 can be used to read bit variables. Function 3 or 4 can be used to read 16-bit registers. To write bit variables, use function 5, and to write registers, use function 6.

Thence, our address will be 1.1 - the first number indicates the read function 1, the second the address of the Communication Write Status parameter from Figure 13.

Figure 16: Entering the address of the measured point for the Modbus Client protocol.

Immediately after saving the measured point, it acquired the value BTRUE (which can be seen in the window header). What does it mean? The communication with the UDC1700 started on the first try and that the D2000 KOM does not need any restarts by default, but accepts the online configuration change.

As another point, I will try working with 16-bit registers - not only reading but writing as well.

I will create a measured point of type Ao - Analog output:

Figure 17: The analog point configuration, with which we will read and change the value of the Setpoint Upper Limit parameter of the controller.

I get the parameter address again from the UDC1700 documentation - Setpoint Upper Limit has address 22.

Figure 18: A part of the table with Modbus addresses of controller parameters.

How to specify an address? If we only wanted to read, the address would be 3.22 (read function 3, address 22). If we want to write into it as well, it is necessary to specify the function for writing (according to Figure 16, it is function 6), so the address will be 3-6.22 (according to the convention functionRead-functionWrite.address).

According to our Modbus protocol documentation, it is possible to enter a value type before the function. The default type is a 16-bit integer (so address 3-6.22 means the same as I3-6.22). The question is - is it correct to interpret the value as signed (I) or unsigned (W)? The controller's documentation is silent about this, so we will use the sign interpretation, which is the default for Modbus.

Figure 19: The configuration of the measured point address for reading and writing register no. 22.

Again, after saving, the measured point came to life and its value is 1000. But, there is the value 100 on the controller display. And if I change it to 99.9 on the controller, the value in the D2000 is 999. What's the problem?

The answer is straightforward - in order to be able to set the parameter by one tenth and at the same time transfer its value exactly, it is multiplied by ten during the transfer. However, this information is not in the controller manual - or at least I did not find it.

Therefore, on the D2000 side, it is necessary to set the inverse conversion on the Conversion tab.

Figure 20: After setting the linear conversion with parameters A = 0.1 and B = 0, i.e. Out = 0.1 * In, the measured point has a correct value of 100.

The last point is to test writing. It can be done from HI, but also directly from the Cnf configuration tool. So, from the context menu of the measured point, select the Value watch option and change the value from 100 to 88 in the window:

Figure 21: The setting of the value of the output measured point directly in the CNF.

After clicking on the Set button, the Transient status flag was set for a fraction of a second (the value is being changed) and after successful communication to the controller, the flag was cleared (and the value time was also updated).

If the parameter Setpoint Upper Limit was displayed on the display of the controller, the change can be seen immediately after writing:

Figure 22: The controller display is showing the value 88.0 of the parameter Setpoint Upper Limit (the SPuL text on the green display of the controller).

The following screenshot shows the listings of the D2000 KOM process. The first highlighted value (1000) comes from the reading. Then a value is written (880) and read again (880). The displayed values are therefore "raw" values from the protocol, before applying the linear conversion.

Figure 23: The D2000 KOM process listings. In addition to the debugging information, the hexadecimal listing data sent and received can be seen, e.g. written value <01><06><00><16><03><70><69><1A>

The purpose of this blog was to show what is needed to establish the Modbus communication in the D2000 system. At the same time, I wanted to illustrate that if everything is well connected (and if don’t make mistakes during the configuration), it is a simple and logical process.

Ing. Peter Humaj

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