View source for JSX

== Protocol ==

Proposal for a protocol to handle a joypad with a variable number of sticks and buttons, plug in the MSX’s [https://www.msx.org/wiki/General_Purpose_port general purpose port].

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

A pulse on Pin 8 is used to start communication then (while Pin 8 still HIGH) consecutive pulse on Pin 6 is used to synchronize data transmission (6 bits per 6 bits).

Pin binding for received data:
<pre>
Bits 7 6 5 4 3 2 1 0
–-–-–-–-–-–-–-–--–-–-
Pins x x 7 6 4 3 2 1
</pre>

== States ==
=== No-communication state ===

Write R#15
: Pin 8: LOW
: Pin 6: ''any value''

Read R#14
: Get 0x0F (JSX idle 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
<pre>
 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)
</pre>
:<u>Note</u>: Number of axis or number of button rows can be zero, but not both. So any device ID is valid except 0x00.

=== Read 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

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 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.<br/>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).

[[File:Joystick.png|512px]]

Note: [https://www.msx.org/wiki/MSX-HID MSX-HID] fingerprint would be: 0Fh, 09h, 0Fh

[[Category:Proposal]][[Category:Proposal/Protocol]]