PWM analog devices for the MSX

The MSX standard has native support for the PWM protocol, and uses it to read analog devices via the General Purpose ports (AKA joystick ports). This document describes how to build yourself some devices and adapters that use it, and how to easily read such devices.

This is the basic circuit of one PWM channel, and will be used as a building block for all devices for the MSX listed on this page:

The variations on each circuit will be:

The amount of PWM channels used
The value for each component
The pinout to connect to the MSX joystick port

Please note that each 74LS123 chip features two PWM channels. When only one of its channels is used, the other must be disabled to avoid interference.

Hardware

1) MSX-Paddle:

It uses a single PWM channel, and can have up to five digital buttons. It's recommended to assemble them with at least three buttons: A, B and START. The case of an old NES, SNES or SMS controller can be used for that, as shown in the pictures below. You just have to install the potenciometer in the place of the directional buttons.

a) Components

C1=22nF
VR1=220K
R2=680R

b) DE9 female connector pinout

     1: (optional) Digital up or shoulder R1
     2: (optional) Digital down or shoulder L1
     3: aX (Q-pin of the 74LS123)
     4: START button
     5: Vcc
     6: A button
     7: B button
     8: B-pin of the 74LS123
     9: GND

All digital buttons must be connected to the pin-8 of the DE9 connector, just like a normal MSX 2-button joystick.
Be sure to connect the inputs of the second channel as described below, to disable it and prefent it from producing spurious oscillations:

A2: GND
B2: Vcc
CLR2: Vcc
C/Rext2: 10K resistor to Vcc
Cext2: 10K resistor to GND

2) IBM-PC DA15 joystick adapter (AKA joyDA15 adapter)

This adapter uses two monostable channels.

a) Components for each channel

C1=47nF
VR1=100K (inside joystick)
R2=330R

b) DE9 female connector pinout and connections

      1: DA15 pin10
      2: 74LS123 pin5
      3: 74LS123 pin13
      4: DA15 pin14
      5: DA15 pins 1,8,9 and 74LS123 pin16
      6: DA15 pin2
      7: DA15 pin7
      8: 74LS123 pins 2 and 10
      9: DA15 pins 4,5, and 12

resistor R2_Y: pin1=74LS123 pin7, pin2=DA15 pin6
resistor R2_X: pin1=74LS123 pin15, pin2=DA15 pin3

3) Atari-2600 dual-paddle adapter

a) Components

C1=4.7nF
VR1=1M (inside the paddle)
R2=3K3

b) DE9 female connector pinout

     1: N/C
     2: N/C
     3: 74LS123 pin13
     4: 74LS123 pin5
     5: 74LS123 pin16 and Paddle DE9_pin7
     6: Paddle DE9_pin3
     7: Paddle DE9_pin4
     8: 74LS123 pins 2 and 10, and Paddle DE9_pin8
     9: GND

resistor R2_A: pin1=74LS123 pin15, pin2=Paddle DE9_pin9
resistor R2_B: pin1=74LS123 pin7, pin2=Paddle DE9_pin5

Software Programming

Analog channels are addressed by their ID, where ID=CH*2+JP.
CH=Channel index (0 to 5), and JP=Joystick port (1 or 2).
The pinouts and channels were arranged to ensure cross compatibility between the PWM devices, and easier integration with non-PWM devices on joylib.

1) Channel mapping

The channels will be mapped as follows. See the Assembly and MSX-BASIC sections on how to read each channel.

a) MSX-Paddle

   Channel
     0            : R1 shoulder button
     1            : L1 shoulder button
     2            : aX (analog X)
     3            : START button
     4            : A button
     5            : B button

b) joyDA15 adapter

   Channel
     0            : D button
     1            : aY (analog Y)
     2            : aX (analog X)
     3            : C button
     4            : A button
     5            : B button

c) Atari-2600 dual-Paddle

   Channel
     2            : Paddle-1 analog X
     3            : Paddle-2 analog X
     4            : Paddle-1 button
     5            : Paddle-2 button

d) Generic template for a dual-axis joystick

   Channel
     0            : aZ (analog Z)
     1            : aY (analog Y)
     2            : aX (analog X)
     3            : aC (analog C)
     4            : A button
     5            : B button

     The digital buttons will have the following output: 00=Pressed, FFh=released.

e) Yamaha MMP-01

Check this article for the channel assignment on this device.

2) MSX-BASIC programming

The Buttons A and B can be read normally with STRIG(N), or also via their respective analog channels as described below.
All channels can be read using the PDL(ID) function. Analog channels will return a value between 0 and 255, while digital buttons will return either 0 (pressed) or 255 (released).
The MSX-BASIC is too limited to be able to reliably identify the connected device. When needed, it's better for BASIC programs to have an option to allow the user to manually select the desired device.

3) Assembly programming

The analog channels can be read with the GTPDL BIOS function and the same IDs used in MSX-BASIC.
It's better to obtain the Digital buttons status by reading the PSG R#14 with RDPSG. The bits will be mapped like this:

Paddle : ?,?,B,A,START,?,TL,TR
joyDA15 : ?,?,B,A,C,?,?,D

The bits marked with "?" may contain random values and must be masked out from readings.

Be careful to handle devices that are different than the standard MSX 2-button joystick, to avoid the infamous bug that some games have, where when a different device is left connected to the joystick port, the game has undesired collateral effects like interpreting a stuck direction.

The connected devices can be identifyed in assembler via their signatures. The best way is to use HIDlib, as it handles both the detection and the disconnection of all MSX-HID devices.

5) Notes

These are hobbyist projects. Build and use them at your own risk.