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

From: Christian Dirks <>
Date: Thu, 16 Feb 2012 14:21:43 +0100
Message-ID: <>
Am 16.02.2012 00:34, schrieb Rob Clarke:
> Q1) As I understand it, I just need to fit the 16 missing drams and 
> decoupling caps?

You also need the BASIC 256 ROMs, or the added 128k will not be used by 
On startup, the kernal tests the installed RAM, but there is no feedback 
how many RAM was found.
Even if RAM is faulty, there is no display.
> Q3) The burn-in tests correctly identify whether the machine is a high 
> or low profile model. ANyone know how it programatically does this?
As already mentioned, it is done by reading PC6 and PC7 from the 6525 
which handles the keyboard.
These are also read by the kernal on startup an thic can be used to get 
the better 8x14 matrix display on a 610. When doing so, the character 
ROM from a 720 is needed.

> 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?
Can simply be generated with a schmitt-trigger like in the c64.
Take a look at the schematics at
how the signal for the cia pin 19 is generated.
For the schmitt-trigger (U23 in the schematics) a 74LS14 can be used.

Christian Dirks

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Received on 2012-02-16 14:00:13

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