Re: Microsoft Z-80 Card on Apple II+

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

In article <PtKdnZmUSY8vVRqjXTWQkQ@giganews.com>, "Bryan Parkoff"
<BParkoff@satx.rr.com> writes:

>    I am surprised that Microsoft Z-80 Card does NOT work on Apple IIgs
>while the speed is set to normal.  It is what I understand that it DOES NOT
>support Apple IIgs ROM 01/03, but it can only support Apple II and II+.
>    Please explain why.

Any slot card must meet the setup time and hold time constraints
of the slot in order to communicate reliably.

The SoftCard uses a rather unusual "syncopated" Z80 clock, and
perhaps pushes some of the timing margins a bit.

The IIgs (and to a lesser extent, the //e) implement the peripheral
bus quite differently than the earlier Apples, and, as a result, shifts
the timing constraints.

It appears that the Softcard and the later bus implementations
move timing margins in opposite directions, resulting in unreliable
communications.

(BTW, it was just such timing constraints that require the //e to
use a 2MHz 65C02 at 1MHz--in this case because of the ASIC
implementation of clock generation, bus logic, and decoding.)

-michael

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