6550 RAM adapters for PET 2001

[crock]

This was something I could have done with a couple of years ago when I was sourcing 6550 RAM's for a PET 2001 restoration.

The seller has been selling composite adapters and 6540 ROM adapters for a while, and he now offers a nice little adapter allows you to replace a 6550 with the plentiful 2114's.

http://www.dasarodesigns.com/product/mps-6550-commodore-pet-2001-ram-adapter/

If this has already been posted, apologies, I couldn't see it in the archives.

cheers, Rob

 

[dave_m]

This was something I could have done with a couple of years ago when I was sourcing 6550 RAM's for a PET 2001 restoration.

The seller has been selling composite adapters and 6540 ROM adapters for a while, and he now offers a nice little adapter allows you to replace a 6550 with the plentiful 2114's.

http://www.dasarodesigns.com/product/mps-6550-commodore-pet-2001-ram-adapter/

This looks like a really neat item, but the one thing I can't see is the jumpers or dip switch to select down to the needed 1K RAM boundary. I see the the decoder chip buried under the 2114 socket, but how does he select the proper 1K chip select for the bad RAM he is replacing? I wonder if the gadget is hard wired for one spot and you are supposed to put the freed up good chip into the bad spot? I guess that would work.

In his web site he gives some great information on different video RAM problems. Good stuff.
-Dave

 

[MikeS]

This looks like a really neat item, but the one thing I can't see is the jumpers or dip switch to select down to the needed 1K RAM boundary. I see the the decoder chip buried under the 2114 socket, but how does he select the proper 1K chip select for the bad RAM he is replacing? I wonder if the gadget is hard wired for one spot and you are supposed to put the freed up good chip into the bad spot? I guess that would work.

Not sure I follow you there; as long as you decode LLHH on pins 18-21 to give you a /CS it should work in any socket, no?

In his web site he gives some great information on different video RAM problems. Good stuff

Indeed, although a couple of things didn't seem quite right; probably me missing something.

 

[crock]

I'm guessing its mapped to only make use of one half of the chip, the rest goes to waste. Thus only replaces one chip.

 

[dave_m]

Not sure I follow you there; as long as you decode LLHH on pins 18-21 to give you a /CS it should work in any socket, no?

Mike,
Right, I see it now. LLHH on pins 18-21 is wired to activate one 74138 decoder output to form the chip select to the 2114. The gadget can go into any spot. Nice!
-Dave

 

[dave_m]

I'm guessing its mapped to only make use of one half of the chip, the rest goes to waste. Thus only replaces one chip.

Rob,
I don't think so. The 2114 and the 6550 are both 1K X 4 RAMs.
-Dave

 

[crock]

Right. I was having a moment. It's been a long week...

 

[MikeS]

Mike,
Right, I see it now. LLHH on pins 18-21 is wired to activate one 74138 decoder output to form the chip select to the 2114. The gadget can go into any spot. Nice!
-Dave

Here's a local boy's version, up to 32K, mounted underneath the PCB to keep the top looking original (the switch and 'LS30 are only to select memory size, 8,16,24 or 32 K.
A little messy, but it works ;-)

 

[dave_m]

Here's a local boy's version, up to 32K, mounted underneath the PCB to keep the top looking original

A new way to surface mount? I can't see where the address and data bus are connected, under the 32K RAM chip somehow? I like that the modification cannot be seen from the top.

 

[MikeS]

A new way to surface mount? I can't see where the address and data bus are connected, under the 32K RAM chip somehow? I like that the modification cannot be seen from the top.

Yup; I believe it sits underneath the 6502 and as it happens 17 of the 28 lines line up directly (different labels, but that doesn't matter).



The PDF of the schematic's too large to upload and I don't know how to easily reduce it but I can email it to ya if you're interested.

Credit for the hack goes to Josh Bensadon BTW. He did something similar for the ROMs but I thought it'd be easier and neater to just use one of Jim Brain's adapters upside down as it were; we haven't tried it but it oughta work.