Re: [stella] The heart of darkness. (was midified VCS)

Subject: Re: [stella] The heart of darkness. (was midified VCS)
From: Chris Wilkson <ecwilkso@xxxxxxx>
Date: Sun, 18 Nov 2001 22:10:51 -0500 (EST)
On Sun, 18 Nov 2001, John Saeger wrote:

> It's a start.  ;-)  So if I'm used to thinking about things as being like
> TTL, how should my thinking change when interpreting this logic?   Are the

First off, start thinking of things in CMOS.  The only major differences are
that output voltages go all the ways to the rails, and there is no static
input current...The inputs are just capacitors to ground.  If you want to
actually build these things from TTL-like chips, I'd recommend using 74HCTxxx
instead of 74LSxxx series.  They are pin for pin compatible, but they are CMOS.
(The HCT stands for Highspeed CMOS TTL-compatible inputs.)

> inputs level driven?  Do the outputs present logic levels, or do they need
> pullups/pulldowns?  Do you know what state an undriven input assumes?  When
> I see a FET like that, I think of it as being like a switch.  Is this a good
> way to think about it in this context?  Do you know if it's logic HI or LOW
> that turns the switch on?

Gates are gates.  Gates are well defined, GENERIC symbols.  The only thing
they need to do is be compatible with each other. my other email,
I mentioned that the transistors are just simple swithes.  I failed to mention
however, that they are closed when the gate voltage is high and open when it
is low.  Sorry.  Because this is a MOS circuit, the inputs (capacitors) will
maintain their levels when the switch opens...but only for a little while.
They need to be refreshed periodically.


> Thanks,
> John
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