Re: (no subject given)

[Rubywand <rubywand@swbell.net>]

Michael A. Balfour writes ...
> 
> comp.emulators.apple2
> Subject: Apple II hardware question
> Reply-To: mab22@po.CWRU.Edu (Michael A. Balfour)
> Organization: Case Western Reserve University, Cleveland, OH (USA)
> 
> This might sound like a silly question, but I really don't understand:
> how does Flashing text work in the Apple II series of computers.  I've
> been looking through Apple IIe schematics, and I don't see any video
> invert signals tied to the output of the character generator ROM.
> 
> My assumption is that the IOU chip is switching between normal and
> inverse text at some unknown constant rate.  Can anybody > confirm/refute
> this assumption?  Any idea what that rate is?  Does anybody have Apple
> II+ schematics where the IOU logic was all contained in TTL logic?
>

 ....

     It has been a while since messing with II+ video circuitry. A quick
look at the diagram indicates that the FLASH chars are produced via a
555 timer IC which produces an ON/OFF inversion at a video shift
register input for characters with the FLASH char codes.

     The approach indicated by the IIe schematic appears to be
different. That is, for FLASH, the IOU chip seems to switch between
NORMAL and INVERSE character sets on the character ROM as you suggest. 
     There appear to be no external IOU R/C timing components. This and
the fact that the IOU appears to supply V Sync implies that the IOU
employs a countdown chain to get ROM address line changes at the flash
frequency which seems to be about 2cps.

 

Rubywand