Some initial info on the LoveBoard:
I spent a few hours staring at it under the microscope and prodding it with a multi-meter. So far I’ve found where all but 3 pins of the big connector go:
| Connection | Row 2 Pin# | Row 1 Pin# | Connection |
|---|---|---|---|
| EMOTOR (Red / A1) | 2 | 1 | EMOTOR (Green / B1) |
| EMOTOR (Blue / A2) | 4 | 3 | EMOTOR (Black / B2) |
| ST490AB - Y | 6 | 5 | MULTIWIRE |
| ST490AB - Z | 8 | 7 | GROUND |
| ST490AB - B | 10 | 9 | H-FAN / P-FAN Shared Ground |
| ST490AB - A | 12 | 11 | HT |
| 5V Supply + (Loadcell + HX717 supply + regulator) | 14 | 13 | NT |
| HEATER 24V - | 16 | 15 | Fans Tachometer |
| HEATER 24V - | 18 | 17 | 3.3V in for filament sensor |
| HEATER 24V + | 20 | 19 | P-FAN +5V |
| HEATER 24V + | 22 | 21 | H-FAN +5V |
The HX717 is connected to what I believe is a ST490AB chip which converts it to RS-422. (The chip says “S490AB MZ306”) The PD_SCK and DOUT pins are hooked up this, effectively ground isolating the chip from the main board. Pictures of the cable show white/blue twisted pairs in this location which is the expected wiring. I’m pretty sure 
I can still talk to it via 2 regular I/O pin and an associated ground wire.
Fans are reversed from how the Chinese boards normally do PWM. Here the + pins are pulsed but on boards like the Octopus its usually the ground pins that pulse. The ground pin is connected to the relay/transistor network on the board so just swapping the wiring is likely impossible. Also using 12V fans is probably also out. (If I’m wrong about this Id be happy to hear it! Also if you have any ideas for a work around) Luckily I don’t think the board relies on the fan power for anything but the associated indicator LEDs so it could all be bypassed.
The unidentified pins (5,15 &17) are probably the Filament Sensor, and 2 signal lines for fan failure. The fan RPM lines go into the relay/transistor network and I think they are reducing the RPM signal to a binary on/off signal so they don’t have to read it with an ADC back on the main board. I cant trace the filament sensor output line at all. I should be able to work out what these do for certain once I get power to the board. But if this is what they are, they are not essential pins.
Update: Prusa published the schematics: https://www.prusa3d.com/downloads/Electronics_drawings/MK4_electronics_schematics.zip
The Filament sensor is connected to the B channel on the HX717, so they must be switching from A to B periodically while extruding to check for filament. I don’t plan to implement support for this as it would be VERY complicated in klipper to have a sensor that doesn’t report data on a regular interval.
The unknown pins on the connecter were the 3.3V supply for the filament sensor, the Fans tachometer and multiwire port.
Also it looks like fan power is direct wired and isolated, so if you swap the pins and use the 2 positive pins as negative pins and it should work, but I haven’t tried it.