From: Marko Mäkelä (marko.makela_at_hut.fi)
Date: 2005-07-24 21:56:21
On Sun, Jul 24, 2005 at 01:42:12PM -0500, Jim Brain wrote:
> To all: I've heard this myth of joyports being read-only often. Is it
> because the joysticks are read-only?
On the plus/4, they are, if I remember correctly. On the plus/4 also
CASS SENSE is read-only, again, if memory serves me right.
On the VIC-20, the CASS WRITE line is shared with the joystick/keyboard.
I started with the C64 and remember someone claiming that touching the
keyboard or joystick while loading would cause errors. In fact, only
the VIC-20 is affected, and only when saving. (If you have a C2N232,
run "c2n -d" and type something on the VIC-20.)
So, because the VIC-20 shares an (in principle) write-only line with
an (in principle) read-only line, we can conclude that the joystick
ports are not read-only. The C64 has very similar joystick port
and keyboard connection.
It should be obvious that the ports are bidirectional if you have a
look at the schematic diagram.
I think I've seen documentation (maybe in the Linux kernel) of some
fancy digital joysticks that use a bidirectional bitbanging protocol.
Maybe they were for the Amstrad or some less widely spread machine.
Marko
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