BBC troubleshooting (remote debugging)?

From: Andrew Davie <adavie_at_mad.scientist.com>
Date: Wed Feb 25 04:24:55 1998

Hi all.

Thanks to the BBC people who have helped me so far in my quest to restore to
life my BBC-B.
I have some detailed descriptions of the problem, hopefully to help you help
me. Actually its kind of interesting, as I'm sure the problem can be
deduced without touching the hardware at all.

But first, I have fairly carefully made sure ALL socketed chips are firmly
pressed down. And, to be sure, I tried the obligatory half-inch-drop test.
More like a 3g WHAM actually. Nothing fixed, nothing further broken :) So,
here's a sequence of events and what happens...

Power on - BEEP
you see just a flashing cursor about 1/3 way down the screen
Interestingly, you seem to always see the cursor in the correct spot on
screen - this is kind of strange as it doesn't follow the problems evident
in the characters - I'd guess that the cursor is some sort of hardware
cursor rather than software, and applied independantly of video memory - am
I right?

So, you see a flashing cursor - I also guess that the top few lines above
the cursor are some sort of boot message for the machine. You don't see
anything but the cursor.
type (AAA, for example) and you see the cursor move horizontally. But no
characters.
If you power up again, and hit about 9 returns, you see a >
that is, the prompt. But, you also see it all over the top half of the
screen repeated every 64 character positions. I counted. The screen is
currently in 40 x 25.
type AAA and you see AAA beside every prompt on the screen (about 9, from
memory).
delete the AAA, back to the >
now type 12345678901234567890123 etc
you can type exactly 31 characters (32 with the prompt included) and then
you can't see what you're typing anymore.
Delete all that.
Hit Returns to the bottom of the screen. A prompt appears, and the bottom
of the screen is also filled with prompts every 64 characters.
now do the 1234567890 etc.
You can type 63 (64 with the prompt) and as you type, the lower screen fills
(ie: beside every prompt) with what you're typing. After 64, you've filled
the lower screen and can't see what you're typing anymore. Typing more
characters DOES NOT erase the characters under the cursor (ie: you're seeing
different video memory from where the data is being stored).
When lines scroll off the top of the screen, those lines appear again at the
bottom of the screen.
I did see some interesting random colours and corruption on the screen, but
only once.
now
type CLS
type 40 As
40 Bs
Cs, Ds, Es, Fs.
you get a beep (limit is 6 lines)
press return
all you see is the flashing cursor (screen is empty)
now
Gs
Hs
Is
Js... you type 8 Js, and suddenly you start seeing what you type another
32 Js
Ks .... you type 32, and the entire top of the screen is now filled with Js
and Ks. the next 8Ks are invisible.
Ls... BEEP (6 line limit)

Ms
Ns
Os
Ps
Qs
Rs.... BEEP
NONE of the above 6 lines appear onscreen

RETurn

> appears in bottom half of screen (7 of them, in the 64 spacing pattern)
type Ss.....
when you get to the last 8, they appear (with the >) beside the Ks where the
last 8 were unseen.


So, quite a puzzle.
64 is a magic number, implying to me that somewhere a 7th bit is misbehaving
on an address.
I'd appreciate any comments and insights on this pattern.

Where do I go from here?

Cheers
Andrew
Received on Wed Feb 25 1998 - 04:24:55 GMT