Re: Pot on Language card: what for?

[mjmahon@aol.com (Michael J. Mahon)]

Linards Ticmanis asked:

>another stupid question. I have two II+ language cards, the standalone 
>versions that do their own refresh and that don't need a cable to one of 
>the RAM chips on the II+ motherboard.
>
>Those cards have one pot each on the PCB. What does this pot do? Adjust 
>refresh cycle time maybe? Which way would you have to turn it for slower 
>refresh, if that is the case?

I would hope not!  The least I would expect of on-card generated refresh
is that it is synchronous with the Apple bus, so it could refresh in the
video access time slot, and always appear ready to the processor.

If that is done, there is no advantage to varying refresh rate.

The design of the video refresh to automatically perform the required
DRAM refresh during alternate cycles, when the 6502 does not access
the bus, was one of the cleverer optimizations that Woz used to make
the Apple II hardware simpler.  (Programmers who don't speak hardware
always complain about the non-linear mapping of Apple video memory
to screen coordinates, but the base address for any line is a single
192-byte table look-at away, or a handful of bit-mashing instructions.
Not a bad tradeoff, I say.)

-michael

 Email:  mjmahon@aol.com
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