2nd Serial on Velleman K8400 board for MMU2

Hello All,

To take my printer to the next level I want to install a MMU2.

I think I got the most of it cleared out for my self. But I have big challenge: The MMU board needs a serial connection from the Printer to handle the commands. I checked which one is available but all the pins suitable are being used... (see attached schema of the main board) According to my information these pins are available for serial communication:

RX0: 00 PE0 Used for host communication (USB )
TX0: 01 PE1 Used for host communication (USB )

RX1: 19 PD2 unused (and no place on the board to connect this one)
TX1: 18 PD3 used for Zstop

RX2: 17 PH0 used for rotary encoder (I dont have schema of the LCD print but I traced the leads and checked the firmware's actual number)
TX2: 16 PH1 used for roraty encoder

RX3: 14  PJ1 used for Ystop
TX3: 15  PJ0 unused (and no place on the board to connect this one)

at this moment I think I have 2 options
- investigate SoftwareSerial (but how?? I don't think it is a good idea to use input interrupts in marlin)
- Replace the mainboard (which I don't like because I also need to replace the LCD etc)

But is there another option? Does anyone have an idea?

On the schematic lower right corner you'll have the connection to the LCD/encoder/SDcard
The connections on the schematic PK4-PG1 are easy accessible, I already have pin header on it for the bed-leveling.

Thanks in advanced, Martijn

Edited 1 time(s). Last edit at 11/29/2019 05:16AM by MartijnD.

From this high res image I found on the internet. It looks like the Tracks for RX2 and TX2 are nice and obvious on the top of the board.



Perhaps cut those two tracks and reconnect them to other general IO pins and update your firmware pins to suit.
Then you can use this serial port once you wire the pins to something useful.

Edited 1 time(s). Last edit at 11/29/2019 05:54AM by Dust.

Thank you for thinking with me. It is a possibility to cut these pcb traces, sadly these PCB traces are already used by the rotary encoder... so if I do this than I have to rewire them as wel. I think it will be a mess as result at the end, wich is not an idea I like.

The most ideal idea I have at this moment is this baord: [github.com] it is the SKR V1.3 it has 2 available RX/TX pairs pins without needing to solder. This upgrade gives me also a nice opportunity to upgrade from DRV8825 to TMC2130.

Does any one had succes with a software serial in marlin yet?

Edited 1 time(s). Last edit at 12/01/2019 10:12AM by MartijnD.