Yet another heat bed issue (1 reply)

My problem : Marlin firmware doesn't power the heatbed.

I'm using an Arduino mega with ramps 1.4 with DiscountReprap 4x20 LCD panel and I'm trying to setup Marlin.

I've tried running a simple sketch on the Arduino (Blink, with pin 8, 9 and 10) and the Arduino is able to turn on the heatbed (pin 8), even with the RAMPS shield installed : the heat bed turn on (and off in loop).

With Marlin firmware v1.0.2-2, I don't see bed temperature on the LCD, and Repetier host and Pronterface both seem to reports 0 degrees.
But on the RAMPS, if I switch the bed and extruder thermistor connector, I get the proper bed temperature reading, so both thermistor are fonctionnal.

Running the M105 gives this :

ok T:30.5 /0.0 B:0.0 /0.0 T0:30.5 /0.0 @:0 B@:0

(30.5 is the extruder temp after a little heating)
M105 with bed thermistor and extruder thermistor inverted:

ok T:23.9 /0.0 B:0.0 /0.0 T0:23.9 /0.0 @:0 B@:0

When I manually turn on the bed from the LCD panel or from Repetier Host, the D8 pin stays at ~0V.

I'm out of ideas... I just bought the Arduino + RAMPS kit cause I had the same problem on a Melzi, but now I'm pretty sure it is software related...