[Coco] Re: Bit 5 of DSKREG

[Richard Batt]

Hi Tim:

I have 3 disk controllers but no schematics for any of them.
However, I found the schematic for the FD-501 in the "Tandy's
Little Wonder" book. This schematic shows the interrupt from
the 1773 being gated before it leaves the disk controller as
the NMI* signal.

The gating is being done by part of U5, a LS02 chip, used as
an AND gate with inverted inputs. If both inputs are low it
outputs a hi. This hi is inverted by a separate gate (U6)to
give the active low output NMI. The AND gate gets one of its
low inputs from the 1773's int out inverted by another gate
(chip unidentified) to give the low input to the AND gate.

The other AND input receives the same signal as the DDEN*
input on the 1773. This is the double density signal to
the 1773, and is active low for dd. When this signal is low
to the AND gate it allows the interrupt from the 1773 to pass
thru to the NMI* output. So the disk controller will only
output an NMI* when the disk controller is in dd mode.

It seems a bit odd to me that the NMI* output would be linked
to dd operation, but assuming the schematic is correct, that
is how it works.

Dick Batt




----- Original Message -----
From: tim lindner <tlindner at ...>
Subject: Re: Bit 5 of DSKREG
Newsgroups: gmane.comp.hardware.tandy.coco
Date: Wed, 28 Jan 2004 10:34:00 +0000

Robert Gault <robert.gault at ...> wrote:
> 
> Tim,
> 
> There is most definitely a problem with making bit 5 both a density flag
> and an NMI flag. This is after all a single bit so it can only have two
> states, on or off. Making this both a density and NMI flag implies that
> the bit can have 4 states which is just not possible.

I am not saying the bit has four states. I am saying the single bit
controls two functions.

It also makes sense to me. A .89 Mhz CoCo should be fast enough to use
polling in single density operation. Though I havent tested it yet.

> There is no indication of this bit being an NMI flag/switch in the 
> "Unravelled" series, "The Facts for the TRS-80 Color Computer" (an 
> earlier unraveled version), or the Disk Basic owners manual.

Since single density operation is so rare on the CoCo, I don't trust
thoes references to get it right. Which is why I turned to the
schematics.

But, again, I could be wrong.

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