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what's wrong with my Hard Drivin repro?
https://forums.nesdev.com/viewtopic.php?f=28&t=10976
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Author:  icemanxp300 [ Mon Jan 04, 2016 10:23 pm ]
Post subject:  Re: what's wrong with my Hard Drivin repro?

R6 left goes to Rambo 15 and right is to ground.

On back side it shows the solder pads for p28 which must be hidden under the rambo chip going to rambo 40 and if jumper would tie in 39 as well.

The front c28 if you jumper it would go to Rambo 22 as is goes to Rambo 40

Attachment:
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Here is the crash thing.
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...

Author:  lidnariq [ Tue Jan 05, 2016 12:25 am ]
Post subject:  Re: what's wrong with my Hard Drivin repro?

I think you found a bug: I can (mostly) reproduce your screenshot in an emulator.

I bet the solder pads next to C28/P28 control whether CHR/PRG is a 28-pin ROM (i.e. pin 28 is vcc) or if pin 30 on a 32-pin package is CHR/PRG A17...

It looks like there was a no-wires way to populate the board to use two 256 KiB 'PROMs, just by adjusting R1/2/3/4 and C28/P28.

Author:  icemanxp300 [ Tue Jan 05, 2016 9:57 am ]
Post subject:  Re: what's wrong with my Hard Drivin repro?

Yeah, you're a bit more advanced in this than me :) While I have learned a lot and come along way I only started learning about anything 7 months ago. Heck back then I didn't even know what an eprom was or for that matter the difference between a resistor and capacitor lol.

It's interesting to learn and the more you know the more it makes sense. I just wonder now that I know all R1 does is ground prg 24 it sounds like that resistor is useless as I lifted pin 24 and grounded to 16. By lifting pin 24 it makes that resistor irreverent correct?

The other thing is since I moved that resistor to R1 which is ground would it even be necessary to lift pin 24? Wouldn't it already be grounded? Sorry I am still a little unfamiliar with the actual role resistors and such play in the whole scheme of things.

Author:  lidnariq [ Tue Jan 05, 2016 12:01 pm ]
Post subject:  Re: what's wrong with my Hard Drivin repro?

icemanxp300 wrote:
It's interesting to learn and the more you know the more it makes sense. I just wonder now that I know all R1 does is ground prg 24 it sounds like that resistor is useless as I lifted pin 24 and grounded to 16. By lifting pin 24 it makes that resistor irreverent correct?
Right. For the ROM to emit data, both pins 22(/CE) and 24(/OE) must be low. But only a single signal comes out from the RAMBO-1. (nesdevwiki:Tengen RAMBO-1 pinout).

So, for a ROM with two enables, R1 ties pin 24 low so that it emits data when it's supposed to.

For cost saving reasons, JEDEC specified 28-pin 128 KiB ROMs. (28 pins: 2 for power and ground, 8 for data, 17 for address, and one for "emit data"). When using one, the equivalent pin is A16 instead of /OE. So to use this shape of ROM, R2 connects A16 instead of ground.

Since R1/R2 control what pin 24 was connected to ... yes, lifting the ROM pin 24 makes R1/R2 irrelevant. You could probably leave both unpopulated now.

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