I would think this is possible, but I would like to see if I could connect the raspberry pie directly to the Ender3 main board and skip the USB. It’s just one less thing that sticks out of both components.
Were you thinking of soldering wires from the existing USB port or something?
If I understand correctly, hi wants to make a direct TTY connection
Yes, I would like to connect the two boards directly. Preferably using DuPont connectors...
Gotcha. Let's see, I suppose some caveats are in order...
When you do serial via the GPIO pins you need to use/consume one of the two UARTs on the Raspi. In order to get the good one, you need to play some games like disabling both Bluetooth and console terminals.
On the Raspberry:
Pin 6: Gnd
Pin 8: BCM 14 Tx
Pin 10: BCM 15 Rx
So you're either looking for a 1x3 header connector that attaches to that collection of pins I've described.
If it's Tx on the Raspi side, then connect that same wire to Rx on the Ender3 board. Likewise, swap the Rx/Tx wire. Gnd goes to Gnd. Try NOT connecting the 5V line and see if it works.
Here's a page which describes adding inline resistors. You should probably review that.
The reason here is that the logic level on the Arduino is 5V. The Raspi uses 3.3V. So see how in that diagram there's two resistors and it's only in the Arduino->Raspi direction? This is to drop the voltage down to something the Raspi can handle. It's suggested that the Raspi can be damaged if you put the full 5V on that Rx pin (10).
It's important NOT to include a resistor for the Raspi->Arduino communication side of things. The Raspi's pushing 3.3V and the Arduino will think "ON" if the voltage is anything above about 2.5V so 3.3V is good enough. If you did add a resistor it might push the 3.3V down into that gray area where it could be below that threshold, so don't.
Of course, most people buy a logic level converter from someplace like Adafruit and use that for the Arduino->Raspi side of things.