Re: [Re: [Re: Using 28F256 in a 747 instead of 2732A??????????]]

[Dig <turbodig@usa.net>]

Ludis Langens <ludis@cruzers.com> wrote:
> There's no ~WE on the MEMCAL.  There is a R/~W on the edge card.  In the
> 727/730/749, there are three unpopulated chip locations meant to add
> 2Kbytes of SRAM.  One of the chips is glue logic that generates ~OE and
> ~WE from the MPU's E and R/~W.
> 
> > I need to look at the CS on the EPROM on the 749... see if it toggles
> > with chip accesses or whether they just pulled it low.
> 
> ~CS is pulled low via a resistor.  The MPU controls ~OE.  Unless I
> remember wrong, for each EPROM access it pulses low for 3/4 of an E cycle.

Dang, that kinda complicates things. I'm wondering, though, if you could
maybe get by waiting for the rising edge of the !OE... once it's off,
you could assume (yeah, I know) that the EPROM was "free", and could 
isolate and write the data.

The big catch here is that you don't know when the MPU is going to want
the EPROM again. You might be in the middle of a series of instruction
fetches, or waiting a while, while it reads an A/D. You really don't need
much time for a single byte, 100 nS or so should be enough. 

Might be a Good Thing to go with the address decode logic anyway... a 
16L8 would be plenty big, and wouldn't take up that much extra board
real estate. You'd know that if the MPU was busy at addresses lower
than $8000, you'd have a little time to play with.

> 
> You might want to take a look at the (unfinished) schematics of the
> 1227748 ECM on my web pages.

I will indeed. Thanks for the URL!

Dig
turbodig@usa.net


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