JSX

Protocol

Proposal for a protocol to handle a joypad with a variable number of sticks and buttons, plug in the MSX’s general purpose port.

This protocol can manage joypads with up to 15 analog axis (one stick requires 2 axis) and 18 buttons.

PSG register 15 (R#15) is use for MSX to device communication and R#14 for device to MSX communication. Data is sent to MSX 6 bits per 6 bits.

Pin binding for received data:

 7  6  5  4  3  2  1  0
–-–-–-–-–-–-–-–--–-–-–-–-
 0  0  P7 P6 P4 P3 P2 P1

No-communication state

Write R#15

Pin 8: LOW

Pin 6: any value

Read R#14

Get 0x0F (JSX fingerprint)

(or any value between 00-3F not already return by other MSX devices)

Start communication and get ID

Write R#15

Pin 8: LOW (in case Pin 8 have been modified by another device)

Pin 6: any value

Write R#15

Pin 8: HIGH (start communication)

Pin 6: HIGH

Read R#14

Get JSX device ID

 7 6 5 4 3 2 1 0
–-–-–-–-–-–-–-–--
 x x A A A A B B
     │ │ │ │ └─┴── Number of button rows (0-3)
     └─┴─┴─┴────── Number of axis (0-15)

Read bytes

Repeat this step for each axis, then for each button row.

Write R#15

Pin 8: HIGH

Pin 6: LOW

Write R#15

Pin 8: HIGH

Pin 6: HIGH

Read R#14

Get 6-bits values

For axis: signed 6-bits value [-32 ; 31]

For boutons: bits field for 6 boutons (0: pressed, 1: released)

Example

For a Joypad with 2 axis (a stick with X and Y axis), and up to 6 buttons, the reading process would be:

• After starting communication, first read to R#14 return the ID.
  ID: 0x09 [00 0010 01] for 2 axis, and 1 buttons row,
• Next reading give X-axis value [-32 ; 31],
• Next reading give Y-axis value [-32 ; 31],
• Next and last reading button states (for up to 6 boutons).

Note: MSX-HID fingerprint would be: 0Fh, 09h, 0Fh