10 Oct
2005
10 Oct
'05
2:45 p.m.
Subject: Re: Fwd: questions about Sanyo MBC 55x collectors
From: Chris M <chrism3667 at yahoo.com>
Date: Mon, 10 Oct 2005 12:25:37 -0700 (PDT)
To: "General Discussion: On-Topic Posts Only" <cctech at
classiccmp.org>
the IBM PC used the 8284A clock generator to pump the
8088 and that chip generates the 30% duty cycle required
by dividing a higher frequency (14.31818mhz) to 4.77mhz.
That higher frequency was high enough for Video clock
generation as well.
The IBM PC also another osc for the baud rate clocks
and the Video had it's own clock.
Many other machines used 3.58 (3.57545mhz) as it was easy
to find and cheap, almost 4mhz and related to US video
color burst. However the primary reason it was cheap
crystal. In some cases the cpu was also limited to 4mhz
or lower and that factored into the clock used.
Allison
The Sanyo ran at 3.58MHz (8088)
so that they could use
CHEAP color burst crsytals for the oscillator.
Almost all the pc's used a crystal that was some
multiple of color burst frequency. Did the Sanyo
actually use a 3.58mhz crystal? The PC had a crystal
that was 4x that, and divided it by 3 to get 4.77mhz. 
10 Oct
10 Oct
9:53 a.m.
Almost all the
pc's used a crystal that was some
multiple of color burst frequency. Did the Sanyo
actually use a 3.58mhz crystal? The PC had a crystal
that was 4x that, and divided it by 3 to get 4.77mhz.
the IBM PC used the 8284A clock generator to pump the
8088 and that chip generates the 30% duty cycle required
by dividing a higher frequency (14.31818mhz) to 4.77mhz. That higher frequency was high enough for Video clock
generation as well.
The IBM PC also another osc for the baud rate clocks
and the Video had it's own clock.
The MDA card has its own clock oscillator (one of those 4-pin oscillator
cans), the IBM CGA card does not. It uses the 14.3xx MHz motherboard
clock for the video timing chain and the NTSC encoder.
In fact the PC/AT motherboard has a 14.3MHz clock circuit on it, fed to
the expansion slots for those cards, like the CGA card, that require it.
IIRC, there's a trimmer capacitor tied to that crystal (on the PC, XT, AT
boards) that you can tweak to get the colourburst frequency right so that
an NTSC monitor or TV will display in colour.
-tony
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