Re: Apple II text blink rate.

[dempson@actrix.gen.nz (David Empson)]

Egan Ford <datajerk@gmail.com> wrote:

> Is the blink rate adjustable?  If not, it is normal for different
> systems to have different blink rates?  Thanks.

The blink rate of flashing text is not adjustable.

In the Apple IIe and later, it is controlled by a counter clocked off
the video scanner inside the IOU chip. It is a fixed rate which will be
identical for every machine. According to "Understanding the Apple IIe"
by Jim Sather, it is 1.87 Hz in the US IIe model. There is probably a
small difference with the International IIe, since its video circuits
are clocked at 14.25 MHz instead of 14.31818 MHz.

In the Apple ][ and ][+, the flash timing is controlled by a simple
analog delay circuit. I don't have a detailed schematic handy, but I
expect it is an R-C delay circuit (resistor and capacitor). There will
be a fair amount of variation between specific machines due to the
margin of error in the resistor and capacity rating (potentially in the
order of +/- 10% from optimum, depending on the rating of the components
Apple used, and it may vary even more as the capacitors age).

-- 
David Empson
dempson@actrix.gen.nz