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.
|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.
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.
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.
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
|21 PIN SCART|
to the TV
|Audio Out||1||-->||-->||2 & 6||Audio in right & left|
|Blue out||10||-->||Level Shifter||-->||7||Blue in|
|Green out||6||-->||Level Shifter||-->||11||Green in|
|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.