PMDX-126 troubleshooting

[SRcnc]

Hello all. I am pulling my hair out troubleshooting my x axis homing issue. First off I bought this machine second hand. It's a HF converted 3 axis mill running on LINUX CNC. I had this issue when I first brought it home but after some fiddling it resolved its self.

The issue is during homing sequence it homes Z and Y perfectly then when it try's to home X the DRO will just keep searching but no movement at table.

Troubleshooting I have done is I replaced all wiring going to X axis. I know the new wiring is correct because if i swap X and Y plugs coming from my controller box the X axis moves and homes like Y axis would and I get no movement from Y axis.

I used a multimeter to test continuity of my parallel port cable and I did not observe any issues even when moving the cable around. All pins have less than 1 OHM even when I move the cable around.

If I check ignore limits I get a no limit switch active message.

I don't understand how this can be since I just milled a part last night. I start it up this morning and boom the issue is back.

Next I swapped my Gecko 251 drives with each other thinking it was a bad drive. The problem did not follow. So that rules out the drives for me.

I did notice on my PMDX-126 there is an led on the J4 connector Pin 3 that outputs to the X drive Gecko. J4 Pin 3 is fed by parallel port cable pin 3 I believe. I watched the LED's while the machine was homing. My homing sequence is Z, Y, X. I watched some LED's on the connector for Z axis turn on and off during homing. I did the same for Y axis. Then when X tries to home it does nothing but the LED stays lit.

 I retested my parallel port cable and it still seems to be fine. all pins have less than and OHM


Any ideas?

[SRcnc]

Okay I disconnected wires to X axis drive and the light is still on after homing. I tested voltage at the STEP and DIRECTION wires out of the PMDX into the drives. Y and Z step and direction wires have 12mv. Step and direction for X have 0V and 12V. I don't know if this is because its not hooked up to the drive or what. I'll test the voltage tomorrow when it's hooked up to the drive. 

[12strings]

There should never be 12V on ANY output of the PMDX-126.   They should go from close to 0V to close to 5V.  How do you have the PMDX-126 J4 wired to the G251?  And can we presume it is exactly the same as J3 and J2 to other G251 boards?

Measuring voltages on the STEP pin can be tricky.  The step pulses are very narrow, and a DC volt meter will end up averaging the waveform with the resulting apparent DC voltage depending on the step frequency and step pulse width.  For homing speeds I would expect it to read close to zero volts.  The direction signal should be easier to measure as it is relatively static, i.e. zero volts for one direction and 5V for the other (or close to those values).  Likewise the LEDs.  The "step" LED may flicker dimly during jog moves, and may or may not have visible flicker during homing.  The "dir" LED should be definitely on or off depending on the direction of motion.

When homing doesn't move the X axis, can you jog the X axis? Or make it move for any type of motion other than homing?

Is this Mach3 or Mach4?

[SRcnc]

Okay guys I think I found my issue. I believe my X direction Gecko driver is burnt up.

I compared voltages on all 12 pins of X, Y and Z drivers. I found an anomaly on pins 5, 6, 7, 8 and 11.

On Y and Z voltages are as follows: Pin 5 - 13v
                                                    Pin 6 - 16v
                                                    Pin 7 - 14.37v
                                                    Pin 8 - 14.5v
                                                    Pin 11 - 6.97v

On X drive voltages are as follows:  Pin 5 - 33.98v
                                                    Pin 6 - 34.0v
                                                    Pin 7 - 33.48v
                                                    Pin 8 - 33.46v
                                                    Pin 11 - 30.92v

I then noticed a burned looking chip on the X Gecko drive. You can see it in the picture.

I'm assuming the Geckos resistance circuit isn't working. I tested all three resistors and they all read around 1.470k Ohms. So I don't think the resistor is the issue.
I guess I need to order a new drive or see if it can be repaired.

Any guesses to why the chip would of burnt in the first place? I don't want it to happen again.

[SRcnc]

There should never be 12V on ANY output of the PMDX-126.   They should go from close to 0V to close to 5V.  How do you have the PMDX-126 J4 wired to the G251?  And can we presume it is exactly the same as J3 and J2 to other G251 boards?

Measuring voltages on the STEP pin can be tricky.  The step pulses are very narrow, and a DC volt meter will end up averaging the waveform with the resulting apparent DC voltage depending on the step frequency and step pulse width.  For homing speeds I would expect it to read close to zero volts.  The direction signal should be easier to measure as it is relatively static, i.e. zero volts for one direction and 5V for the other (or close to those values).  Likewise the LEDs.  The "step" LED may flicker dimly during jog moves, and may or may not have visible flicker during homing.  The "dir" LED should be definitely on or off depending on the direction of motion.

When homing doesn't move the X axis, can you jog the X axis? Or make it move for any type of motion other than homing?

Is this Mach3 or Mach4?

Thanks for your reply. I did some more troubleshooting and I got the following.

On J3 which is my Y axis I got PIN 1 - 5.03v
                                            PIN 2 - 0.802v traveling toward home switch and 5.02 when going to home position.
                                            PIN 3 - 5v constant.

On J4 which is my X axis I got PIN 1 - 5.03v
                                            PIN 2 - 0.730v constant since x axis has no movement I did not expect a change of state.
                                            PIN 3 - 5v constant.

So at this point I concluded that it was most likely not the PMDX-126 and started looking at the drives when I found my problem.