The Arcade Joystick button 2 with resistor and zener diode

Rookie (20)

08-02-2022, 00:19

I own several The Arcade joysticks and know that some of The Arcade Turbo joysticks aren't MSX compatible. I got 4, from 4 different sources and not sure at all which one is MSX compatible.

I read from https://www.msx.org/forum/msx-talk/hardware/joystick-port that basically all triggers are connected to pin 8 (OUT on MSX, GND on most other systems). So far that is the case when measuring but with the second fire button there is something strange. My multi-meter is showing no continuity but resistance. I opened my joystick and tried to figure out the schema.

In the schema there is:

1. A resistor (~250 Ohm) between the 2nd button and pin 7
2. A I think zener diode between pin 7 and 9.

Meaning, when button 2 is pressed pin 8 is connected to pin 7 with a 250 Ohm resistor, and pin 7 is connected to pin 9 via the 250 Ohm resistor and the zener diode.

Anyone that an idea why the resistor and zener diode are used? If this somekind of protection for when the joystick is plugged into a non-MSX machine or is this specifically for the MSX?

The circuit board states this is revision 22-2631

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By ramonsmits

Rookie (20)

08-02-2022, 00:41

I also opened another Turbo. That seems to be revision 22-2630 and that design does not have any traces to pin 9.

However, it has a jumper between button 2 and pin 7. The jumper in my unit was not set. Meaning, it simply will not work.

Unfortunately, you cannot see a difference between these revisions from the outside. However, the 22-2630 has a microswitch for the first fire button where as my 22-2631 has a leaf switch.

By ramonsmits

Rookie (20)

08-02-2022, 01:09

Opened another one, according to the circuit board revision 22-2631, with microswitch firebutton, no jumper, but button 2 wired to pin 7 with a 275 Ohm resistor. I opened the remaining 4th Turbo and that is identical.

However, on further inspection button 2 isn't wired to pin 7 on these but to pin 9 (!).

Also, on these 2 units the color coding of the DB9 connector is different. Black is pin 8, yellow is pin 9 which on the first two units was exactly the opposite. I thought the wires were rewired but they still match the same pins on the connector of the circuit board.

By Pokun

Expert (72)

08-02-2022, 22:29

Setting the second button on pin 9 is the Sega and the NEC PC-6001 method of extending the Atari joystick to 2-buttons, so that joystick is probably for a Sega system or other computer using the same standard.

I have the Sega SJ-300M which has a slide-switch for switching between both standards (the red-button "SJ-300" is missing this slide-switch). However since Sega defined button 1 to always be to the left of button 2, the joystick is a bit awkward in some 2-button games as MSX joysticks typically has a button layout with trigger 1 to the right of trigger 2.

Wire color coding is seldom a standard thing. I wouldn't rely on them without checking with a multimeter first.

By ramonsmits

Rookie (20)

10-02-2022, 17:27

@Pokun

So I understand pin 9 to support "button 2" for Sega compatibility based on http://wiki.icomp.de/wiki/DE-9_Joystick . According to that pin 9 is unused but then I don't understand the zener diode.

Pin 9 on MSX is GND, wouldn't it just be safe to connect to that or is that a specific purpose?

For sure then that joy stick is not compatible with C64/Amiga as it would connect pin 8 (GND) with 7 (VCC +5V).

Would be nice if I could mod the switch to make the joystick switch between MSX and C64/Amiga mode but also prevent issues if the joystick was incorrectly inserted.

If I would swap pin7 with 9 then the zener diode sits on pin 7. This likely prevents the VCC +5V to cause any damage right?

On MSX button 2 would then connect pin 8 (OUT) to pin 7 (Button 2) and pin 9 (GND)

On Amiga button 2 would then connect pin 8 (GND) to pin 9 (Button 2) and the diode would prevent connecting both to ping 7 (VCC +5V).

Would that work?