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Pc engine region mod with 74hc157 .circuit plan inside

Started by bigsanta, October 08, 2010, 10:30:35 am

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bigsanta

I have a few of these ICs74157s   lying around ,so i went ahead and de soldered the 8PDT that was inside my pc engine,opting instead for the IC method. but can the 74157s be made to work with the PC engine region mod ,or do you need the 4551 ICs

How would i connect the final wires to the circuit (the dotted lines) ,is my circuit right or wrong?


NFG

You need a tri-state chip that can 'float', which the '157 cannot do.  Basically, the '157 outputs either ON or OFF, but the system needs to be able to shut off the cart output entirely so it doesn't interfere with the rest of the console's operation.

So no, this isn't gonna work for you.


bigsanta

Quote from: Lawrence on October 08, 2010, 10:58:08 am
You need a tri-state chip that can 'float', which the '157 cannot do.  Basically, the '157 outputs either ON or OFF, but the system needs to be able to shut off the cart output entirely so it doesn't interfere with the rest of the console's operation.

So no, this isn't gonna work for you.


The ICs in question MC14551B ,HEF4551B/CD4551 are proving quite difficult for me to find over here in the UK ,are there any other family of ICs which i could otherwise use.?

l_oliveira

October 22, 2010, 01:44:42 am #4 Last Edit: October 22, 2010, 01:46:14 am by l_oliveira
Two 74LS245 should work with ROM ONLY hucards.

Due to the bidiretional nature of the bus analog switches like the CD4053 have better chance of success.

Tiido Priimägi

74AC645 is what you want to use. Its a 3-state octal transceiver, should fit the bill perfectly. Just tie the Direction with !WR or !RD and !Enable line with !CE signal of the huCard and you're good to go.
Mida sa loed ? Nagunii aru ei saa ;)

l_oliveira

Quote from: Tiido Priimägi on November 02, 2010, 03:50:13 am
74AC645 is what you want to use. Its a 3-state octal transceiver, should fit the bill perfectly. Just tie the Direction with !WR or !RD and !Enable line with !CE signal of the huCard and you're good to go.

Well... That work for ROM HU-CARDS.  CD System cards and ROM HU-CARDS with memory mapper won't be writeable that way...

Maybe the pair of 74LS245 could have their dir pins tied to the R/W control line. And an pair of ports of an 74LS32 could be used to select which chip would be active while still keeping the ability of  "muting" the chip output if the PCE CPU is accessing the video chip or internal RAM chip.

What makes this mod a bit tricky is the fact that the card slot is connected to the main CPU BUS and we can't "latch up" the bus with an device that cannot be disabled/arbitrated by the CPU (the card is, using it's Chip Enable or /CE pin).

So adding the two OR ports would then "add" the input from a 2 position switch to each one of the buffer chips, allowing us and the PCE CPU to enable one at a time, only when it needs access to the PCE card slot.