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From: dth@cs.brown.edu (Dzung T. Hoang)
Newsgroups: comp.sys.atari.st.tech
Subject: Re: TOS ROM switch
Message-ID: <73937@brunix.UUCP>
Date: 30 Apr 91 00:13:48 GMT
References: <281c4c3e.26ba@petunia.CalPoly.EDU>
Sender: news@brunix.UUCP
Reply-To: dth@cs.brown.edu (Dzung T. Hoang)
Organization: Brown University Department of Computer Science
Lines: 28

In article <281c4c3e.26ba@petunia.CalPoly.EDU> sslee@polyslo.CalPoly.EDU (Steven Lee) writes:
>
>I'm finally thinking about getting the TOS 1.4 ROMS, however I would
>still like to get access to the old TOS 1.0 ROMS I have now.  (Yes,
>there are some programs I have that insist on TOS 1.0.)
>
>Anyway, what I would like to do is add a simple switch to flip between
>the ROMS.  Sounds easy, right?  I'm sure others have done something
>like this and could fill me in on the details of what problems I
>may encounter.
>
>Thanks,
>Steven

I propose a solution:

Program both TOS ROM sets into 27512 ROMs and use the MSB of the address
to select between TOS versions.  This would involve programming TOS 1.0
into the bottom (or top) half of the '512 ROMs and TOS 1.4 into the other
half.  Pin 1 of each 27512 can be used to select the TOS version.
The 27512 has the same pinout as the 27256 except for pin 1, so the existing
ROM sockets can be used, except that pin 1 of each '512 needs to be bent
90 degrees and soldered together and connected to a switch from either
+5V or GND.
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Dzung T. Hoang
dth@cs.brown.edu
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