[coco] advice needed

[RJRTTY at aol.com]

In a message dated 10/31/05 12:34:31 P.M. Eastern Standard Time,  
robert.gault at worldnet.att.net writes:

I can't  remember any software using the monitor sensing line, but that 
doesn't  mean much. The only problem might be that the PIA is configured 
for input  on that line. You would need to change it to output. That 
means you will  need to reconfigure any time the RESET button is pressed 
for the PIAs will  be reset based on the ROM.

Lets say that you burn a new ROM which makes  $FF22 bit 3 an output. Now 
any software that wrote to $FF22 will probably  have to be changed. None 
of the software using $FF22 expects bit 3 to be  output so it may not 
keep bit 3 high. There is a fair chance that any  software adjusting the 
video PMODE settings, or single bit sound could  turn off your monitor.





OK....I tried the break key and reset and they both turn that line into an  
input.
This turns the monitor back on.    This happens because  there is a pull up
resistor on the PIA line that pulls it up when the PIA  signal becomes an 
input
 or "hi z" eventually causing the I2C data line to go high.
 My converter software uses that signal to drive the I2C  data line that
is high by default.  By definition, all I2C devices release (go high  Z) the 
data line when
they don't need it so that other devices can be on the same  line.  This is
called "wire anding".   This also means
that line has a pull up resistor to hold it high until another device  needs 
it.
  The I2C software I have written for the unit releases
the PIA  line by leaving it as an input by default between  data transmitions
to the converter.
 The micro controller does not need or use that signal. DECB and the  service 
manual
also define that line as an input.    Simply writing a high  signal to bit 3 
at &HFF22
will not blank the monitor as might happen in software that alters VDG  video
modes.    It has to be deliberately programmed as an output  AND written
to AND held down permanently.  So, I see little chance of a  conflict.  Why
would any software set it as an output and hold it down unless it  was
driving a special feature on a monitor. (like in our situation).
 
   I think I will leave it as is for the time  being.   If a problem develops
I can always provide an upgraded controller at no cost to the  customer
and without the need to modify the PC board in any way.   The  controller 
is socketed by the way.
 
thanks for the input
Roy