RS-485 is known for its ruggedness and reliability, but there’s one invisible problem that can silently destroy your devices—Common Mode Voltage (CMV).
🔍 What is Common Mode Voltage?
CMV is the voltage difference between the ground of two connected devices. In an ideal world, all grounds would be at exactly 0V relative to each other. But in reality? They’re not.
If this voltage difference gets too high, it can:
❌ Corrupt data – Devices see distorted signals instead of clean data.
❌ Cause intermittent failures – Works sometimes, fails other times.
❌ Damage transceivers – Exceeding RS-485 chip limits can burn out communication circuits.
What Causes Common Mode Voltage?
⚡Long Cable Runs – Voltage drop builds up across long distances.
⚡Devices powered seperately that dont share the same ground.
⚡Nearby motors, VFDs, or high-voltage lines induce stray currents.
⚡Floating Grounds – If signal ground isn’t connected.
⚡Ground Loops – Connecting multiple grounds incorrectly.
How Common Mode Voltage Destroys RS-485 Devices
RS-485 transceivers only tolerate a limited range of common mode voltage, typically -7V to +12V (-15V to 15V for Senquip devices).
🔴 CMV < 7V – You may see data corruption or occasional failures.
🔴 CMV > 12V – The transceiver goes into latch-up mode or burns out.
🔴 CMV > 50V – You just fried your RS-485 circuit. 💀
Real-world worst case? 270V measured between two “grounded” systems. That’s an RS-485 execution on sight. 😵
How to Protect Your RS-485 Network
✅ Connect Signal Ground – Always run a third wire for a stable reference.
✅ Measure Before You Connect – Use a voltmeter between device grounds.
✅ Use Isolation – Optical isolators break dangerous ground loops.
✅ Surge Protection – Add transient voltage suppression (TVS) diodes.
✅ Proper Shielding – Use twisted-pair, and ground shielding at one end.
Ever had an RS-485 transceiver mysteriously die? Common mode voltage might be the culprit. Drop your experiences below! 👇