Logic-level bed FET connected to RADDS pin - default to "on" when firmware not booted?

I am just starting to get the RADDS that i bought almost a year ago hooked up to my Tronxy X5S, which is a relatively large corexy printer, thus it comes with a fairly high wattage heatbed.

They've shipped a bunch of different versions of functionally defective heatbeds over the few years they have sold this printer, but i got lucky and bought in a window when they were shipping one that can be properly configured for 12v or 24v, and they only wired it wrong at the factory.

I now have an 800w 24v supply cobbled out of a pair of old compaq proliant hot swap supplies, so the bed doesn't draw quite as many amps reconfigured for 24v as it did in 12v configuration, but I still think it may push the limits of the onboard bed fet on the RADDS.

I have one of the smaller bed mosfet modules that has "board" and "signal" control inputs. On a regular RAMPS this works great connected to any random pin for bang-bang or a hardware pwm pin for pid.

It works connected to a servo pin or aux pin on the RADDS too, but my i noticed that during bootup and during flashing procedures, the pin seems to default to ON.

So i have some concerns about the bed getting indiscriminately heated, potentially in an error situation as well.

Is there a simple way around this? From what I have been able to glean, this may simply be due to how the Arduino DUE is, so i am guessing that it perhaps doesn't matter which pin i connect to, it's just going to be that way.

I am handy with a soldering iron and have a large stock of misc electronic parts if there is an electronic way around this.

Thanks much for any insight.

Edit: fwiw i am using Marlin 2.0-bugfixes and i have a background in embedded systems and software quality assurance so i am not completely lost in source code, but i'm not really a coder either.

Edited 1 time(s). Last edit at 03/12/2019 06:36PM by Timpanogos Slim.

Hello

Which external MOSFET module do you have exactly I wonder?

Their are a few different types ..

MOSFET modules

Most all MCU's have their pins in high impedance mode when in reset/boot-up mode, so if the pin has an external pull-up then that would turn a FET on during that period (which you do not want). But it all depends on how you have it wired and on the external MOSFET schematic etc.

Quote
Pippy
Hello

Which external MOSFET module do you have exactly I wonder?

Their are a few different types ..

MOSFET modules

Most all MCU's have their pins in high impedance mode when in reset/boot-up mode, so if the pin has an external pull-up then that would turn a FET on during that period (which you do not want). But it all depends on how you have it wired and on the external MOSFET schematic etc.

I have one of these

[smile.amazon.com]

One of the jst inputs is marked "sig - gnd" or similar. I have those connected to pin 35 and ground on aux1 at the moment, with bang-bang heating. Behaved the same on the servo 3 connector.

The outputs of the Due default to high when no firmware is loaded, so unless either a pulldown resistor or an inverting buffer is used to drive the MOSFET, the heaters will default to on. The Duet controllers use inverting buffers to avoid this happening.

Edited 1 time(s). Last edit at 03/13/2019 03:21AM by dc42.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].