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      How to Run a Multimeter Test for Modbus Connection Checks

      Overview

      If you are receiving errors in your Installer app that your Modbus system isn’t communicating as expected, you can confirm whether the installation is correct by performing a multimeter resistance check.

      This simple test helps identify wiring issues, missing resistors, or other connection faults.

       

      Step-by-Step Instructions

      1. Power Off the System
        Ensure the entire Modbus system is powered down before testing. You should ensure the following elements are powered off

        1. The heat pump

        2. The Homely Hub

        3. Any additional Modbus adaptors e.g. DCOM board

          This prevents inaccurate readings and protects your equipment.

      2. Verify Connections
        Confirm that all Modbus devices are connected correctly and that the communication wiring is complete.

      3. Set the Multimeter
        Switch your multimeter to its lowest resistance (ohms) measuring range.

      4. Measure Across the Bus at both ends of the Connection
        Place the multimeter probes on either side of the 120-ohm terminating resistor on the Modbus network.

         

        Understanding the Results

        After taking your measurement, compare your multimeter reading against the expected range: 

        Resistance Reading (Ohms)

         

        Meaning Recommended Action

         

        40–70 Ω

        ✅ Correct installation No action needed

        0–20 Ω

        ⚠️ Wiring short detected Inspect cabling and connectors for shorts

        20–40 Ω

        ⚠️ Possible power interference Turn off power to electricity meters and retest

        70–130 Ω

        ⚠️ Missing or incorrect termination Check that the 120-ohm resistor is correctly installed

        130+ Ω

        ⚠️ Broken cable or missing resistors Check all Modbus cables and terminators
         
         
        Advanced Check: Isolating the Fault

        If the system still shows communication errors, test with the Modbus cable disconnected between the heat pump and the Homely Hub:

        • If both ends read around 120 Ω, each resistor is present and the cable may be broken.

        • If the heat pump side now reads greater than 120 Ω (or OL for open loop), the resistor on the heat pump is likely faulty.

        This test helps identify whether the issue lies with the cable or the termination resistor.

         

        When to Contact Support

        If your reading remains outside the expected range after performing the checks above, please contact Homely Support for further assistance.