Date   : Tue, 13 Sep 2005 18:53:20 +0100
From   : Jules Richardson <julesrichardsonuk@...>
Subject: Re: 2nd processor fault finding

Richard Gellman wrote:
> Rob wrote:
> 
>> Hi All,
>>
>> OK I was lucky enough to pick up a 6502 second processor off ebay.  It 
>> came as a board only; no PSU or cables.
>>
>> My problem is I can;t get it to work... I've made a ribbon by cutting 
>> down an old 40 conductor IDE cable, but when the board is connected to 
>> the beeb, it locks up.. (specifically, a B presents no display and a 
>> constant beeb, and a Master boots up fine, but locks up with no 
>> keyboard input accepted.)
>>
>> I've tried it with the 2P powered on and off.  I've even swapped the 
>> tube ULA and swapped it with the one on a Master internal 6502, and it 
>> worked in there.  The odd thing is I get the same symptoms with the 
>> tube ULA removed from the board, when my reading of the circuit 
>> diagram indicates that nothing on the ribbon connects anywhere else!  
>> The ribbon alone connected to a beeb doesn't affect it at all, which I 
>> would have thought it would if it were a problem with the cable.  I'm 
>> presuming I only need a straight through cable..?
>>
>> Any clues anybody, or shall I get back onto the seller - he did 
>> advertise it as working.
>>
>> Rob.
> 
> 
> Yes, the cable should be straight through. Its interesting to note the 
> lack of keyboard response - since the TUBE connector shares no lines 
> with the slow peripheral bus (of which the keyboard shares with other 
> devices). If this is a definite fault, I would investigate the BBC 
> Master circuit board as well.

How come your board was advertised as working yet came with a missing 
cable? That sounds a little suspicious on the part of the seller (unless 
they mean "it was working x years ago before I tore it all to bits")

I'd check power supply voltages first I think, followed by a visual 
inspection for shorts / bad solder joints. Then check RAM chips to see 
if any are obviously running hot and indicating chip failure (they don't 
always die this way sadly!). Probably put a 'scope on the CPU reset and 
clock lines next if you have one and make sure you're getting activity 
there.

When you say you can get the system to boot but with no keyboard 
response, does it actually report the 2nd processor on the display? Or 
do you mean it gives the usual Master boot messages from the Master 
itself and then stops without allowing keyboard input? (the latter's the 
exact same symptoms I get with the 80286 copro in the ABC310 machine - 
that with a B+ I/O processor rather than a B or a Master board. Wonder 
if it's the same fault...)

cheers

Jules