I am designing a board that has 2 DB9 plugs/connectors. They are used to supply power(Vcc and GND) and data signals respectively. Now, I wont be using all the pins in the DB9. Some will be left floating.

I will then be trying to get FCC/CE certifications for the board. Now, I know, at least in MCUs and such ICs, pins are not left floating. They are eithey pulled up to Vcc or down to GND. Do, I need to do any such thing to some of the unconnected DB9 pins ? If so, what are the criteria or design changes I need to make to them ?


IC's (as a rule of thumb) don't like floating pins but connectors are just totally passive in nature and don't care.

A floating input pin of an IC can be problematic - not only can it cause logic errors (if part of a logic device) but it can cause analogue errors. This is generally the case when the pin should have been connected to a defined voltage point such as 0V.

However, some chips will have information in their data sheets that tell you not to do anything with certain pins and you have to respect the IC supplier's instructions. I've seen this on a few voltage references.

Connectors don't care but you might decide that to reduce the possibility of EMI susceptibility that unused pins are grounded to the case thus reducing the "aperture" created by a bunch of unconnected pins.

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  • \$\begingroup\$ You might ground them through a medium value resistor that could tolerate say 24V applied to it by some strange cable harness without causing a short circuit to ground. \$\endgroup\$ – KalleMP Oct 27 '15 at 19:15
  • \$\begingroup\$ @KalleMP if it's for emc reasons then grounding them thru a 10nF capacitor would probably be better. \$\endgroup\$ – Andy aka Oct 27 '15 at 19:18
  • \$\begingroup\$ I suspect it is for overzealous reasons and either would work fine. Using both and a ferrite filled EMI suppression connector would be even more paranoid but not likely to make much difference either. Connecting the spare pins to long traces and placing our resistors/capacitors on the other side of the board would be a bad idea though. I have never seen anyone do this for an unused pin yet but perhaps it is the norm in TEMPEST rated equipment. \$\endgroup\$ – KalleMP Oct 27 '15 at 19:22

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