Under what circumstances will a passive HDMI-to-VGA adapter work?

I have bought a no-brand, passive HDMI Male to VGA Female adapter, to link up a PC with a more dated monitor. It did not work.

After doing some reading, I have learned that buying such an adapter is often discouraged because in many cases it will not convert the signal from Digital to Analog, or the Motherboard/CPU on a specific computer might not be compatible with that particular adapter.

So my question is: seeing how there are so many of these passive adapters on the market, are there circumstances under which they actually do suffice to connect an HDMI output to a VGA input? Or are they generally just a rip-off?

Example of the kind of adapter I bought:

1 Answer

are there circumstances under which they actually do suffice to connect an HDMI output to a VGA input?

There are no "passive" HDMI to VGA adapters. It requires active signal conversion. HDMI uses digital data sent over differential pairs, VGA uses analog voltage levels sent over single signal wires per colour channel.

I use a cheap plug and play, line-powered, HDMI to VGA adapter to connect a Raspberry Pi 4 to an old VGA monitor. It works well, needs no external power, no drivers, no setup, but it is not passive ("Supports output up to UXGA/1080p with 10-bit DAC").

^{Not much larger than a VGA connector but large enough to contain active circuitry powered from HDMI pin 18}

Compare

PIN CONNECTIONS FOR TYPE A HDMI CONNECTOR
HDMI PIN NUMBER SIGNAL
1 TMDS Data 2+
2 TMDS Data 2 shield
3 TMDS Data 2-
4 TMDS Data 1+
5 TMDS Data 1 shield
6 TMDS Data 1-
7 TMDS Data 0+
8 TMDS Data 0 shield
9 TMDS Data 0-
10 TMDS Clock+
11 TMDS Clock shield
12 TMDS Clock-
13 CEC
14 HEC Data-
15 SCL (Serial Clock for DDC
16 SDA (Serial Data Line for DDC
17 DDC / CEC / HEC Ground
18 +5 V Power (50 mA max)
19 Hot Plug Detect (1.3) / HEC Data+ (1.4)

with

VGA Pinout Configuration
Pin No. Pin Name Description
1 RED Red video (75 ohm, 0.7V peak-to-peak)
2 GREEN Green video (75 ohm, 0.7V peak-to-peak)
3 BLUE Blue video (75 ohm, 0.7V peak-to-peak)
4 ID2 / RES Monitor ID Bit 2 / Reserved
5 GND Ground
6 RGND Red Ground
7 GGND Green Ground
8 BGND Blue Ground
9 KEY +5V DC output from graphic card
10 SGND Sync Ground
11 ID0 / RES Monitor ID Bit 0 / Reserved
12 ID1 / SDA Monitor ID Bit 1 / I2C bi-directional data line
13 HSYNC Horizontal Sync
14 VSYNC Vertical Sync
15 ID3 / SCL Monitor ID Bit 3 / I2C data clock

So far as I can tell, there is nothing in common that could usefully be passed through using a passive connector.