Question: When designing a product that uses a DE9 connector, is it normal to just classify it as an RS-232?

Background: I am seeing a lot of references to RS-232 while reading engineering documents in the avionics sector that really don't line up to what is defined in the EIA RS-232 documents (I've checked quite a few revisions). These products do use a DE9 connector, but it in no way aligns with RS-232 recommended pinouts. Stuff like using a pin to transfer power to the device and maybe just one pin for TX and one for RX. It's making me feel like maybe I'm not understanding it correctly. Why aren't these connections considered proprietary implementations and named accordingly?

I understand this is a DE9 connector, not a what is often referred to as a DB9.

  • 4
    \$\begingroup\$ There really isn't any such thing as DB9. Standard PC serial ports use DB25 (for old ones with all the status lines) or DE9 (the most common ones). Nothing ever used "DB9". \$\endgroup\$
    – Hearth
    Jun 8 at 19:20
  • 4
    \$\begingroup\$ @Hearth this is true, I should be referring to it as a DE9, but about half the docs say DB9. \$\endgroup\$ Jun 8 at 19:24
  • 3
    \$\begingroup\$ DB9 would refer to a connector with nine pins in the B-sized shell, which would have a ton of wasted space. I can't imagine anyone ever manufactured those. \$\endgroup\$
    – Hearth
    Jun 8 at 19:26
  • 8
    \$\begingroup\$ "I understand this is a DE9 connector" My inner nitpicker loves you for that. \$\endgroup\$ Jun 8 at 21:15
  • 4
    \$\begingroup\$ ...but let's not all pretend that we haven't seen those very things referred to as "DB9" many many MANY times. It drives me crazy too, but at some point you have to just go with the flow, communicate, and solve problems without getting overly pedantic. \$\endgroup\$ Jun 8 at 21:53

2 Answers 2


It is common to name "RS-232" anything with a DE9 connector with pins 2,3,5 connected as per "IBM PC convention". It may appear unfortunate, but there's very little to be gained by omitting the fact that yes, there are two data lines and a signal ground that match the requirements of the RS-232 standard.

Usually, it's only those three pins that are used for new designs, with everything else left for legacy uses. Software flow control is easiest to implement in terms of hardware resources, so in practice those three pins is all that's needed.

As a matter of good practice, all other pins should have directions and acceptable signal levels within RS-232 specs by default, or be floating. A special cable can be used that can be electronically detected. Once such a cable is detected, the other pins can be used as desired, still keeping in mind the connector specs such as voltage ratings at various altitudes etc. Detection of the special cable ensures that turning on those pins in an electrically-incompatible manner won't cause equipment damage or otherwise unsafe conditions.

  • \$\begingroup\$ Whoa! thank you for explaining this, this makes it much more understandable as to why they are labeled as RS-232. \$\endgroup\$ Jun 8 at 20:40

Just because a device has a DE-9 connector, it may have nothing to do with RS-232 standard.

The RS-232 standard allows only two connectors, and DE-9 is not one of them.

So strictly speaking, and technically, all RS-232 interfaces with DE-9 connector or any other connector than the allowed connectors are custom/proprietary implementations.

That said, when IBM made the dual RS-232 interface card for the IBM 5170 (the PC/AT might be more familiar term), one of the ports had to use a smaller connector to fit into card slot, so this custom implementation developed and became a de facto standard.

And even if it does have a custom connector, if it still implements RS-232 interface electrically but with a non-standard physical connector, be it DE-9 or 8P8C modular jack or 3.5mm TRS connector, in a pinch and in common language it can still be called an RS-232 port. After all, it only takes an adapter or a cable to end up with a standard connector.

And even more technically, RS-232 devices may still be incompatible - the standard does not define a protocol, only an electrical interface for binary data. While you may safely assume that asynchronous start-stop framing protocol with NRZ line coding is used as an UART is very common, it's not defined by standard. You could use any binary data encoding like Morse code, FM, Manchester or EFM if you wanted, and it will technically still be an RS-232 interface, but it would seriously limit the ability to communicate with other equipment due to bit protocol and line code.

So again, the de facto protocol is just made with UARTs, but not mandated by the RS-232 standard.

  • \$\begingroup\$ Yeah, and thats exactly the issue I have been running into. I do cybersecurity, and I constantly have to explain that just because it looks like a "DE9" doesn't mean you are going to get a terminal back when connecting to it, most of these devices don't even contain the required chipsets to give you a shell. \$\endgroup\$ Jun 8 at 20:57
  • 2
    \$\begingroup\$ @SamuelBarthelemy If that's the question, why not ask it directly? Not all DE-9 connectors are serial ports. Not all 8P8C sockets are Ethernet. Not all DIN connectors are MIDI. Not all XLR connectors are AES/EBU or DMX. Not all 3.5mm TRS jacks are for headphones or microphones. The list is endless. \$\endgroup\$
    – Justme
    Jun 8 at 21:25
  • \$\begingroup\$ I broadly agree. To qualify as RS-232 it should have the right voltages, the right sort of connector, and strictly should be operated in the right context i.e. between a data-set and data-terminal with due consideration of which connector is chassis and which tail. \$\endgroup\$ Jun 9 at 4:45

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.