VGA/Dsub-15 cable - pins shorted internally?! - Page 1
Forum section: electronics / beginners

author: igendel
author_rank: Frequent Contributor
timestamp: June 24, 2024, 01:56:57 pm
content: I have a male-to-male 1.5m VGA cable, 15-pin DSub (apparently it's a Samsung bn39-00244k , so far couldn't find any technical info on it). I thought it will be good for transferring a bit of power and data between two circuits (personal hobby project), but I didn't imagine that a few of the pins would be internally shorted together within the cable itself! I tested this using a multimeter's continuity tester when the cable isn't connected to anything on either side. I think there are 5 shorted pins overall but it's hard to tell without more accurate equipment.So, can you please help me -1. Sanity check: does this make any sense?2. Is it a feature ofall15-pin VGA cables?3. Where can I find a reliable mapping of such internal connections?Thanks!

author: darkspr1te
author_rank: Frequent Contributor
timestamp: June 24, 2024, 02:42:33 pm
content: Google vga pinouts will return a great deal of answers likehttps://www.nextpcb.com/blog/vga-connector-pinoutYes some ground pins are internally connecteddarkspr1te

author: igendel
author_rank: Frequent Contributor
timestamp: June 24, 2024, 03:00:41 pm
content: Quote from: darkspr1te on June 24, 2024, 02:42:33 pmThanks, yes, I've seen this, but there are other pinouts too, which don't specify such a clear connection, so I was wondering if it's in some official VGA spec or just a "convenience".Some pinout legends specify "Ground, Red Ground, Green Ground, Blue Ground, Sync Ground", that seem to match the 5 pins I've identified - still, it's strange that they're specified as separate entities if they're physically connected in the cable itself!

author: jmelson
author_rank: Super Contributor
timestamp: June 24, 2024, 03:24:17 pm
content: Cheap VGA cables just have a bunch of wires in a foil shield, no attempt to provide controlled impedance. The image on the monitor will have terrible reflections and "ghosts". Good VGA cables are generally thicker and have at least 3 coax cables inside for the RGB signals. Even better ones have 5 coaxes, including the sync signals.Jon

author: soldar
author_rank: Super Contributor
timestamp: June 24, 2024, 04:00:09 pm
content: Three wires are used for monitor ID capabilities, pins 4, 11 & 12. i think the standard is that the monitor itself should pull whichever to gnd but I have found sometimes pulling one or more in the cable is needed. Maybe, probably, pins 4 & 11. I am talking from memory. I have a whole bin full of VGA cables but I am not going to spend time.All the other pins and wires are well documented.https://en.wikipedia.org/wiki/VGA_connectorhttps://en.wikipedia.org/wiki/Display_Data_Channel

author: igendel
author_rank: Frequent Contributor
timestamp: June 24, 2024, 04:54:25 pm
content: Quote from: soldar on June 24, 2024, 04:00:09 pmPulling by the video source, or built into the cable? The latter would make my situation a lot worse :-)Quote from: jmelson on June 24, 2024, 03:24:17 pmFor my current application, "a bunch of wires" is actually preferable... I don't need the impedance matching, just wires which don't share pins with other wires...

author: jzx
author_rank: Regular Contributor
timestamp: June 24, 2024, 06:03:41 pm
content: If you want the cable for power transmision, the ones with coaxial cables (the good ones for video) can be bad, because normally the cores of the coaxial cables are very thin, to maintain the impedance with no too big size.You can always measure the resistance.Sometimes the grounds are shorted in the extremes to save a wire or two.

author: igendel
author_rank: Frequent Contributor
timestamp: June 24, 2024, 06:33:49 pm
content: Quote from: jzx on June 24, 2024, 06:03:41 pmThanks, that's an important warningIn my case I'll be delivering 60-70mA at 24VDC. I picked a random pin on the cable - my multimeter (not too accurate in itself) says the resistance of the wire is 0.2. Should be ok - I don't mind a little voltage drop.Quote from: jzx on June 24, 2024, 06:03:41 pmThat's what I was afraid of. Back to the drawing board

author: CaptDon
author_rank: Super Contributor
timestamp: June 24, 2024, 07:18:50 pm
content: You may be better off with the bigger DB15 like used on joysticks and so forth. How many wires do you really need? A DB9 may work where pin 5 is often the shield and a ground tied together, the other 8 pins a separate.

author: igendel
author_rank: Frequent Contributor
timestamp: June 24, 2024, 09:51:27 pm
content: Quote from: CaptDon on June 24, 2024, 07:18:50 pmWell, initially I just wanted to repurpose stuff I already had lying around, that's why I used that cable in the first place. The units in my project require 10 wires minimum between them (two for power, 8 for various signals).