Fw: Re: Fw: Re: Modern myths

From: Todd S Elliott (eyethian_at_juno.com)
Date: 1999-04-23 02:16:15

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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