TG16/PCE Mod w/ IC's?

Started by Lost Monkey, November 22, 2004, 11:28:29 AM

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Lost Monkey

I was looking at the switchless Saturn mod last night and it got me wondering whether the TG16 region mod could be done in a similar fashion using 2 IC's and the Run+Select reset feature...

I don't know enough about the 74157 IC's to know whether they could be used for such data transfer though...

Any thoughts?


The Saturn has a simple jumper pair that needs to be changed from either +5v or GND ot the other.  These jumpers will only ever be on (5v) or off (0v).  The status of these jumpers is only checked during powerup, which is why holding the reset button works, you can release it after the region jumpers are checked and the system boots into whatever region you've chosen.

There are three reasons this won't work for the PCE.

1. The TG/PCE has eight lines in the cart port that need to be flipped, so that one system has lines 1-8 and the other is 8-1.  The PCE's cart port is accessed at all times, so clearly we can't use a pad switch to do the job 'cause then you couldn't play the game while you force the chip to reverse the lines.

2. The pad cycles through different outputs, even if you COULD hold the buttons down at all times the pad alternates between UDLR and I/II/ST/SL, so that you holding (for example) START all the time would only result in the switching chips being held in one region of the other half of the time.

3. The cart port needs to be disabled from time to time, because the system shares the same data bus between several devices.  This is what the /OE line is for on the cart port, it disables the output so other devices on the bus can talk to each other without interference from the cartridge. It's called a Z state, neither high nor low.  For all intents and purposes the cart port has simply disappeared, electrically speaking.   The 74157 chip is not capable of tri-state operation, it can only output ON or OFF.  You could use another chip, there are several switching chips that CAN tri-state, but then you run into the first two issues again.

You could add some additional logic inside the system that activates one region or the other for every second powerup...

Lost Monkey

Thanks Lawrence!

My confusion was because of a lack of understanding of the IC's used in the Saturn mod.

I didn't realize that the state change was dependent on the reset button being held, I thought that each time the reset button was pushed, the state would change and remain until the next time it was pushed... the same way the traditional switch mod works.    :unsure:

(Glad I didn't start hacking up the Hucard slots yet!)

Dave Shadoff

I dunno about that particular mod, but you can use IC's to reduce the number of 'poles' on your PC Engine switch from 8 to 1.  That is, instead of needing an 8PDT switch, you can use a SPDT (or even SPST) switch together with a series of "CMOS Bilateral Switches".

These are generally Triple Bilateral Switch IC's on a 16-pin DIP package (or at least they were 15 years ago - I don't know about packaging now).  You arrange 3 IC's (8 of the switches, with one extra unused) in the same 8PDT format, but tie all the toggles together for a single logic signal as input.

"Back in the day", I used 4053's (I believe) - or the substitute 74HC4053.  They should still be available, but may not be in the original 0.1"-center DIP package.