Re: Speed of AE Z80plus ?

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

Steven N. Hirsch wrote:

>Quadrajet1 wrote:
>>   I do recall the AE Z-80 Plus had 2 different models.  One had a red LED
>and
>> one had a green one.  The green LED model was the fastest of the two.
>
>Must be placebo effect.  These were a complete knockoff of the Microsoft 
>  Softcard and they all ran at 2x (IIRC) motherboard clock speed: 2Mhz.

Yes--the absence of a real-time clock on the Apple II engendered a
lot of subjective speed comparisons which were often misleading.

>Given the way it interacts with the host processor, there's not a lot of 
>flexibility here.  No doubt Michael Mahon will have the details on the 
>top of his head <g>.

Ha!  ;-)

I never owned an AE Z-80 Plus, so I wasn't sure if it was one with
on-card memory or if it used the Apple memory.

If it used the Apple memory, as the Softcard did, then you are
absolutely correct--there is very little flexibility.  The Apple bus
allows memory accesses no more frequently than once per
(Apple) cycle, or about 1MHz.

The Softcard used a very clever syncopated clock derived from
the 2MHz Q3 clock and the divided-down 7M clock, so it ran
with "peak" speed of 3.58MHz and average speed of 2MHz.
This timing allowed the 6502 to "keep alive" during the DMA
accesses the Softcard made.

The details on the Softcard were published in Electronic Design
in the late 1980s, but I can no longer lay my hands on the article.

Because the Z-80/8080 processors are much more CISCy and
firmware-intensive, they take about four clocks to get done what
a 6502 can do in one, so Softcard-derived designs always seemed
somewhat sluggish compared to native Apple speed.  (Unoptimized
screen I/O that switched to the 6502 on each character compounded
the sluggishness.)

On-board memory allowed broad clocking flexibility, as you
note below...

>CP/M boards which communicated using port IO were generally faster, 
>since they needed no lock to the 6502 clock.  I was involved in the 
>development of the CardZ180, which ran a Hitachi HD64180 at 9Mhz.  One 
>of these in an accelerated IIgs was quite snappy!

An easy rule-of-thumb to tell the speed of a Z-80 card is to look for
a crystal oscillator on board.  If there is one, that's the clock speed.
If there isn't, it's 2MHz.

-michael

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