My Atari ST to VGA Adapter

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I had to build an ST to VGA adapter because my SM124 monitor suddenly ceased to function in 2020‑05. My adapter only supports the ST "high" resolution 640x400 pixels, monochrome, not the two colour modi.

Long time ago, I had put my ST in a custom case and equipped with a "Rhothron" compatible DB‑9 connector (and the SM126 with a DB‑9 plug). Because of this, "Rhothron" is referenced in the middle column.

Atari Connector (DIN? 13-pin)	"Rhothron" DB-9	VGA DB-15
	1 unused?
1 Audio output (yellow)	3
2 H+V-Sync
3 Port A, bit 6, Sound-chip
4 Monochrome sensor (black)	8
5 Audio input
6 Green output, 75 Ohm
7 Red output, 75 Ohm
8 +12 V, 10 mA max. (black?)
9 H-Sync (red)	5	13
10 Blue output, 75 Ohm
11 Mono chrome output, 75 Ohm (clear+shield)	2	1 red, 2, green, 3 blue
12 V-Sync (green)	4	14
13 Ground (black)	6, 7, 9	6, 7, 8, 10
		4 Monitor id bit 2
		5 n.c.
		9 no hole (protect)
		11 Monitor id bit 0
		12 Monitor id bit 1
		15 n.c.

The column VGA DB-15 shown is according to German periodical c't 1990 no. 9.

The graphics from Hallvaard Tangeraas from 2002‑09‑30 calls the monitor id bits "Sense". Pin 15 is Sense 3 (id bit 3) in addition. Pin 5 is called Self-Test (TTL ground).

VGA according to en.wikipedia.org:

Pin	Short Name	Description
1	RED	Red video
2	GREEN	Green video
3	BLUE	Blue video
4	ID2/RES	Reserved since E-DDC, formerly monitor id. bit 2
5	GND	Ground (HSync)
6	RED_RTN	Red return
7	GREEN_RTN	Green return
8	BLUE_RTN	Blue return
9	KEY/PWR	+5 V DC (powers EDID EEPROM chip on some monitors), formerly key
10	GND	Ground (VSync, DDC)
11	ID0/RES	Reserved since E-DDC, formerly monitor id. bit 0
12	ID1/SDA	I²C data since DDC2, formerly monitor id. bit 1
13	HSync	Horizontal sync
14	VSync	Vertical sync
15	ID3/SCL	I²C clock since DDC2, formerly monitor id. bit 3

The Atari ST has a 13‑pin female connector. The DB‑9 connector at the case outside is female as well. Hence the Rhothron to VGA adapter has to have a male DB‑9 connector and a female DB‑15 plug.

The wiring inside the adapter:

Rhothron Pin 4 (monochrome sense) shall be wired to ground (6, 7, 9), such that the VGA monitor is recognised as Atari ST monochrome, 640x400.

To match the 75 Ohms impedance of the three VGA inputs to the ST's monochrome output, three resistors of 150 Ohms need to be put in series (because 150 Ohms + 75 Ohms in parallel equals 75 Ohms). Hallvaard Tangeraas recommends 47 Ohms or a 100 Ohms adjustable resistor.

I tried it out using an Oscilloscope. Three times 150 Ohm is OK, but three times 100 Ohm seemed to provide faster rise times, so I kept 100 Ohms.

I left the monitor id bits 0‑4 open. The monitor does not actively use them, rather a "graphics card" would, querying the monitor.

The audio output has an impedance of ? and does not have a DC component. I fitted it with a 3.5 mm female connector.

Atari ST H-sync. and V-sync. have a much too low output impedance for the VGA cable. Hence there would be serious reflections if just wired 1:1. Hence there shall be a series termination resistor in each of the two lines. I experimented: for the high to low transition 56 Ohms seemed optimal, for low to high 39 Ohms. I settled for 47 Ohms. On the web you can find the advice to rather use capacitors to dampen the reflections. I do not recommend this as it would also distort the timing, especially for H-sync.

Rhothron DB-9 male, Solder Side

  H   V   A   M
 --------------------
\ 5   4   3   2   1 /
 \  9   8   7   6  /
   ---------------
    G   G   G   G

VGA DB-15 female, Solder Side

     R   G   B       G 
 ------------------------
\    1   2   3   4   5   /
 \ 6   7   8   9   10   /  <- all G, except 9
  \ 11  12  13  14  15 /
    ------------------
             H   V

Monochrome VGA Adapter Monitor Check

Check against Fujitsu B24-9 WS 24" LCD monitor

Native resolution=1980x1200: I cannot determine the exact resolution shown for the ST signal but about 80% of the screen width is used and similar for the screen hight. The monitor does interpolate obviously

Check against MD9915AF 15" LCD monitor

Native res=1024x768. Result: does not work. The monitor just does not seem to be prepared to do 640x400. It should be able to do 640x480 though, according to its manual.