Cassette Sense on Plus/4

From: Rob Clarke <>
Date: Tue, 24 Jan 2012 10:40:21 +0100
Message-ID: <>
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

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
: )

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.

All the documentation I found implies that the default on a plus/4 should
be to use P7 of the cpu, but all the 6 Plus/4 boards I have all use the
same method as the C16/C116 and go via $FD10-D2. So by default, J8 is
closed and J9 open.

Were Plus/4's ever shipped using the CPU for cassette sense by default or
was it only ever an after market change? Maybe it was regional? I have
never actually seen an NTSC Plus/4 so maybe it was the standard in the US?

Anyone know?


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Received on 2012-01-24 10:00:18

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