Date   : Thu, 07 Dec 1995 22:18:29 +0000 (GMT)
From   : James Fidell <james@...>
Subject: Re: Reading from the Serial ULA

Phil Blundell wrote:

> 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.

You are a man of rare insight, sir.

I've checked the circuit diagram and there is no R/'W line, only a chip
enable line.  There aren't even any address lines (because there's only
one register).

> 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.
> 

Possibly the ACIA might be able to do this ?

James.

-- 
 "Yield to temptation --             | Work: james@...      
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