How to restore a true soft reset on the SMS model II NTSC console

[CrossBow]

I have a client that has sent their SMS II for me to do some work on. One of the requests was to see if I could add a separate Reset switch to the console because for some odd reason, Sega didn't think that was needed on the smaller model II SMS? Now, I've not looked into the reasons for why you want this that much, but I understand that some cheat options for games require the use of the soft reset function that the original models have?

Well, in looking through schematics and other online stuff, all I could really find was info on how the soft reset function works on the PAL model of this console. That wasn't going to help me but as luck would have it, the NTSC model II SMS is literally just a smaller version of the model 1. Same chips...same schematics.

So, in looking at the mode 1 schematics you find that the reset switch on the console was tied to pin 22 of the gate array IO chip. That pin doesn't appear to be connected to anything else on the system. In the model 1 console, when you press the reset switch, it just grounds pin 22 on the gate array chip and that sends a soft reset command to the rest of the system. As a result, the actual reset line on pin 26 of the Z80 CPU isn't really used on the model 1 SMS console.

On the model 2 SMS, Sega instead decided to use a diode/resistor combo to cause a safe quick short circuit to the Z80's reset line when you power the console off/on. Or rather, when you apply power the console, it also sends a hard reset to the Z80 at the same time and that is it. That is the only way to reset a game on the SMS II console is by cycling power. 

Well, you can easily restore the soft reset function by using two lengths of wire and a momentary switch that you install on the console somewhere.

Simply solder one end of a length of wire off pin 22 of the gate array IO chip (marked as 315-5216). This pin is marked and easy to find and is soldered to the board but has NO traces running to it. The other end of that wire you solder to one side leg of your switch. Then solder the other length of wire from the other leg of the switch to a point attached to ground. 

That's it... that is all you have to do! I made a quick video that demonstrates this in action below.

 

Edited December 22, 2023 by CrossBow