The Ninja Tap is an adapter that allows to use up to 4 joysticks per MSX port (MSX Wiki).
There is not much information available on such adapter, let alone the schematics.
I wanted to build a clone of such adapter then I have started to disassemble a driver code and translations of japanese documentation available to figure out the connections.

Protocol

The adapter uses four CD4021s to shift in the controller data from the four controllers:

disassembling the GETNIN function :
The sequence for reading the 4021s is the following
0) the sampling start with pin 8 (CLOCK = HIGH) and pin 6 (PARALLEL/!SERIAL = LOW)

1. pulse (go high, then low) the PARALLEL/!SERIAL line (pin 6). That will latch the input of the keys to the 4021 internal flip flops. Notice that the CD4021 output is already loaded at the Q8 output.
2. Pulse (go high, then go low) the CLOCK line (pin 8). (note: If this pin weren't initialized high, the first bit would be missed)
3. shift in one data bit  from TAP 1 at the UP line (pin1) into register H
4. shift in one data bit  from TAP 2 at the DOWN line (pin2) into register E
5. shift in one data bit  from TAP 3 at the LEFT line (pin3) into register D
6. shift in one data bit  from TAP 4 at the RIGHT line (pin4) into register C
7. Repeat steps 2 through 6 for until all 8 bits are read
8. Process the data.

    The sequence of bits is the same used by NES controllers, being possible to make an adapter that uses 4 NES controllers with only one inverter (which makes me wonder to reuse a JoyMega adapter to create a Ninja Mega Adapter)

Oddity

The adapter was designed to be detectable by the driver, and uses the inverted state of signal at pin 8 which is applied to pin 7. The driver flips the state of pin 8 and check if inversion occurs on pin 8.

The problem is that pin 7 is an open collector and the detection routine pulls this pin down (instead of let it float  )  during the detection, which means that the external inverter will have to fight (and win) against the internal 7407 open collector gate, otherwise the detection will fail.

Here's the code

;======================================
; Connection Check Sub.
; return with B=B+1 if no hardware detected
; otherwise return with B=B+4 if hardwar detected
CHECK:    CALL    PORSEL   ; 
    INC    B          ; B = Max player number + 1
    AND    0C0H       ; A is state of port 15. 
    OUT    (0A1H),A    ; 678=L  for both joystick ports (!)
    EX    AF,AF'      ; save state of Reg15 with all output pins low
    
    LD    A,14        ; select register 14 
    OUT    (0A0H),A
    IN    A,(0A2H)    ; read state of joystick port
    AND    20H            ; 7=H ?   return if bit is high (!) (shouldn't be forced high, its being driven low internally )
    RET    Z           ; if return here, A = state of pin 6, and B=B+1 from the function call
    
    LD    A,15        ; select register 15
    OUT    (0A0H),A
    
    EX    AF,AF'      ; restore state of Reg15 with all output pins low
    OR    030H        ; make bits 4 and 5 high (pin 8 of both pulse pins)
    OUT    (0A1H),A    ; 8=H  for both output ports
    
    LD    A,14        ; select register 14
    OUT    (0A0H),A
    IN    A,(0A2H)    ; read inputs  
    
    AND    20H            ; 7=L ? is pin 7 low
    JR    NZ,CHECK1   ; no, return with A=0

    INC    A           ; Complement bit 0 of register A (read from pin 6) 
    INC    B           ; B = B+3
    INC    B
    INC    B
    RET
; No NTAP
CHECK1:    XOR    A
    RET