How to Measure Voltage on Communication Lines in TFT Electronic Firefighting Monitors

Electronic firefighting equipment relies heavily on clean, consistent signals across their communication lines. If you’re running into comm issues in your system, a solid first step is to check voltage levels on those communication lines.

Here’s a quick, step-by-step guide to measuring voltage on your TFT monitor’s communication lines and narrowing down the root of your system issue.

Tools You'll Need

  • 3mm Allen Wrench
  • Multimeter with DC Voltage Capability (12-24V range)
  • Basic Hand Tools (if needed for access)

Step 1: Power Down the System

Before doing anything else, cut the power to your monitor. Safety first. You’ll be working inside the enclosure, so this step is critical. 

Step 2: Open the Monitor Enclosure

Use your 3mm Allen wrench to remove the four front bolts. We recommend temporarily securing the lid to the left side using the right two bolts to keep things neat.

Watch the ribbon cable! It connects the membrane keypad to the communication board. If it pops out, you can reconnect it.

Step 3: Identify Your Communication Wires

Inside, you’ll see the communication board and three motor boards.

  • The blue and white twisted pair at the top of the comm board is your RS-485 communication line.
  • This line carries a reference voltage of ~3.2V to ground, which is a key indicator when troubleshooting.

Step 4: Initial Isolation Test

Let’s say your monitor is functioning locally (i.e., motors work fine), but there’s a comms issue somewhere else in the system. Start by isolating:

  1. With power off, disconnect the blue and white comm wires from the monitor.
  2. Re-apply power and check the LEDs:
    • Solid green on the power LED = good.
    • Flashing LEDs on motor boards (except nozzle) = also good (no nozzle is connected in this video).
  3. Test the monitor’s functions manually or via wireless controller to confirm it’s working.

If everything checks out, the monitor is NOT the problem—move on to other components in the system.

Step 5: Reconnect and Continue

  • Turn off power again.
  • Reconnect your blue and white communication wires.
  • Power back on.

This time, you’ll be testing the voltage at different points in the system—like your interface box, joystick, or additional monitors—all connected via the same RS-485 line.

Step 6: Voltage Testing with a Multimeter

  1. Set your multimeter to DC voltage.
  2. Identify where the communication lines plug into the system.
  3. Turn off power and unplug the communication connector from the circuit board.
  4. Power back on.

You’re now reading voltage directly from the circuit board to ground.

Reading the Voltage:

  • Place your black lead on a known ground.
  • Place your red lead on each comm line terminal (white and blue).
  • You should get ~3.2V on each line.

If you see that, the board is communicating correctly.

Step 7: Check the Plug and Connected Wires

While the communication plug is disconnected:

  1. Measure voltage from ground to the wires in the connector.
  2. If both blue and white wires show 3.2V—great. That side of the system is clean.
  3. If one wire is 0V or reads low, you’ve likely found your problem.

If You Find a Bad Reading:

Here’s how to interpret it:

  • Board voltage is good, but the connector voltage is bad?
    • The issue lies downstream—check the next device (joystick, interface box, etc.).
  • One wire reads 0V on both board and plug?
    • Fault is likely with that circuit board—replace it or dig deeper into that component.

Keep Isolating Until You Find the Fault

The key to RS-485 troubleshooting is isolation and repetition. Power down, unplug, test voltage to ground, then move to the next device in the chain. This process will lead you to the faulty wire, connector, or board.

 

Final Notes

  • Always cycle power when connecting or disconnecting communication lines.
  • RS-485 is a robust system, but it’s only as strong as its weakest link—loose connectors or damaged cables can cause major headaches.
  • Knowing that 3.2V is your “good” voltage helps you quickly eliminate variables.

Need More Help?

If you’re still stuck or have a unique configuration, our support team is ready to help. Contact us for personalized troubleshooting and service help!