JSX-C

Protocol

Proposal for a protocol to handle joypads 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.

Pin 8 is used to synchronize data transmission:

• Leaving Pin 8 LOW for long enough (>1 ms, about 3580 t-states) resets the joystick's internal counter,
• then successive pulses on Pin 8 (LOW > HIGH) are used for data transmission (6 bits per 6 bits).

Internal counter is used to send successive data: first the device ID, then each of the input data.

States

Idle state (no-communication)

Write R#15

Pin 8: LOW (for >1 ms)

Reset internal data counter

Read R#14

Get 0x0F (JSX idle fingerprint)

Note: It could be any value between 00-3F not already return by other MSX devices)

Communication state

Write R#15

Pin 8: LOW

Write R#15

Pin 8: HIGH (request data)

Read R#14

6-bit data

Pin binding for received data:

Bits 7 6 5 4 3 2 1 0
–-–-–-–-–-–-–-–--–-–-
Pins x x 7 6 4 3 2 1

Device ID

First received data is the JSX device ID:

7 6 5 4 3 2 1 0
–-–-–-–-–-–-–-–--
 0 0 A A A A B B
     │ │ │ │ └─┴── Number of button rows (0-3)
     │ │ │ │         0: No button
     │ │ │ │         1: Up to 6 buttons
     │ │ │ │         2: Up to 12 buttons
     │ │ │ │         3: Up to 18 buttons
     └─┴─┴─┴────── Number of axis (0-15)

Input number can be computed as:

• Axis: ID / 4
• Button rows: ID & 0x3
• Buttons: (ID & 0x3) * 6

Note: Number of axis or number of button rows can be zero, but not both. So any device ID up to 0x3F is valid except 0x00.

Data bytes

Repeat this step for each axis (if any), then for each button row (if any). Number of axis and button rows is given by the device ID

Get 6-bit 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 a stick with X and Y axis (2 axis), and 4 buttons (1 row), 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 give button states (for up to 6 boutons).

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