Megadrive/Genisis switchless mods???

[cde]

Hi all, just got a asain pal megadrive, the one that looks like a jap ntsc but is in fact the same as uk pal ones, except for the wide cart slot. So i thought i would take it apart and do the 50/60hz and region mods, found some great guides and also found a mention of a switchless?? mod. somethig to do with pressing the reset buton and a changing colour LED to indicate the mode,50/60hz.. Does anyone remember this or was it just made up..:DOH:


Here is a link, look at the first megadrive on the list??

http://www.consolepassion.co.uk/sega...e-consoles.htm

[Calpis]

There's nothing special about the mod, for switching between 4 options you just need a 2 bit counter. Then one of the bits is just wired to a bicolor LED... "We are the only company in the world that does these modifications" -- that's a bit of a stretch!

I'm not positive but I think the reset signal is debounced, which means it's directly suitable for a counter without an additional debouncing circuit. Most likely the cleanest signal is directly at the CPU, which is 68000 pin 18.

Edit -- fixed:


You may want to use a 1K resistor on the LED since the chip is supplying the current, and some won't source enough for a LED. You can move the LED arrangement to either jumper 1 or 2, and if you want the colors switched, just flip the LED. When you're connecting the jumpers, make sure the old ones are removed and that the chip outputs go to the signal side of the jumper, not GND!

[cde]

Thats great thanks for the info:clap: .. i understand most of the schematic and am quite good at soldering, so am eager to try this out. One thing i dont understand is, what sort of IC is that and where can i get it from, or is it one of the megadrives chips?? sorry for the lack of knowledge, thanks

[Calpis]

The chip is any 7474 but if you want the LED, you should go with a 74HC74/74HCT74 instead of a 74LS74 since it can power the LED on it's own.

[devzone]

Great simple mod, looks like the dual d flip flop is wired up as a 2 bit binary counter giving 4 possible link selections but the led only indicates the least significant bit. should there be another dual color led on the other bit too ?

[cde]

Thanks for that, found the chips you said from trusty old ebay..28pence:lol:
just a few more things to get and ill try it as soon as i can.. ill post back with my results "GULP"

[andoba]

There's nothing special about the mod, for switching between 4 options you just need a 2 bit counter. Then one of the bits is just wired to a bicolor LED... "We are the only company in the world that does these modifications" -- that's a bit of a stretch!

I'm not positive but I think the reset signal is debounced, which means it's directly suitable for a counter without an additional debouncing circuit. Most likely the cleanest signal is directly at the CPU, which is 68000 pin 18.

Edit -- fixed:


You may want to use a 1K resistor on the LED since the chip is supplying the current, and some won't source enough for a LED. You can move the LED arrangement to either jumper 1 or 2, and if you want the colors switched, just flip the LED. When you're connecting the jumpers, make sure the old ones are removed and that the chip outputs go to the signal side of the jumper, not GND!

That is what I call being AWESOME! I am using that for sure!!

[cde]

Ok, got the 74cht74 IC and some veroboard and started soldering and a bit later, well.. it works. flawless PAL to NTSC switching. Two problems though.
1. Cant find a BI colour led yet so did it without one (my fault fix later)
2. The system boots as PAL and works fine, when i press the reset button it switches to NTSC, no probs but there are some strange screen artifacts at the top of the screen and the game speed seems to slow when you try to play. If you short the 5volts (YOU REALLY SHOULD NOT!!) it resets to PAL, then do it again its sets to NTSC again but this time its perfect, with no slowdown or artifacts. I have tried this many times and it is consistant.

My thought is that the reset is not enough to shut down the cpu before starting again in NTSC, a 5volt short causes a hard reset of the cpu thus fixing the above issues. I have another megadrive fitted with normal switches at the back and sometimes if you select NTSC during PAL gameplay you get the same results. Does anyone know of a way to hard reset the cpu from this circuit, or can i make it NTSC as default and swithch to PAL..

GREY = PIN 18 CPU RESET
RED = 5 VOLTS
YELLOW = JUMPER 1
BLUE = JUMPER 2

[Calpis]

Where's the wire to ground? You need to connect the chip's power rails! (pin 7 to GND, pin 14 to +5V)

The flip flop shouldn't be in a determined state at powerup (though it's possible the chip is made to), I think the problems are due to the fact that you're making the flip flop oscillate (and when it's stable you're shorting the output!).

The only way to initialize it to a specific state is to use the reset signal, but since you're already using reset for the counter, the counter operation would break as it would always be reset to the initial state. Basically to do this you'd need more logic gates or build a power on reset circuit separate from the reset signal.

[cde]

Thanks for your help on this, CALPIS your knowledge knows no bounds...after putting in the earth and 5 volts to the pins you said it now works great, no ghosts or slowdown. this really is a great little mod and i really enjoyed doing it. :thumbsup:

I will post later with a pic when i get my bi colour led and show the corrected (cough) wiring....:pray:

[Jamtex]

Be nice to modify the circuit to allow a multicolour colour (RGB) LED to be used. Having Red for JAP/60, Blue for Eng/60 and Green for Eng/50 and off for the useless JAP/50 mode.

[Calpis]

That would be possible with just an additional 7400 because of the flipflop's inverted outputs.

FF1 FF2
/Q /Q
/Q Q
Q /Q
Q Q

Connect the NAND's Y outputs to the LED's cathode through a resistor (anode tied to VCC), and there you go. You can use a 74LS00 on this one because TTL can sink enough current for an LED.

[devzone]

Be nice to modify the circuit to allow a multicolour colour (RGB) LED to be used. Having Red for JAP/60, Blue for Eng/60 and Green for Eng/50 and off for the useless JAP/50 mode.

As requested by Jamtex:


colors will be
mode0: red
mode1: green
mode2: magenta
mode3: cyan

you can change which output pins (5,6,7,8) are wired to the TRI-COLOR LED to change the colors if you wish.

[Calpis]

That will obviously result in the high bit LED being on during half the cycle though (desirable?).

Here's what I meant in the last post:


top LED = 0 .. bottom = 3

BTW, I looked up the jumpers:
Language: 0 = JPN, 1 = other
System: 0 = 625/25, 1 = 525/30

[devzone]

That will obviously result in the high bit LED being on during half the cycle though (desirable?).

Yeah the circuit I gave is for a single tri-color RGB led as requested by Jamtex and will give the following colors

mode0: red
mode1: green
mode2: magenta
mode3: cyan

[cde]

Ok, got some bi colour led's and wired one in as per the diagram and nothing... no red or green.. played about and found that connecting it to jumper 2 and pin 8 of the ic got the led to switch perfectly every time i pressed reset... BUT:noooo:

Now it does not switch from PAL to NTSC anymore it switches from
NTSC to NTSC-422, according to my tv's display??? and sometimes will not display at all on the NTSC-422 thingy.. i presume its going out of range of my tv.
By me connecting the led up this way, is some current getting through and not letting the video switch properly:confused:

[devzone]

Ok, got some bi colour led's and wired one in as per the diagram and nothing... no red or green.. played about and found that connecting it to jumper 2 and pin 8 of the ic got the led to switch perfectly every time i pressed reset... BUT:noooo:

Now it does not switch from PAL to NTSC anymore it switches from
NTSC to NTSC-422, according to my tv's display??? and sometimes will not display at all on the NTSC-422 thingy.. i presume its going out of range of my tv.
By me connecting the led up this way, is some current getting through and not letting the video switch properly:confused:

You are using a resistor in series with the led right ? if not then you will be overloading the chip outputs and this will cause incorrect login levels to the megadrive

[cde]

the resistor i was using was too low, 100 ohms.. added another and it works great now, thanks again you guys for all you help and input on this mod.. really enjoyed it and really enjoying my now multi talented megadrive.

NTSC RED

PAL GREEN

[cde]

At last..:clap:

[devzone]

great news, I would recommend a little more than the 2*100R that you are currently using as the leds will currently be drawing around 15mA from the chips which is a little high although the higher the value the dimmer the leds :(