Due to the similarity from the similarity of the waveforms from MSX reading the digitizer using the PAD( ) function and the NES (SNES) controller protocol, the adapter circuit can solely consists of a straight cable.

The connections for the NES and SNES controllers are shown below. A wire connects the pin 1 to ground is also required since it tells the MSX BIOS routines that the digitizer is being touched.

A short patch cable was used to build the adapter.

A small code was used to test the interface

And here's a video showing the code at work

Here's a sample of  code that shows how to use the controller in BASIC programs.

10 a = pad(0) ' dummy
20 p = not pad(1) ' read X coordinate 
30 if (p & 1) then ….  ' Right
40 if (p & 2) then ….  ' Left
50 if (p & 4) then ….  ' Down
60 if (p & 8) then ….  ' Up
70 if (p & 16) then ….  ' Start
80 if (p & 32) then ….  ' Select
90 if (p & 64) then ….  '  B on NES, Y on SNES
100 if (p & 128) then ….  ' A on NES  B on SNES 

Limitations:

Since the MSX digitizer works with 8bit transfers, only the 8 first buttons from SNES controller are read.