RepRap Part 2 : It moves

It’s been a while since I assembled the Gen7 Electronic board and the extruder. I ordered the extruder together with the steppers from the Netherlands. The hot end v6 is a very nice design compared to the original, where you had to wrap the heating wire around the nozzle yourself. But it’s too big to fit through the hole of my wooden X-Axis sledge from the gffr kit. So, for the moment the extruder is tied to the X-sledge with a clamp.

When I assembled the electronics, I flashed the Teacup firmware onto it but I was never able to connect to it with the reprap host software on the computer. When I connected to it with a serial terminal, I saw a “0” about once a second.

Now after some months I found the time again to make some progress on my RepRap. Also ReplicatorG showed no sign of a successful connection. So I thought to be sure, I would flash the firmware again. But this time I didn’t succeed in doing so, not even with the ICS programmer. Looks like my avr suffered from a corrupted bootloader. That’s a known bug resulting from random operation during power down. Traumflug guided me through the process of flashing the bootloader on IRC. After that, flashing the firmware worked again. Traumflug also told me that neither the reprap host nor ReplicatorG are too well suited for operating with Teacup. He suggested to use printrun instead.

Now with printrun I established a connection. But the machine wouldn’t move. The stepper performed one click, and then stopped. The first thing I suspected was that something was wrong with the endstops. So I started to measure on the Gen7 board. There was no voltage other than the standby power coming from the power supply. I learned that I can force the PSU to normal mode by pulling the green wire of the ATX connector to ground. That’s what the firmware makes the AtMega do, when it sends commands to the steppers or the heater. So, by forcing the PSU to stay on, I could measure again. All the voltages seemed clean and right where they ought to be. Now with the propper voltage, I could also successfully test the endstops. When I enabled the nozzle heating from printrun, it really started heating, and I could read the temperature back. That almost ruled out the possibility of a faulty PSU. But as soon as I tried to move any stepper: one click and done.

So I checked the pin assignment again as there was a change between rev1.0 and 1.2 or the board. I have rev 1.1 and the pins are the same as with the current rev 1.31. Removing the pololus one by one didn’t change a thing, and everything I measured seemed to be right.

The AtMega would support on chip debugging, but the first requirement that I could not meet would be an AtMega JTAG cable. I don’t know if one of my JTAG cables would work. But after my recent experience with JTAG, I figured it was easier to add some more uart debugging to the firmware. With this in place, I could confirm that it resets as soon as it should move a stepper.

Next, I disassembled the board, and checked again visually for shorts. Then I measured the resistance of adjacent pins at the AtMega. And indeed, between pins 27 and 28 which drive the extruder stepper, I measured only 35 ohm. Looking closer with a magnifier glass, I saw something that might be a small solder bridge, but it was so small that it was really hard to see. After scrathing that junction with a knive, the resistance faded away.

Finally, my reprap moves. I can move the axes one by one, and the Y axis homing works. The other two axes have mechanical problems at the moment. X slips as the glue was not good enough. On Z the overhaning pulley jumped off at the first bit of force. So, I improved the attachement of the two pulleys by honing a small surface on the stepper axle, thus improving the torsion. The glue is still drying…

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