6540 Select Signals (Was: Re: 6550 pinout?)

From: Jim Brain <brain_at_jbrain.com>
Date: Sun, 23 Aug 2009 10:14:38 -0500
Message-ID: <4A915CDE.2000006@jbrain.com>
donni wrote:
> That pinout is not entirely correct.
> Pin 2, 3 and 4 are labeled CS3, CS4 and CS5 in that picture.
> That should be: _CS3, _CS4 and _CS5.
> -donni
> ----- Original Message ----- From: "Spiro Trikaliotis" 
> <ml-cbmhackers@trikaliotis.net>
> To: "CBM Hackers" <cbm-hackers@ling.gu.se>
> Sent: Sunday, August 23, 2009 9:19 AM
> Subject: Re: 6550 pinout?
>> Hello Jim,
>> * On Sun, Aug 23, 2009 at 01:30:45AM -0500 Jim Brain wrote:
>>> Anyone have a link handy for a 6550?  I'm not seeing anything.
>> http://www.cbmhardware.de/cbmpet/cbm2001mos.php?Lang=D&from=pet2001
Spiro, thanks, it appears I found that link via:


but, the php3 in there caused a redirect.

Donni, I modeled my 6540 adapter after your via (!CS3,!CS4,!CS5), but 
both of our views tend to contradict the poster "ghaytack" in the above 
thread.  For context:

"The 6540 were somewhat strange (but useful!) mask programmed ROMS made 
by Commodore. They are basically 1Kx8 but have 28 pins! This comes from 
the fact that they have several chip select pins. *To make life easier 
for system designers the function of the chip select pins could be 
chosen as part of the mask programming option* so you could get four 
different 6540's where each has a different combination of active high 
and active low select pins.

This made things a doddle for the designers of the PET as they could 
miss out a chunck of addresss decoding for the ROMS and connect the 
lower 13 address lines (for 8K) directly to the chips with the A12 
through A9 connected directly to the select pins of ALL the chips. One 
6540 could have (say) pin 26 as an active high CS while it's neighbour 
could have pin 26 as active low CS. "

Can anyone shed any light on the discrepency


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Received on 2009-08-23 16:00:05

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