RE: A newly working Commodore 720 - many questions...

From: Hoffmann-Vetter, Martin <>
Date: Thu, 16 Feb 2012 02:05:17 +0100
Message-ID: <005601ccec47$0cd41300$>

> Q1) As I understand it, I just need to fit the 16 missing drams and
> decoupling caps?

Yes, that's all!

> Q2) WRT to the differences in the PLA between the lo and high
> profile.

The logic is the same. Only the pinout is changed! All the PLAs have the
logic to drive four banks of ram.

> Could someone explain where the differences are?

Only the pinout and the layout. And that's the problem with a 8088 board in
a 610!

> Q3) The burn-in tests correctly identify whether the machine is a high
> or low profile model. ANyone know how it programatically does this?

There are two pins at the keyboard 6525. One is for identify the high or low
profile board. The other is for 50 or 60 hertz.

> Q6) I think modern ATX supplies have now dropped the -12V rail. From
> looking at the schematics this is only used for the RS232 port. As I'm
> not likely to use this any time soon, will I have any issues if I omit
> the -12V rail?

Or you look for an old AT supply.

> Q7) The CBM-II PSU's all supply a 50/60hz signal from the PSU, which I
> think is just used for the TOD clock on the CIA. As I'm unlikely to
> find this on a modern PSU, is there a really simple way of generating
> this signal?

Yes, use an optocoupler.


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