Date   : Tue, 05 Dec 1995 16:39:39 +0000 (GMT)
From   : Philip Blundell <pjb27@...>
Subject: Re: Reading from the Serial ULA

On Mon, 4 Dec 1995, James Fidell wrote:

> The serial ULA has one (apparently) write-only register.  As I'd kind of
> expect, reading from this register returns zero.
> 
> However, if I turn the cassette motor on (which effectively sets bit 7
> of this register) and then read the register again, I get the value zero,
> but the cassette motor is turned back off.
> 
> Can anyone explain this bizarre behaviour ?

If the register really is write-only, it could be that the R/'W signal is
just ignored.  Assuming the designers didn't bother to factor this into
the chip enable decoding, *any* access to that address will be seen by the
chip as a write.  If you're actually trying to read, the databus will be
in a high-Z state, so you will write something indeterminate in; 
apparently 0, in this case.  This could be entirely wrong though, I'm 
just guessing.

AFAICR, there is another register which turns the cassette motor *on* 
when read.  I vaguely remember that aiming MZAP or something at a 
particular part of SHEILA would make the cassette relay click on and off 
at quite a rate.


--
philip blundell trinity college cambridge 01223 356256 pjb27@...