[Coco] Question about the PIA RAM Size input

[coco at yourdvd.net]

Also, important to note in the CoCo 3, it is pulled high with a 4.7k
resistor and is connected to pb6 of the keyboard pia (since ramsize
isn't used in CoCo 3). Perhaps a better pin to use would be pin 10 of
the coco 3 rgb connector. it is connected to pb3 of the 6821 and is
held high by a 4.7k resistor. it's intended purpose was to allow auto
detection of an RGB monitor, but wasn't implemented for some reason. It
is initially an input. on coco 1 and 2's it was connected to css of the
6847, but since the coco 3 has no 6847, it was available for other use.
Another method of control would be using the cassette relay to activate
a circuit to control something else. -r

> -------- Original Message --------
> Subject: Re: [Coco] Question about the PIA RAM Size input
> From: Mike Pepe <lamune at doki-doki.net>
> Date: Mon, April 30, 2007 9:08 am
> To: CoCoList for Color Computer Enthusiasts <coco at maltedmedia.com>
> 
> coco at yourdvd.net wrote:
> > actually, depending on the coco (1 or 2) you have and the installed
> > memory, depends on how it is jumpered, whether it be to gnd, vcc, pb6
> > of the keyboard pia, etc. I wouldn't do it - say it is jumpered to gnd,
> > and you output a logic 1 to it - or vice versa - probably wouldn't be
> > good. Look over some schematics of various coco models - the jumpering
> > scheme is different on the three coco 1 and 2 model schematics I just
> > glanced at...r
> > 
> 
> I'm going to negate you here a bit, but your logic is sound- I guess 
> just to clarify things for Darren.
> 
> The PIA is a pretty robust device, it would survive having one of its 
> outputs shorted to gnd/+5 pretty much indefinitely, at least according 
> to the data sheet. However that's really not the issue- What do you 
> intend to attach the output to is the better question. If whatever it is 
> pulls the PIA line high or low when the PIA is in input mode - and that 
> state matches the expected state - then you're golden. Obviously the 
> loading of the output is fairly important. Again, it really depends on 
> what you're attaching it to.
> 
> So, to clarify, if the expected state of the ram size input is 1, and 
> when you disconnect the jumper and attach the device, read the PIA bit 
> and if it's a 1, no issue. (yes, a gross simplification)
> 
> Now, if you patch the BASIC ROM to ignore the state of that PIA bit and 
> correctly set the SAM registers for the memory size you have installed, 
> what you do with that particular PIA bit is a don't-care scenario.
> 
> -Mike
> 
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