Date   : Sun, 09 Jul 2006 15:51:07 +0100
From   : Rob <robert@...>
Subject: Re: interfacing3

At 09:23 09/07/2006, you wrote:


>How may I "safely" interface to it? Without breaking anything, esp. the
>BBC's VIA chip, I want to detect when a voltage in the range of about 2V
>appears. Can I put a transistors collector and emmitter into the
>userport between a dataline and ground and connect my 2V logic to the
>base?


The pinout of the user port can be found at the back of the original 
BBC Micro User Guide.  It's basically direct connection to the 6522 
port B, so only the internal pull-up resistors in place.   I don't 
see why your circuit shouldn't work, although I can't comment on the 
values of the resistors.  Start with high values to limit the current 
being drawn, and reduce if necessary.  I'm sure you can find some 
example interfacing circuits on the net.

b.t.w. reember to set the ports as inputs, and I guess that you 
should be using OSBYTE &97/97 to read/write from the user port 
registers... especially if you want to use it on a machine with a co-processor.

Rob