Are all DB-9 (serial COM port) to 10 pin header ribbon cables the same? - Page 1
Forum section: electronics / general

author: Freesurfer
author_rank: Regular Contributor
timestamp: February 07, 2023, 10:23:48 pm
content: I want to use the serial port on my motherboard (ASUS Z170-A). There is a 10 pin header near the bottom of the motherboard where you are supposed to plug in a ribbon cable to run a DB-9 port to the back of the PC.I happened to have a cable in a junk bin, but I am unsure if the pinout is correct. The 10-pin connector has a notch and will physically fit on to the motherboard header. But the pinout does not seem to align (see below). Do you agree? But even then, looking for other cables online they seem to be the exact same cable. So I wonder, are all this header serial port cables pretty much identical, or are there different pinout schemes out there? I am even wondering if the MB manual could have it wrong.I asked this question in another forum where others pointed out that the cable is keyed and should fit one way etc. The cable does indeed fit, but by closely looking at the wires and pinout diagrams I notice this:The first pin is correct: Assume the pink wire on the ribbon is connected to pin 1 on the IDC connector "O_DCD1" you can see that it runs to pin 1 on the DB9 (DCD) (correct)But the second is wrong: Assuming the first pink wire correct, the second wire on the ribbon cable must be connected to "O_RXD1" due to how the IDC connector is constructed. But that second wire on the ribbon cable, runs to "pin 6" "DSR" on the DB9 connector. (wrong)

author: coromonadalix
author_rank: Super Contributor
timestamp: February 09, 2023, 03:28:27 pm
content: I would say they are not fully compliant between brands, some of them use the non connected 10 th pin to do plug / presence detection / enabling the portI banged my head a few times before catching this oneSome brand use the same trick for the power button, the usb plugs too etc ....Sometimes keyed and others are not ....

author: mankan
author_rank: Regular Contributor
timestamp: February 09, 2023, 07:01:42 pm
content: There are two types of cables, seehttps://www.scantips.com/serial-db9.html. I had the same issue myself recently and ended up buying two different unspecified ones. Both were different but one had the "Asus" layout and worked fine.

author: Freesurfer
author_rank: Regular Contributor
timestamp: February 16, 2023, 11:21:13 pm
content: Thanks, that's some useful and clarifying info! So I guess the manual is correct then and I need to find a compatible cable. I tried searching ebay or things like "ASUS serial cable" "ASUS COM port" but none of them specified any compatibility nor showed any pinout diagram. I guess the only way to be sure is to rewire the one that I have.

author: mankan
author_rank: Regular Contributor
timestamp: February 18, 2023, 06:19:12 pm
content: I ended up buying two different ones on chance,https://www.amazon.se/dp/B006DYQNIKandhttps://www.amazon.se/dp/B001Y1F0HW. I think the StarTech one was the Asus layout but I'm unfortunately not certain.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: February 19, 2023, 09:08:27 am
content: If you are careful, you can open the IDC header, pull the cable out, cut of 10mm or so, separate the wires and the put them back in any order you want.It's a bit finicky to do, but as this is an electronics forum you can do it to "expand your skills"

author: Ian.M
author_rank: Super Contributor
timestamp: February 19, 2023, 10:15:00 am
content: Its easier to simply remove the back cover from the D connector and re-wire it there. Simple soldering and no fiddling with a single use IDC connector!