Posted By
unclouded
on 2020-04-06
04:03:46
|  Re: C16 Expansion Port to User Port
Aha! So:
$FD1X addresses a second 6529B on the Plus/4 but on the C16 simply pulls D2 low via the buttons on the C1531 regardless of whether it's a read or write to the address within the $FD10 to $FD1F range?
On the Plus/4:
- P2 of the user port is shared with the Cassette Sense line, so the C1531 must be disconnected or no buttons pressed in order to use P2 of the user port (either as an input or an output)
On the C16:
- A write of $04 (or any value where bit 2 is 1) to $FD1X when a button is down on the datasette could damage the CPU?
- Plus/4 software that expects a second 6529B (on the C16 cartridge) at $FD1X will write to addresses in the range $FD10 to $FD1F causing potential bus conflict between the CPU and the C1531 when the CPU tries to drive D2 high while the C1531 drives it low (when a button is down)
- A cartridge that depends on a hardware mod to re-route 'CST SENSE' from 'CN3' before use would potentially damage an unmodified C16
- A cartridge that maps the second 6529B to an unused address range would require (minor) changes to the Plus/4 software but if any changes to instructions that write to $FD1X were missed then potential damage again
Complicated! Did I understand?
P.S. Your English is much better than my Italian!
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