why do MD & MD compatible (3rd party) pads work on the SMS but not on the MARKIII? did they change the port definition between the MARKIII and SMS generation? if yes, why? any 3rd party pads suggestions, which will work on the MARKIII?
It's something to do with the way that the MK3 reads the joystick ports, remember the SMS / MK3 only use grounded data lines (6 buttons plus the ground) where as the Megadrive uses an IC to get the extra buttons and this causes problems with the MK3. The easiest way to get a MD pad to work on a SMS is to remove all the components from the PCB and just resolder the wires to the correct inputs. Like this Japanese website shows you...
but the strange thing is, the SMS doesn't have any problems with using a MD pad or 3rd party like the blaster. only the MK3 does. so there must be a difference between the SMS and the MK3 controller ports. but thanks for the link, will read it. altough, I won't modify anything, since I have the SMS to play the games on as well.
The control ports are exactly the same, however hardware wise the way the SMS and MK3 decode the joypad input to the port address is different and that's why anything apart from direct grounded pins don't work.
ok, this explains now the situation quite well. but still strange that the MD pad works on the SMS. so this could mean, the SMS already used a multiplexor? or could the MD be used as a transparent pad as well, if there is no multiplexor seen on the hardware (opposite) side? for example, the amiga also didn't use any multiplexor and the MD pad could be used without probs.
The issue is that MK3's pin 9, which is the multiplexer's select signal, is tied to GND (which also forces left and right low and maps A/start to 1/2). On the SMS, the pin appears to have become an input on the TTL-level IO ASIC. TTL inputs float high (it's floating since it's also an input on the MD pad and nothing is driving either of them, no pullups or anything) but the CMOS multiplexer in the pad requires so little input current that even through the ASIC's high input resistance, there's enough current to reliably drive the multiplexer input high, putting the pad in the proper state.
