Hi All.
Been printing now for about 12 months.
Always had some occasional stops mid print.
Installed SD card reader and printing from SD was reliable until about 2 weeks ago.
As far as I'm aware, nothing changed in my setup.
I dont think its overheating. Have printed much bigger jobs before and the drivers and motors are fine to the touch.
Besides, there's nothing in the pololu to feedback error conditions to the CPU. And not missing any steps or anything. Build quality is good otherwise.
Only change I can think of is we have the print temp higher than we used to with this new PLA we just got.

So, the question is - how do we debug this ? When I woke up this morning, another print half done on the bed.
After some G1 commands, no response. Tried to send M122 (debug I think), but no replies from the printer, then about 10 mins later the G1 command was working.
Obviously, something in the firmware gets into a knot. (using marlin firmware on Sanginololu, with custom PTC table and higher head cutoff temp so we can print ABS)

I've looked around, but can't find much on determining the firmware status when a fault occurs.
BTW - I usually disconnect the USB from the printer when printing from SD, but the last few it hasn't made a diff.

Any ideas / advice would be welcome.

First let's address the SD card. Make sure it's not full and you are saving the entire gcode file.

Now the more common problem.
You said you left it alone overnight. Can you confirm it "stopped" printing? I mean, when you came back to it, was the print head still touching the print? Or was it higher up in the Z axis? like it had finished the print.

You said you used a "new" roll of PLA and a higher temp. PLA is more likely to burn inside the extruder tip and cause a clog. The new roll of PLA may also be contaminated with something that causes a clog. With your higher print temp and the PLA, it will be easy to check if the filament stopped extruding. With the hot end turned on, back the filament out and see if a "groove" was cut into the side of the PLA as if the extruder rubbed it away in one spot.

It's easier to diagnose a mechanical / physical problem than a software problem.

I agree with KingRahl. I had an instance where I thought my print only finished about half and I was completely baffled looking at the gcode, etc. Then I firgured out that it actually had gone through the motions all the way to the end, just the filament stopped extruding due to a clog.

Thanks for the hints both.
So, the print head was still in touch with the job where it stopped.
My default slicer settings used to home the X axis after a job, but on tall jobs it always crashed. So we have changed this to a Z+5 (relative mode) at the end and switch off the heaters.
In the few cases I had recently, the head was still hot and touching the model.

Anyway, some progress has been made. Last night I had a complete print. Same model as before, same SD card, and no issues.
While it was running though, I setup a CRO on the 12V from the power supply, and also onto the PTCs, in case a temp problem was causing a reset / stop.
Plenty of noise on the temp sensors - I'm surprised, but I dont think its a prob.
Did see the 12V drop to 10V for a short spike, so I guess that's the issue. As I said, that job completed, but much lower than 10V would be a problem.
Anyway, now I'll setup a 12V gell cell as a small UPS on the power supply and see what happens.
Have to up the output voltage on the power supply to keep the battery float charged, but it will definitely rule out any PS issues in the future.

Also found some buffer issues in earlier firmwares which caused mid print stops. They seemed to be repeatable though and my issue is not. But I have ordered a 1284 CPU and will load the latest Marlin firmware too. I want the auto leveling feature and LCD enabled which wont fit into my current CPU.

Let you know how it goes.

Well, battery backup is now installed. I can cut the main power for a few seconds without issue.
But the printer still stops mid job. (Was printing from the PC though, not SD).
Printer seemed unresponsive after the stop. Needed a reset to get working again.
Will try again on SD.

Anyone know how to JTAG these Sanguinololu boards ?

In the last few days, I have upgraded the Sanginololu board to as 1284p cpu and loaded all the latest marlin firmware from github.
We'll see if the problems persist.
One thing interesting though - when I pulled out the CPU to change it, the jumper to disconnect the USB reset was not actually removed.
Seems that the cheap jumper on my board dissassembled itself. I pulled out the plastic part and left the metal connector in place, so its possible that USB was still sending some resets. Although I can't explain why this would happen if the USB was disconnected when SD printing.

Anyway - running the big print job again finally after firmware updates. From SD card. No errors yet - touch plastic.
Just hope that the PLS doesn't snap again. Crappy brittle stuff this is !

Are you using twisted wire on your steppers? The wires in each churchy should be twisted together for the entire length. If not, it will create interference which can cause the printer to stop.

Thanks for that hint kevbme.
Obvious when you think about it, but no I havn't.
On another note, the cpu upgrade (and Marlin upgrade) seem to have made a difference, although it may still be too early to tell.
At least one long print job is done without issue.
And I've added the LCD and panel now.
Get another roll of filament today and we can test some more of those large jobs.