[Coco] MC-10 Questions...

[jdaggett at gate.net]

Mike

Since the joystick porrt measures voltage across a resister,you need to know 
the resistance or the current through the resister. 

To setup the joystick port to read a resistance value you need to measure the 
voltage drop across an unknown resister that has as its current source a known 
constant current.

The real problem is that the Coco3 does not really measure the voltage. It 
programs a middle value, between 0 and 63,  into a DAC and the joystick port 
and the output of the DAC are fed into a comparator. To determine the exact 
position, the DAC values are adjusted until the comparator output changes 
state. Kind of an indirect method. Besides the six bits in the DAC are woefully 
very inaccurate.

james
  

On 7 Aug 2004 at 22:36, KnudsenMJ at aol.com wrote:

From:           	KnudsenMJ at aol.com
Date sent:      	Sat, 7 Aug 2004 22:36:32 EDT
Subject:        	Re: [Coco] MC-10 Questions...
To:             	coco at maltedmedia.com
Send reply to:  	CoCoList for Color Computer Enthusiasts 
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> In a message dated 8/7/04 9:09:35 PM Eastern Daylight Time,
> farna at att.net writes:
> 
> > The other gauges, fuel level, engine temp, oil pressure, and
> > possibly volts 
> (
> > haven't figured out how to do that one yet, the other three are
> > simple variable resistances that I know the ranges for) could be
> > slower. The oil pressure will have a back-up warning light. The
> > numbers displayed will have to be on a graphics screen to be big
> > enough to easily see. I'd probably use 
> a 
> > mechanical hour meter under the hood and forget mileage, unless I
> > can find 
> a 
> > stand alone odometer instead of an hour meter. 
> >  
> >  Heck, I've forgot how I measured resistance now!! I remember using
> >  the 
> > joystick D/A inputs, but that's all. Got the idea from a game that
> > used a series of push buttons for a Jeapordy style buttons connected
> > to one port. 
> I 
> > think each button had a different resistance and the CoCo could tell
> > which button was pressed. Might have to have the gauges work in 5 or
> > 10 unit increments, but would be close enough. 
> 
> I think you're forgetting something -- that the Coco's joystick inputs
> measure Volts, period.  Resistor hookups are just voltage dividers of
> a known external reference voltage, into a dependent voltage that
> feeds to the A/D joystick inputs.
> 
> To monitor your car's battery voltage, which ranges between 12 and 15
> or so, use a pair of fixed resistors to divide it by 3, so that range
> becomes 4 to 5 V (remember, the Coco joysticks measure from 0 to 5 V,
> no more).
> 
> I think a competitor, maybe Apple, used a similar scheme to the
> Coco's, but had one of the resistors built into it, so the joystick
> was just a variable resistance.  Not nearly as flexible as our Coco
> (but then, what was/is?) --Mike K.
> 
> 
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