Hello there guys, I've successfully created some repros.
So much fun ! lol, except i've only made single rom 8bit ones.
I have a question, because my next try, will be multi eprom.
I know that, to use more than 1 EPROM in a single rom donor cart, the donor cart needs to have a mad 1 right?
(if I'm wrong, how to use multi eprom on a non mad-1 chip donor cart?)
Then, if we need more than 1 extra eprom, we need 74hct139 right?
Please correct me if im wrong.
Well, I opened a donor cart here, SHVC-1A3B-12.
This doesn't have a MAD-1 . It has an HD74LS139P.
I am confused because, it looks to me it does the same job the mad-1 would, but it isn't a mad-1.
How do I wire to eproms with this?
Thanks !
You need to search the forums. This topic has been discussed many times with diagrams and pictures.
The 139 decoder works like this: it only allows 1 of 4 EPROMs to be enabled at any time. So it is a rom selector. If more than 1 rom is enable at the same time, that's called buss contention....when you have more than 1 rom on a databuss, only 1 can be enabled. So the "OE" signal is transferred to any 1 of the 4 output lines. And when the OE is high, it disables ALL the roms connected to the 139. This is so the SRAM can be activated and used. Rom enabled-- SRAM disabled. SRAM enabled--rom disabled. The mad1 chips is just a decoder like a 139 with added battery memory retention circuitry. The mad1 passes the OE signal to the rom or roms just like a 139.
Every additional address line doubles the roms size. Your 8mbit EPROMs has a high address line of A19. So a 16mbit device would have a high address A20, 32mbit .... A21, 64mbit .....A22
So if you want a second 8mbit EPROM to add, then you have to have a decoder so only 1 rom is enabled at any given time. So you would connect address A20 to the decoder as 1 of the inputs and when ever A20 is selected, it disables the first rom and enables the 2nd rom...
Anyway, search the forums, you should be able to find your answers
Markfrizb wrote:
You need to search the forums. This topic has been discussed many times with diagrams and pictures.
The 139 decoder works like this: it only allows 1 of 4 EPROMs to be enabled at any time. So it is a rom selector. If more than 1 rom is enable at the same time, that's called buss contention....when you have more than 1 rom on a databuss, only 1 can be enabled. So the "OE" signal is transferred to any 1 of the 4 output lines. And when the OE is high, it disables ALL the roms connected to the 139. This is so the SRAM can be activated and used. Rom enabled-- SRAM disabled. SRAM enabled--rom disabled. The mad1 chips is just a decoder like a 139 with added battery memory retention circuitry. The mad1 passes the OE signal to the rom or roms just like a 139.
Every additional address line doubles the roms size. Your 8mbit EPROMs has a high address line of A19. So a 16mbit device would have a high address A20, 32mbit .... A21, 64mbit .....A22
So if you want a second 8mbit EPROM to add, then you have to have a decoder so only 1 rom is enabled at any given time. So you would connect address A20 to the decoder as 1 of the inputs and when ever A20 is selected, it disables the first rom and enables the 2nd rom...
Anyway, search the forums, you should be able to find your answers
Thanks ! thanks for the detailed info.
Thanks so much.