4

Why would someone want to bridge the GND wire to the metal shield of a DE-9 connector? Take a look at the picture below.

de-9

A 10 position flat ribbon cable is soldered to a DE-9 connector (male). On position 5 (GND) a short piece of wire is soldered, which branches off and is soldered on the metal shield/housing of the connector.

Why would you want to do that?

On the other side of the ribbon cable is a 10+10 IDC socket that connects to the COM port header of a PC motherboard.

Anindo Ghosh
  • 50,188
  • 8
  • 103
  • 200
Samir
  • 151
  • 1
  • 9
  • I am really not that knowledgeable as it may seem. But I am learning, and I like paying attention to details where others ignore them. But I had to use the wrong "db9" tag like everyone else here. =) I am not allowed to make new tags, otherwise I would have tagged it "de9". – Samir Aug 20 '13 at 17:40
  • What about DB25 connectors? They have GND on pin 18-25. Should one of them be branching off in a similar fashion and making contact with the metal shield of the connector? The connector above is part of a (PC I/O breakout plate) kit for the COM and LPT headers. The DB25 has no bridges like that. Weird... they both came from the manufacturer of the motherboard. Looking at the part number for the LPT/DB25 plate, it seems to be an older revision (compared to number noted in motherboard manual). – Samir Aug 20 '13 at 18:12
  • 1
    @Sammy DB9 tag has been retagged to DE9. – Anindo Ghosh Aug 20 '13 at 20:08

1 Answers1

3

I'd do it to try to improve EMC shielding. Others may do it for something else of course.

Andy aka
  • 434,556
  • 28
  • 351
  • 777
  • Electromagnetic compatibility, i.e. reduce electromagnetic interference (EMI)? Or are these two different things (EMC vs. EMI)? In practice, what is the benefit of this? And is this really necessary in this application? Will the COM port and any peripheral device I connect to it function/communicate normally if I were to remove this wire? – Samir Aug 20 '13 at 17:33
  • @Sammy EMC or EMI - same thing to me. It could be that whatever connects to the ribbon-cable-end of the wires may be a tad flaky and solidly grounding pin 5 to (presumably) the PC end has a benefit. I cannot answer your last question in your comment because I'd be guessing BUT someone attached it for a reason and I've seen it done for EMC reasons. – Andy aka Aug 20 '13 at 18:35
  • That's why I'd do it, AFTER I made sure I had reason to believe I was only grounding the attached cable shield on one side. – Scott Seidman Aug 20 '13 at 20:48