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Plasma Troubleshooting

Common plasma and MASSO DTHC problems and how to fix them. For setup, see Installing MASSO DTHC and Plasma I/O.

1. Arc OK not working

SetupCheck
External Plasma - ARC OK inputThe input changes from Low to High when the arc starts. See External Arc OK input.
DTHC arc detectNo Plasma - ARC OK input is assigned – it overrides DTHC arc detection. Run the Arc Detect Voltage calibration; without it the plasma may start and then stop after 5 seconds.
No Arc OK signal availableAssign a spare input as Plasma - ARC OK and invert it so it always shows High.

2. THC not controlling height correctly

SymptomCheck
THC not working with MASSO DTHCNo THC-Up or THC-Down inputs are assigned – they override DTHC control. Use MASSO DTHC module is ticked in the plasma Multi-Head settings.
Z moves too fast or too slowThe F value in M667 (Z speed under THC), and the THC Feed slider on the Overrides tab (normally 100%).
Z movement unstableReduce the THC feed rate in the G-code, or with the slider while cutting.

3. Probing

SymptomCause / fix
Probing happens twiceThe G-code is wrong. Use the correct MASSO post processor.
Torch height wrong after probingCheck the Torch Touch Offset in the plasma Multi-Head settings.
Probing alarm immediatelyThe input may already be High because of wiring or input settings.
Probing alarm before the torch reaches the materialCheck the Z-axis soft limits.

4. Alarms and messages

MessageMeaning
Clean material surface and press Cycle StartG200 S1 is active, which pauses after piercing so the surface can be cleaned. Use G200 S0 to turn it off.
No signal received from Ohmic touchThe Plasma-Touch input triggered before the Ohmic sensor.
Probe ErrorZ reached its limit before detecting the material.
Torch HitA breakaway was detected. If no breakaway input is assigned, the Plasma-Touch input acts as the breakaway signal.

5. MASSO shows DTHC disconnected

#CheckExpected / action
1Input 9 on MASSOSwitches continuously between High and Low.
2DTHC green LED, plasma offOn solid. If not, check the 24 V supply. If power is present and the LED is off, power-cycle the DTHC. If it continues, the DTHC may be faulty.
3GND to DATA on the DTHC3–4 VDC. If 10–13 V, check wiring continuity and that the power supply grounds are connected together.
4Voltage at MASSO Input 9If voltage is present but the input doesn't switch, test the optocoupler – see Replacing Damaged Optocouplers.

6. Arc voltage reads 0 or incorrect

#Check
1Measure across ARC+ and ARC− on the DTHC. The input is polarity sensitive: ARC+ must be positive, ARC− negative.
2Multiply the reading by the divider ratio set on the DTHC rotary switch. MASSO shows arc voltage only above 30 V. E.g. 1.5 V at 50:1 shows 75 V; at 16:1 the same 1.5 V gives 24 V, which shows as 0 V.
3If nothing shows, test the DTHC with a 1.5 V AA battery or 3 V coin cell on the ARC input, rotary switch at position 0 (50:1). The plasma doesn't need to be on. Nothing connected: green LED on solid. Voltage applied: LED flashes quickly and the voltage shows on screen.
4If the LED doesn't flash or no voltage shows, power-cycle the DTHC and test again. If it still fails, the DTHC may be faulty – contact MASSO Support.

7. Plasma doesn't turn off on M5

CheckDetails
Trigger modeSet the plasma unit to 2T (runs while the trigger is held). 4T (press and release to start, again to stop) doesn't work with CNC control.
Electrical noiseNoise can hold the relay on. Mount the DTHC at the plasma source, not in the control cabinet.
Dry runRun with the plasma off, overriding Arc OK if needed. If the relay works correctly, the problem is likely noise. Restore the settings afterwards.

8. Can't change to T112

No E-Stop warnings on screen.
The machine is homed.
Open MDI from the F2 screen, enter T112 M6 and press Run.

Last Updated On 07 October, 2026

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