MTX512 - Screen Trouble

[SchmitS]

Hello

When I turn on my MTX there is a character displayed instead of a space on a few lines of the screen.
Before going further into research, RAM or decoder, have any of you already encountered this problem ?

Thank you in advance for your good advice.

Best regards

MTX512 - Trouble.jpg (1.55 MiB) Viewed 102031 times

[Dave]

VRAM Fault

Look here

http://www.primrosebank.net/computers/m ... epairs.htm

Regards
Dave

[Martin A]

There's a write up of VRAM test and repair here http://primrosebank.net/computers/mtx/r ... repair.htm

The reverse quote has an ASCII code in hex of #60. a space has a code of #20.

That #40 difference suggests that there is a rogue bit appearing on D6.

At the time TI designed the VDP, they numbered thier address and data busses the opposite way to modern converntion, so what we would call D6, they would have referred to as D1. RD1 is connected to the VRAM chip at 7G, so that would be the one I would test first.

[SchmitS]

Thank you for your replies

I found the same result that bit 6 was corrupted at 1 on some adresses.
I was hesitating between CPU or VDP RAMs. This means that BASIC uses VRAM as a buffer for keyboard inputs.

Now I have to find new one to replace them.

Have a nice day.

6=36h >> v=76h

MTX512 - wrong caracter.jpg (1.37 MiB) Viewed 102012 times

space=20h >> @=40h

MTX512 - Cursor.jpg (1.34 MiB) Viewed 102012 times

Basic input

MTX512 - Basic Line.jpg (1.5 MiB) Viewed 102012 times

[Dave]

Keyboard scanning ……..

http://www.primrosebank.net/computers/m ... de_kbd.htm

[Martin A]

As far as I've been able to work out, yes, the OS does use the input line on the screen during some part of the command processing. The manual says there is a keyboard buffer at #FB4B i'm not sure though at what stage that gets filled.

If the CPU ram misbehaves, then you'll likely just have a non starting MTX.

[SchmitS]

Hello

It works again, I exchanged VRAM 4116 by 4164.
-5V is no more needed.
I also repaired 2 defected keys.

MTX512 - VRAM.jpg (2.04 MiB) Viewed 101792 times

MTX512 - Board.jpg (3.13 MiB) Viewed 101792 times

MTX512 - Alpha.jpg (1.47 MiB) Viewed 101792 times

MTX512 - Graphic.jpg (1.22 MiB) Viewed 101792 times

[1024MAK]

Nice

Mark

[Martin A]

Shame it's being force to show Toado though !