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Help to program 16 bit EPROMs on a 8 bit programmer

Posted: Thu Jan 13, 2022 4:39 am
by Fisher
Hi.
I've been trying to program 16-bit EPROMs, but my programmer only supports the 8-bit ones.
It's a Genius G540.
I tried to find some kind of updated software that could support at least a 27c4100 with no luck.

I tried to use an SNES DIY game PCB to do the work:
Botton
Botton
Top
Top
So far so good, I was able to successfully read a Sonic 2 ROM:
Read results
Read results
Unfortunately, I can't program using this adapter.
Changing the pull-up resistor networks to pull down resulted in the EPROM being all 00's, which probably means that the programming sequence is doing well.
Maybe the multiplexers I'm using, 74HC257 aren't bi-directional, are they?
The datasheet says nothing about that...
Does such multiplex exist?

Another idea I had was to use a couple of 74HC245 bus transceivers.
If I do this, should one get the regular A0 signal and the other an inverted one?
What about the DIR pin?
Should I add a 78L05 to the VPP pin to select the direction or there's some better way to do it?

Thanks in advance.

Re: Help to program 16 bit EPROMs on a 8 bit programmer

Posted: Thu Jan 13, 2022 11:46 am
by lidnariq
Fisher wrote: Thu Jan 13, 2022 4:39 am Maybe the multiplexers I'm using, 74HC257 aren't bi-directional, are they?
Correct.
Does such multiplex exist?
Yes, an analog multiplexer such as the 74'3257, 4053, or 4551.
Should I add a 78L05 to the VPP pin to select the direction or there's some better way to do it?
Normal voltage regulators won't work with only 1.5V of headroom during programming, let alone when it's in verification mode and the programmer only provides 5V in the first place. Better to find a part (4000 series?) that can operate at 6.5V.

Re: Help to program 16 bit EPROMs on a 8 bit programmer

Posted: Sat Jan 15, 2022 2:14 am
by TmEE
Usually there's a byte mode on 16bit EPROMs, that will allow you to read/write everything with only 8 bit databus.

Re: Help to program 16 bit EPROMs on a 8 bit programmer

Posted: Sat Jan 15, 2022 4:20 am
by Fisher
That's an interesting information.
Unfourtunatelly, if I've read the datasheet correctly, it seems that the /BYTE pin is also the VPP pin.
Unless there's a trick that I didn't figure out yet I'm busted. :cry:
The EPROM is a ST 27c800.

Re: Help to program 16 bit EPROMs on a 8 bit programmer

Posted: Sat Jan 15, 2022 6:52 am
by TmEE
Ok, that prevents this idea...

But you can still program the chip one half at a time. Erase turns zeros into ones, and programming operation turn a one back to zero... so you can tie one byte high, while you program the other and do things in two passes.

Re: Help to program 16 bit EPROMs on a 8 bit programmer

Posted: Mon Jan 17, 2022 3:26 am
by Fisher
That's a nice Idea.
I think I have floating around an old MegaDrive GameGenie PCB.
Maybe just putting connectors where the old 8-bit ROMs are may do the trick.
I just need to remember where did I left it if it still exists in some way. :|