New odd problem on the master

[vexorg]

I thought I had most of the issues sorted on the master, and only mode 7 wasn't working. I had left it for a bit due to other work.

I was expecting just to sort the mode 7 problem and it's behaving a bit odd.
I only get "Acorn MOS" and the * below it. it doesn't respond to any command, just simple goes to new line with *.
Holding 'R' on power up doesn't clear the memory, the R key us working, Used to say "This is not a language"
And sometimes the screen is blank at first with break needed for "Acorn MOS"

If you press break again sometimes the "Acorn MOS" is wrapped over, so the the A and * are on the other edge of the screen.

I thought it could be the battery connector as it was poor, so replaced that with new 3 pin header, no different.

Has anyone seen that before I start from the beginning again on possible faults?

[wiggy]

We've seen before a weird case where the CMOS apparently resets to all ones rather than all zeros (or vice-versa - whichever it shouldn't be, anyhow) - and then locks up.

What does *STATUS show? Or are you not getting that far?

The other "common" issue with Master weirdness is bad contacts on the MOS ROM, particularly if there's been a carrier board in the socket at some point.

[vexorg]

Nothing in response to any command:

and pressing break a few times gets the weird wrap over, sometimes 1 character, sometimes 2:

And still works the same with no response to commands, just shifted.

I had lifted and re-seated the MOS rom too. It was very tight, flats crewdriver digging into the plastic at the end before it would move.
Pins and socket looked fine, looks like the double leaf socket.

[vexorg]

I've ordered a cmos ram/rtc chip, and will get the scope out to see what's going on with it

[arnoldemu]

The weird wrap over looks like the the CRTC 6845's Register 8 Display skew has been set for mode 7 but it's displaying a different mode.
EDIT: Check the 5050 'teletext chip' is enabled to output.

[vexorg]

Mode 7 never worked right when it was all kind of working before. Just a blank screen when mode 7 was selected.

[vexorg]

Got hold of a cmos chip as the issues all seem to be to do with startup.
Not looking the healthiest underneath, looks like the cmos chip has rusted

Some cleanup with IPA

And new socket to finish

There are some improvement with the new cmos chip, it now say clearing cmos memory when I hold R on power up.
Then it's much the same.
I've also found ctrl-F and break goes into terminal mode,
And ctrl-D is DFS 1770 mode, but with no cursor or responding to keyboard.
And powering up pressing break gives the ACORN MOS and cursor as above.

[vexorg]

I've found the * commands work in terminal.
So STATUS looks like everything is set to 1, and CONFIGURE isn't setting them.
LANG is 15, FILE is 15, DELAY 255, TV 255,1. etc, all look like max values (or all read as 1)

Is there a documented boot sequence somewhere the shows what happens at start up?
Just trying to see if it's reading the cmos as it should or if the cmos is wrong.

[wiggy]

Yep, that's the "stuck CMOS" bug. I had one which did this only sometimes - reset it three or four times and you'd get to all zeroes and everything happy. Others have had to swap the CMOS chip.

Beebmaster wrote a program (viewtopic.php?p=364601#p364601) to try to figure out what was going on, but I'm not sure that we every got anything useful back.

There's also the incompatibility between pin 1 on -A suffix and non-A parts to check for: viewtopic.php?p=373236#p373236

[vexorg]

Will need to have a look at the lines more closely, and the LS259

[vexorg]

Problem fixed!!

The new cmos chip was the -A variant with MOT pin 1. This was floating around 0.5v.
If tied to 0v then it rarely booted up, hard to get the ACORN MOS message.
Left floating was random effects, and CMOS all read as '1'.
Pulling the pin 1 to 5v fixed it, cmos reset put everything to zero.
*Configure set everything and it stayed, power off and on and cmos settings still good.

Just need to get mode 7 fixed to finish it off, blank screen for mode 7.