Date   : Sat, 28 Jul 2001 12:10:52 +1200
From   : Michael Foot <mikef@...>
Subject: Re: Help with BBC B fault

With a multimeter, I've now tested the 555 against the 6502 on both a working
BBC and the faulty one.

During power on and reset (pressing break) all pins from the 555 to the 
6502 pin 40 (reset line) read the same on both machines. However, on the 
faulty machine, pin 1 of the 555 to pin 4 (IRQ) of the 6502 stays high during 
a power on and a reset. On the working BBC this goes high, then low, then
high again. Pressing break sends this low before it returns high again. On
the faulty BBC this does not change.

So, it appears the IRQ line is stuck high all the time. What could be the
cause of this?

Thanks,

Mike.


-- 
Michael Foot <mikef@...          >
http://www.voyager.co.nz/~mikef