Re: Cassette Sense on Plus/4

From: Gerrit Heitsch <>
Date: Wed, 25 Jan 2012 18:41:19 +0100
Message-ID: <>
On 01/25/2012 03:47 PM, Rob Clarke wrote:
>> As you pointed out, using P7 would disrupt the IEC bus if you pressed
>> PLAY on the datasette during data transfer. I assume that's the reason
>> it was changed later when the PLA was added. I don't have a ROM listing
>> of the 264 ROMs, but I'd be surprised if the -04 and -05 KERNAL-ROMs
>> would still be able to use CPU-P7 for the tape sense.
> Having tested it on a +4 where I moved the jumper to J9, the cassette does
> not then work on an -05 kernal. I have a huge excel spreadsheet with *all*
> the known kernal ROMS dumped into it. It's all in hex and not a
> dissassembly but it does highlight the differences between adjacent ROM's.
> If I look at the differences between 05 and 01, I am not even sure the 01
> kernal supported it, the differences do not seem to related to handling
> $01.
> I seems peculiar that the +4 board is the only board to have the jumpers
> when it was one of the later evolutions from the original 9-IC TED.

The question is, what was the sequence in the 264 development? I think 
the C16 was the last one. I wouldn't surprise me if the layout of the +4 
board was done while the change to the PLA and extra logic was 
relatively recent and they wanted to keep their options open.

BTW: The C16 board also has 2 unexplained jumper blocks on the board 
next to U11 (74LS125) labeled D1 and D2. They are not mentioned in the 
schematics. Anyone have an idea what they are for?


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Received on 2012-01-25 18:00:28

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