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

[Rodney V Hamilton]

In article <d2uh1j$mbu$1 at sea.gmane.org>, 
Torsten at Dittel.info says...
>
>> And I love the idea of using the extra pin on the CoCo CM-8 port.
>
>I guess a pin which is planned to be a "just input" to the PIA and which 
>detects the presence of a device by its electrical ground uses a pull up 
>resistor to create a "high" level while no device is attached. Would 
>this prevent the pin in any way from being used as an output? Hardware 
>people?
>
>Torsten

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 used this pin to control the INTENSITY line of an RGBI monitor
by adding 560 ohms in parallel with R62 and using the SCREEN
command to turn it on or off via the CSS (color set select) bit.
  (SCREEN0,0 for off/dim,  SCREEN0,1 for on/bright)

So yes, an internal modification WILL be needed to use this pin.

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