Date   : Mon, 03 May 2010 05:41:50 +0100
From   : afra@... (Phill Harvey-Smith)
Subject: 68B54 memory locations and ANFS puzzle

paul aslin wrote:
> Actually even the 16 possible addresses aren't fully accessible to
> existing econet/ethernet cards as none physically plug into A2 and
> A3. Hence are limited to 4 addresses.
> 
> With a bit of extra hardware its possible to use the remaining 12
> addresses for an internal upgrade, say a hard drive controller.  

That is true however because A2 and A3 are not passed on to the Econet 
card it will act on accesses &FEA0-&FEAF, irrespective of the state of 
A2/A3.

What this means is that you would have to decode A2/A3 and then gate the 
   !CS signal to the ADLC in such a way as it was only active when both 
A2/A3 where low.

This should be doable with something like an LS139 and an OR gate.

Cheers.

Phill.

-- 
Phill Harvey-Smith, Programmer, Hardware hacker, and general eccentric !

"You can twist perceptions, but reality won't budge" -- Rush.