Atari ST Monitor Output

Pin Dir Description
1 --> Audio Out
2 --> Composite Sync Out
3 --> General Purpose Out
4 <-- Monochrome Detect In
5 <-- Audio In
6 --> Green Out

Seen looking
into the ST
monitor
socket
Pin Dir Description
7 --> Red Out
8   See notes below
9 --> Horizontal Sync
10 --> Blue Out
11 --> Monochrome output
12 --> Vertical Sync
13   Ground

Notes:

Monochrome Detect input is pulled to 5V by a 1K resistor.
When active (pulled down to 0V) the Atari configures itself to drive a monochrome monitor.

Resolution Low Medium High
Colours 16 4 2
Pixels (X) 320 640 640
Pixels (Y) 200 200 400
Resolution (horizontal) 452 653 895
Resolution (vertical) 585 585 585
Video Bandwidth / MHz 10 18 18
Slot pitch / mm (typ) 0.64 0.41 0.31
Vertical Scan rate / Hz 50 or 60 71.25
Video levels 1V pk-pk, 75R output impedance
Audio levels 1V pk-pk, 10K output impedance
Sync pulses 5V active low, 3K3 output impedance

The Atari SC1224 Colour monitor has a 50/60 Hz scan rate.
The Atari SM124 Monochrome monitor has a 71.25 Hz scan rate.

Information above checked against:
"The Concise Atari ST Programmer's Reference Guide" by Katherine D. Peel, ISBN 1 85181 017 X.

Signals have not been verified by me experimentally. 1V pk-pk video and audio levels are standard, but I have read on another web page that the ST video levels give an over-saturated picture when fed to the RGB inputs of a TV's SCART socket. That page suggested putting 150R resistors in the RGB signal paths to attenuate them. The video output impedance is 75R which is correct, so series resistors will change this. Try adjusting your TV brightness setting first.

That page also indicated that pin 8 was "Plus 12V pull-up", as does "The Complete Atari ST", while Katherine Peel's book indicates pin 8 is Ground. In an effort to resolve this I applied a voltmeter to pin 8 at the ST, and found it was indeed 12V.

Plugs and sockets

These can be bought from Maplin, but the one I bought had solid pins which are more fiddly to solder to than hollow pins that you can stick the wire inside them while your hands hold the solder and iron. Also it is a very cramped space to work in. If one of the inner wires becomes unstuck, you have to undo many outer wires to reach it.

Atari ST to SCART cables

These are available ready made from CPC (while stocks last), but I have no information about the internal wiring as yet. I have requested the information from them.

Atari ST to SCART adapter

I've made one of these myself, and can report that the picture quality is noticeably better. This is good for detailed work but for some images (e.g. games) you may want the pixels to blend into each other a little, and not to appear as a distinct pixel. Putting a switch in the Fast Blanking signal path allows switching between RGB and Composite video input.

Soldering wires in the 13-pin connector is very fiddly, so you should examine each joint with a magnifying glass to check there are no tiny short-circuits. Especially around the 12V output pin!

13 PIN monitor
MALE to the Atari		 Pin       Plug
pin		 21 PIN SCART
to the TV
Audio Out 1 --> -->   2 & 6 Audio in right & left
Ground 13       5 Blue Ground
Blue out 10 --> Level Shifter --> 7 Blue in
Ground 13       9 Green Ground
Green out 6 --> Level Shifter --> 11 Green in
Ground 13       13 Red Ground
Red out 7 --> Level Shifter --> 15 Red in
Ground 13       18 Fast Blanking Ground
Vertical Sync 12 --> 75R --> 16 Fast Blanking
Ground 13 ---     17 Composite Video ground
Composite Video Out 2 --> --> --> 20 Composite Video in
+12 V 8 --> 12V to 5V regulator
for level shifters etc.		      

Fast blanking can alternatively be driven to 2V by 100R tied to the adapter's 5V rail. This will allow you to do without the Vertical Sync co-ax cable.