Atari ST Display Basics

[IngoQ]

I just checked pin 8 on STE & STM is via 1.2K resistor. Assuming a typo Ingo ?

In only have a STE, the information about the 520ST having GND connected on Pin 8 is based on this pinout:

http://info-coach.fr/atari/hardware/int ... _connector

Since it is obviously wrong I corrected it.

I did a little research, and found that many sites show this wrong pin out. This page did a little research where this mistake actually came from, and it seems to be based on the Book "The Concise Atari ST Programmer's Reference Guide" by Katherine D. Peel, ISBN 1 85181 017 X.

[IngoQ]

Yep, here it is:

2018-03-18 14_19_32-The_Concise_ST_68k_Prog_Ref_Guide.pdf.png (342.92 KiB) Viewed 10478 times

[IngoQ]

What confuses me further is, that Atari calls pin 8 in their schematics "mode switch". Since it is only connected to 12V this does not make any sense to me. One could assume that this is the signal that pin4 needs to switch to mono mode. But this would be GND, so why "mode switch"?

[exxos]

Yep, here it is:

So we have a bit of a mystery then... At this point I would assume the image has the wrong info there.. Though unless someone comes across a ST with no 12V on pin 8.. then there must be some "early early" design out there which pre-dates even the schematics..

[IngoQ]

Found a possible explanation... SCART uses a 12V signal to differentiate between RGB and composite inputs. As you can see i.e. here, many SCART cables use pin 8 to provide these 12V. And on SCART connectors, this is actually called "mode switch".

So my theory would be, that this was actually intended by Atari. But this would leave us with absolutely no reason, why any machine would NOT have 12V on this pin.

[troed]

(I've started to doubt what my 520STM does. I'm currently running it on 5V only waiting for a picoPSU to arrive, so it obviously cannot put out 12V .. )

I guess we'll need to find someone with a 260ST to see what they had on pin 8 Now with the picoPSU connected to my 520STM I can verify that it has 12V, as the schematics say it should.

/Troed

[displaced]

Hi! Just a small addendum/correction:

I have a UK 520STFM and Pin 2 definitely has a composite signal, despite this being indicated as STE-only.

I don’t have the board revision to hand, but it’s the odd one with the TOS ROMs under the PSU, with the RAM chips below (where ROMs are on most boards)

[rubber_jonnie]

Hi! Just a small addendum/correction:

I have a UK 520STFM and Pin 2 definitely has a composite signal, despite this being indicated as STE-only.

I don’t have the board revision to hand, but it’s the odd one with the TOS ROMs under the PSU, with the RAM chips below (where ROMs are on most boards)

Is it one of these: https://www.exxosforum.co.uk/forum/viewt ... f=56&t=717
?

[displaced]

Is it one of these: https://www.exxosforum.co.uk/forum/viewt ... f=56&t=717
?

That's the one -- a C103253. Mine's a C103253 Rev. 1

[rubber_jonnie]

Is it one of these: https://www.exxosforum.co.uk/forum/viewt ... f=56&t=717
?

That's the one -- a C103253. Mine's a C103253 Rev. 1

Yeah, seems to be a bit of an oddity that one, I ended up using mine as a test board for my PAK 68/2 accelerator. I will say it has been very stable and reliable.