On 01/24/2012 10:40 AM, Rob Clarke wrote: > Hi all, > > I have just built a diagnostic harness for the Plus/4, C16 and other TED > based machines. Partly because I needed one and partly just to see if I > could. > > As part of the test suite I check the operation of all the cassette lines > using a loopback connector. (You can find the whole thing documented here > : http://inchocks.co.uk/commodore/Diag264/HTMLManual/Diag264.htm ) > > One of the things that has mystified me somewhat is the cassette sense. On > a C16/C116/C232 the sense is linked to the input of a buffer/driver which > is in turn enabled by access to the address range $FD10 - $FD1F, the > output being fed into D2. > > In a Plus/4 the 6529 on the user port sits in this address range and > although the sense line is still routed into P2 (by J8), pressing play on > the cassette would obviously have disrupted any activity on the user port. > > To work around this, there is the option on the board (J9) to instead > route cassette sense to P7 of the CPU I/O port, which is normally used for > the serial port DATA IN line, so any serial communication would have been > equally screwed up. As far as I can deduce from what I know about the history of the 264, using P7 of the CPU for tape sense was the original design of the C116. C16 and Plus/4 came later. As you can see in http://plus4world.powweb.com/hardware/390132 the C116 prototype shown there doesn't have the PLA and therefore has no decoding logic for any I/O-devices (it also lacks the banking logic). So the only built in I/O was the port in the CPU which had to handle everything. Remember, it was supposed to go against the ZX spectrum an compared to that, it had plenty of I/O... 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. It also does annoy me that on the Plus/4 tape sense disrupts the user port, but if you need more I/O, you could use Pin 18 (F0) of the PLA as a chip select for a 6522A or similiar. It should appear in $FD2x. I think that space was supposed to be used for the speech synthesizer. Gerrit Message was sent through the cbm-hackers mailing listReceived on 2012-01-24 17:00:03
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