Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!rutgers!ucsd!sdcc6!sdcc10!cs163afu
From: cs163afu@sdcc10.ucsd.EDU (Some call me...Tim)
Newsgroups: comp.sys.atari.st
Subject: Re: cartridge port addressing
Summary: Cartridge CANNOT have a write...
Message-ID: <116@sdcc10.ucsd.EDU>
Date: 24 May 89 03:31:56 GMT
References: <419@carroll1.UUCP>
Reply-To: cs163afu@sdcc10.ucsd.edu.UUCP (Some call me...Tim)
Organization: University of California, San Diego
Lines: 38

In article <419@carroll1.UUCP> dnewton@carroll1.UUCP (Dave Newton) writes:
>
>   A lot of people (including myself) have wanted ATARI to put expansion
>slots on their machine.  What I am considering is this:  get a whole big
>bunch of people to come up with a standard interface, publish it, and get
>hardware for it.
>   What I need to know is this:  A bus error (illegal address) is generated
>when writing to the cartridge port.  Is this error hardware or software
>generated? 

The error, so I'm told, is generated in hardware.  It's hardwired
into one of the custom chips of the ST--probably the MMU, though
someone tried to tell me it was the GLUE chip.

A better interface would be DMA based--1.3 Megabytes per second is a
sufficient transfer rate for almost anything.  A card cage with a
simple bus based on the DMA port would be best.

Of course those of us with Megas have a bus expansion option that
could easily be brought out into a card cage, with some simple
electronics.  Depending on what kinds of loads you expect, it may
also need its own power supply.

One alternative that works for simple expansions (home grown ones
come to mind, but a multi-joystick expansion uses this path) is
through the parallel port.  For some reason, it's bi-directional--
you could communicate with a device plugged in there.  

I've yet to do any hardware hacking on the ST (other than installing
many ROM sets and a 4096 color board), though I've done some hacking
on other computers...but the DMA port is somewhat intimidating,
probably because of my lack of documentation covering it...

_________,_________
  /im |\/|ensch	    |	"Life is full of infinite absurdities, which,
      '	 	    |	strangely enough, do not even need to
tmensch@ucsd.edu    |	appear plausible, since they are true."
					--Luigi Pirandello.