Adapting a printer (expansion board heater control)

[Shoki]

Hey

I'm trying to adapt a semi-industrial printer to a duet board.

There are two electronic cards on the printer for now :

• One is a regular marlin printer
• The other is handling all the power (bed, chamber, 220V input, emergency stop, etc).

I would like to replace the first one with a Duet and keep the second one so I don't have to redo all the mains wiring, order some relays etc...

There is only a 5 pin cables between the two cards. And it's the only part i'm unsure about the transition to Duet.

The pins are labeled like this :
B -> Bed -> which goes from 0 to 3.3V when bed is turned on
C -> PTC -> which goes from 0 to 3.3V when chamber is turned on
G -> GND which for me is just the ground (and i used it to measure the other)
V -> 5V which is just 5V
P -> ACCK I have no idea what this one is/does and it's just always at 3.3V

Here are my questions :

1 - Any idea what ACCK means ?
2 - The original board seems to be switching the 3.3V and not the ground since there is only one ground for both the bed and the chamber conrol. From my understanding of Duet boards, it's switching the GND and not the 3.3V (Vin).
Any idea how I can wire this secondary board on a duet to make the two heaters (bed and chamber) work ?

Thanks in advance for the help.

[Phaedrux]

The shared ground will be a problem as you've identified correctly that the Duet switches on the negative side.

For the bed and chamber though, I assume there is a relay being switched?

[dc42]

@Shoki as your heater switching board needs only 3.3V signal on the B and C pins you can use two io_out pins from a Duet 3 board to control them. These switch in the normal sense.

[Shoki]

@dc42 Thank you very much this will work !

[Shoki]

@dc42
Your solution worked.
However there is a behavior I don't really like.
When the printer is off both bed and chamber are off.

When the printer is on and duet is running, both chamber and bed are controled by the duet and can be on and off depending on temperatures and control values.

The problem is while turning on the machine (while config.g loading) or when I restart the machine using the emergency stop button, the machine is turning on both the bed and the chamber heater.
For me this is a problem as if the duet where to fail (bad SD or something else) while powered, both heaters would turn on uncontrolled...

Any way for the default state to be low ?

[dc42]

@Shoki try adding 10K pull down resistors between each of the IO_Out pins (the ones you use to control the heaters) and ground.