1-Wire to Modbus TCP

Scenario

An IOZER basic reads three 1-Wire temperature sensors and writes the measurements to a Beckhoff BC9020 flag memory area over Modbus TCP. The PLC then processes the received values as temperatures in degrees Celsius.

On IOZER, datapoint configuration with forwarding enabled is sufficient. No Blockly or JavaScript program is required for this transfer. The Structured Text code below runs on the BC9020.

Goal

After configuration:

• IOZER reads three temperature values every five seconds,
• each temperature is transferred as a signed 16-bit value in tenths of a degree,
• the BC9020 receives the values at %MW2, %MW4, and %MW6, and
• an additional IOZER datapoint reads a life sign back from the PLC.

Hardware used

• IOZER basic (IOZ-B-10) with a suitable power supply
• Three 1-Wire sensors; two DS18B20s and one DS2438 in this example
• Beckhoff BC9020 with an operational K-bus configuration
• Shared IP network for IOZER, the BC9020, and the engineering computer
• Computer with access to the IOZER web interface and TwinCAT PLC Control

Requirements

• Devices and sensors are installed according to the safety information and electrical connection instructions.
• I/O & Interfaces are active on IOZER.
• The BC9020 is reachable; this setup uses IP address 10.0.1.19 and TCP port 502.
• The flag addresses used below are reserved in the PLC project and are not overwritten by other logic.
• Communication intervals and the BC9020 watchdog settings are coordinated; see step 5.

Implementation

1. Configure the 1-Wire sensors

Open I/O & Interfaces → 1-Wire. Choose Start discovery, add the three sensors, and enable temperature value T for each one.

The example setup uses the following assignments:

Use your own sensors' addresses. For the DS2438, explicitly select temperature value T; the none (raw data) template shown in the screenshot supplies it without an additional sensor template.

Save and apply the configuration. Check the measurements and their update times. Further details are available under 1-Wire.

2. Create the Modbus TCP connection to the PLC

Navigate to I/O & Interfaces → Modbus. Set Modbus TCP → Mode to Master (Client) and add the BC9020 using Add slave.

Slave 1 is the connection to the BC9020. IOZER operates as the client. According to the Beckhoff documentation, section 6.2.1, the BC9020 Modbus unit can be freely selected; Unit ID 1 is a choice for this example and is not displayed in the screenshot.

Save and apply the configuration. Under Modbus status, check that Slave 1 is Connected. A successful read or write then confirms register access.

3. Prepare registers and the TwinCAT program

The Beckhoff flag memory area for Modbus TCP starts at 0x4000, decimal 16384. This setup uses:

Modbus register addresses count 16-bit words. The absolute flag addresses in this BC9020 project count bytes. Therefore, register 16385 at %MW2 is followed by register 16386 at %MW4. Enter the decimal addresses from the table directly in IOZER, without an additional 4xxxx prefix.

In TwinCAT PLC Control, create a Structured Text program named MAIN with these declarations:

PROGRAM MAIN
VAR
    lifesign AT %MB0 : BYTE;
    lifesign_high AT %MB1 : BYTE;

    temp_1_x10 AT %MW2 : INT;
    temp_2_x10 AT %MW4 : INT;
    temp_3_x10 AT %MW6 : INT;

    temp_1 : REAL;
    temp_2 : REAL;
    temp_3 : REAL;
END_VAR

Enter the following in the implementation area:

lifesign_high := 0;
lifesign := lifesign + 1;

temp_1 := INT_TO_REAL(temp_1_x10) / 10.0;
temp_2 := INT_TO_REAL(temp_2_x10) / 10.0;
temp_3 := INT_TO_REAL(temp_3_x10) / 10.0;

The life sign increments each PLC cycle. Because IOZER reads a complete 16-bit register, the additional lifesign_high variable reserves the second byte and keeps it at 0. IOZER writes the three temperature words; the PLC program converts them to REAL temperatures.

Build and download the project to the BC9020, then start the PLC. Save a boot project if required for subsequent operation.

4. Configure temperature datapoints with Modbus targets

Under Datapoints, create three enabled datapoints:

Select classification Temperature and leave Read only off to allow forwarding to the data target.

In the Data source & target tab, first configure the source:

Configure the target of each temperature datapoint:

Float describes the IOZER datapoint in degrees Celsius; Int16 describes its representation at the Modbus target. IOZER uses inverse scaling when writing:

Register value = (Datapoint value − Offset) / Scale
25.5 °C / 0.1 = 255
−5.2 °C / 0.1 = −52

The register value is rounded to an integer. In the PLC, INT_TO_REAL(RegisterValue) / 10.0 converts it back to degrees Celsius with a resolution of 0.1 °C. Use Int16 in IOZER and INT in the PLC so that negative temperatures are handled correctly.

In Automatic mode, a single 16-bit holding register is written using function 06. The F3 label in the datapoint overview identifies the holding register area and is not the write command being executed.

Configure Temp 1 completely, then use Copy datapoint. In each copy, change the label, technical name, sensor source, and Modbus address according to the tables.

Save and apply the configuration. Forward to data target automatically writes the first valid source value and subsequent value changes. Unchanged measurements do not cause another write command. See Datapoint forwarding.

5. Read a life sign back from the PLC

Create an additional datapoint to read a value that changes cyclically in the BC9020:

The PLC counter is declared as a BYTE and wraps after 255. The value read therefore does not have to increase continuously. The life sign checks the return path and PLC program execution; also verify temperature transfer in TwinCAT.

Beckhoff documents a watchdog enabled by default at 1000 ms. The read interval shown leaves no timing margin. Coordinate the interval and watchdog with a margin. Reads can service the watchdog in telegram mode; they are insufficient in write-telegram mode. Temperature forwarding writes only on changes. See Beckhoff Modbus TCP watchdog.

6. Verify the transfer

Check the IOZER datapoint overview. The three temperatures and the life sign should show valid values with status OK.

Open the online view of MAIN in TwinCAT. Check the raw values and the temperatures calculated from them. The supplied screenshot shows:

The screenshots were taken at different times and can therefore show different measurements. Compare the current displays during your test:

1. Change the temperature of one sensor, for example by warming it with your hand.
2. Wait for the next 1-Wire poll and forwarding operation.
3. Check the corresponding IOZER datapoint, the correct PLC memory word, and the REAL temperature.
4. Repeat the test for the other two sensors.

A changing life sign alone does not confirm successful writes to the temperature registers. The PLC online view provides direct feedback. This register layout contains no separate validity or timestamp value for the temperatures; if updates stop, a previously written value can remain in memory.

Troubleshooting

Result and extension

IOZER integrates three 1-Wire temperatures into the PLC project. Each value occupies one holding register and is available in the BC9020 as an INT in tenths of a degree and as a calculated REAL value. Add further sensors using additional datapoints and free, non-overlapping register and flag addresses.