View source for JSX-B

== Protocol ==

Proposal for a protocol to handle joypads 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'''.

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).

Pin 6 and 7 are used as 2-bit transfer mode identifier:
* <tt>00</tt>: ''Reserved''
* <tt>01</tt>: ''Reserved'' 
* <tt>10</tt>: Get Device ID
* <tt>11</tt>: Serial read of joypad input data (and increment internal counter)

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 6: ''any value''
: Pin 7: ''any value''
: Pin 8: LOW (for >1 ms)

Read R#14
: Get 0x11 (JSX idle fingerprint)
: <u>Note</u>: It could be any value between 00-3F not already return by [https://www.msx.org/wiki/MSX-HID#List_of_known_fingerprints other MSX devices])

=== Device ID ===

Write R#15
: Pin 6: ''any value''
: Pin 7: ''any value''
: Pin 8: LOW (for >1 ms)

Write R#15
: Pin 6: LOW
: Pin 7: LOW
: Pin 8: HIGH (send device ID)

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 up to 0x3F 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 6: ''any value''
: Pin 7: ''any value''
: Pin 8: LOW
::Pin 8 have to stay LOW for more than 1 ms for the first read, then less than 1 ms for the next ones

Write R#15
: Pin 6: HIGH 
: Pin 7: LOW
: Pin 8: 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]]