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| From: Marko_Mäkelä
Date: 2002-01-29
Subject: Re: 264 series kernals
|
On Tue, 29 Jan 2002, Richard Atkinson wrote:
> Particularly regarding the tape routines - I think many people expect
> to see different bit timings in the early versions, perhaps using
> double clock mode. There must be some reason why the 264 series
> machines ended up with bit periods twice as long as the other CBM tape
> formats
I have gathered some experience when I developed the C2N232 device
(http://www.funet.fi/pub/cbm/crossplatform/transfer/C2N232/C2N232.html). I wouldn't say that the bit periods are twice the ones used on other Commodores. According to my measurements on the plus/4 I got from Levente
(I haven't checked the KERNAL version), the pulse times in 8-microsecond units are
37..53, 55..72, 75..93 for other Commodores, and
60..62, 119..121, 239..241 on the (PAL) plus/4.
The PAL C64 values are 45..50, 65..69, 87..90. Multiply these by
8 microseconds to get the actual times. It's not exactly double, isn't it? I really wonder why the longest pulse on the plus/4 is that long,
twice the length of the medium pulse.
> perhaps Commodore was originally intending to run them in double clock
> mode, but found that the memory refresh accesses (or TED badlines
> even) upset the timing accurracy?
Could be. Also, the 264 series doesn't have an high-to-low sensitive CASSETTE READ line, like all other 8-bit Commodores.
Marko
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