Jim Butterfield's earlier reply. Enjoy.
-Todd Elliott
--------- Forwarded message ----------
From: Jim Butterfield <fjb@freenet.toronto.on.ca>
To: Todd S Elliott <eyethian@juno.com>
Date: Thu, 22 Apr 1999 10:42:14 -0400 (EDT)
Subject: Re: Fw: Re: Modern myths
Message-ID: <Pine.3.89.9904221041.A25343-0100000@queen>
> > It did this by watching the
> > retrace bit in the VIA chip (bit 5 at address $E840/59424)...
> He probably means bit 6 at 59408.
No, I meant bit 5 at 59424. It was in the VIA chip, not the PIA. Users
effected the speedup by changing the directional register at 59458.
> At any rate, that is the circuit that was changed.
?? Claiming that no other circuits were changed ?? The same VIA I/O
register contained a number of bits related to the IEEE-488 interface,
such as NRFD (both in and out), DAV, ATN, and NDAC, as well as some
cassette tape controls. The EOI signal was elsewhere.
> This line [bit 6 at 59408] was shared with the EOI line of the IEEE
> interface.
Shared with what? The retrace signal certainly wasn't there.
> This interfered with disk operation so Commodore seperated [sic] the
> two functions. The timing function was replaced with changes to the
> display timing circuitry.
?? Where did this come from ??
> ... The remaining EOI remains as it was before except for being
> disconnected from the display circuitry.
Assuming the fantasy that it ever was connected.
> The TTL devices "trying to output different levels" simply do not
exist.
> Anyone can see this for themselves. The schematics are available at
> http://www.funet.fi/pub/cbm/schematics/computers/pet/index.html
Haven't looked, but I trust that schematics for all revisions are
supplied there.
> > There was another one that came in when Commodore introduced the CRT
> > controller chip to later models of the CBM line ("Fat 40" and 80xx
> > units). Users playing POKE games with this chip (at $E880/1, decimal
> > 59520/2) could vary the frequency and sizing of the screen raster.
> > Keep in mind that the business end of this was transistor circuitry
> > driving a flyback yoke; take the frequency too far out of line and
> > the yoke's inductance could start to cause damage. I heard a number
> > of reports of damaged yokes (surprisingly .. I would have thought
> > that the driving circuitry would have been more susceptible to
damage).
>
> The danger of damaging the computer by doing this is grossly exagerated
> [sic]. It is theoretically possible to damage a display in this way,
> but I know of NO VERIFIED instance of this happening.
We are talking of events many years in the past. I can assure you that
it's true that more than one user told me that his computer had been
damaged as a result of toying with CRT controller settings.
Some of these could undoubtedly be tracked down, but I don't see the
point. For my part, when I was told of such cases of damage, I decided
not to play with these registers (other than the relatively standard
values that are set up for 40/80, text/graphics options). I would find
it curious if Mr. Levak is advocating uncontrolled experimentation in
this area, assuring all comers that damage is impossible.
>From the Commodore Reference Sheet on the 6545 CRT Controller:
"NOTES:
1. Registers are write-only.
2. Avoid extreme changes in Register 0. CRT damage could result...."
> When it comes to choosing between the information of a qualified
> engineer, and gossip and rumor.... Well, there isn't really any
choice.
When it comes to choosing between the information of someone claiming to
be a qualified engineer, and Commmodore technical information sheets, I
know what choice I will make.
--Jim
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