[Coco] Re: [Color Computer] Re: Adapter from Coo3 to SVGA

[RJRTTY at aol.com]

In a message dated 4/5/05 6:27:47 PM Eastern Daylight Time, 
Rodney_Hamilton at GBRonline.com writes:

<< Actually, that pin is already programmed as an output by default.
> It is connected through a 4.7K resistor to pin#10, which also has
>a 4.7K pullup resistor to +5V.  This is fine for use as a monitor
> sense input line, but to use it as an output you'll need to reduce
> the value of R62 from 4.7K to 500 ohms or less to reach digital
> logic levels.
 
I dont fully understand what your saying.    The I2C is a serial
interface with +5 V and 0 V logic levels with a pullup resistor
because the chips that are not being addressed are in a
high impedance input state.
 
> So yes, an internal modification WILL be needed to use this pin.
 
I frankly dont see why.   With the cassette port, I used the relay
with a 4.7k resistor connected to the +5V supply to one side of relay
as a pull up and the other side of the relay connected to ground.
This was my data line and it had no trouble driving the video
chips and the video chips had no trouble driving the data line
depending on data going out or data going in.  The PIA pin
should work as well.

> Note also that if this pin is used as the I2C data line and the
> frame sync is used as the clock, the Averlogic video chip will
> need to use some form of handshake sequence for init mode or
> the normal use of the screen mode and color set bits will make
> pin#10 look like an endless stream of all zeros or all ones.
> It the video chip uses a 'sync' sequence, no problem.
 
> Rodney
  >>

I am going to use the vertical sync to drive a comparator input
with an apropriate RC filter to convert it into a %50 duty cycle
to be used as the I2C clock.
The coco3 will know the clock is high in the converter as the
vertical sync signal is transmitted and it will know the clock
is low after a set time has passed.   Software will keep track
of it.    The device being addressed will use the clock in
the normal way.     Since the coco3 controls ( or monitors
would be more accurate) the I2C clock,  the master and
slave device will always be in sync.

Roy