Fun with Supermicro motherboard serial headers

or, “LOL, standards” ^

TL;DR: Most motherboards have a serial header in an IDC-10 (5×2 pins) arrangement with the pins as a row of even numbered pins (2,4,6,8,X) followed by a row of odd numbered pins (1,3,5,7,9). Supermicro ones appear to have the pins in sequential order (6,7,8,9,X and then 1,2,3,4,5). As a result a standard IDC-10 to DB-9 cable will not work and you’ll need to either hack one about or buy the Supermicro one.

Update ^

A comment below kindly points out that Supermicro actually is using a standard header pinout, it’s just that it’s a competing and lesser-used standard. It’s apparently called Intel/DTK or crossover, so that may help you find a working cable.

Are we sitting comfortably? ^

I bought a Supermicro motherboard. It doesn’t have a serial port exposed at the back. I like to use serial ports for a serial console even though I am aware that IPMI exists. IPMI on this board works okay but I like knowing I can always get to the “real” serial port as well.

The motherboard has a COM1 serial header, and I wasn’t using the PCI expansion slot on the back of the chassis, so I decided to put a serial port there. I bought a typical IDC-10 / DB-9 cable and plate:

IDC-10 to DB-9

Didn’t work. Serial-over-LAN (IPMI) worked alright. On COM1 I would get either nothing or a run of garbage characters from time to time. I wasted a good number of hours messing with BIOS settings, baud rates, checking if my USB serial adaptor actually worked with another device (of which I only have one in my home), before I decided to sit down and check the pin numbering for both the header and the cable.

Looking at the motherboard manual we see this:

x10sdv board com1 pin layout

And the cable?

IDC-10 to DB-9 pinout

Notice anything amiss?

The cable’s pins go in a row of odd numbers and then a row of even numbers:

2 4 6 8 X
1 3 5 7 9
    -

The X is the missing pin (serial uses 9 pins) and the - indicates where the notch for the connector would be: next to pin 5 in this case.

The header’s pins go in sequential order:

6 7 8 9 X
1 2 3 4 5
    -

As a result all but pin 1 are incorrect.

You actually need a Supermicro cable for this. CBL-0010L is the part number in my case. CBL-0010LP would be the low profile version. Good luck finding it mentioned on Supermicro’s site, but your favourite reseller will probably know of it. As it was I found one on Ebay for £1.58+VAT, and it works now.

After knowing what to search for I also found someone else having a similar issue with a Supermicro board.

You could of course instead hack any existing cable’s pins about or fit an adaptor in between (as the person in the above link did).

Thanks Supermicro. Thupermicro.

9 thoughts on “Fun with Supermicro motherboard serial headers

  1. Nice work on that deduction process… However, that sort of bullshit is obviously completely intolerable- and needs to be brought to SuperMicro’s attention [so that they change it, maybe?]

    Or at least (in a friendly way) threaten to initiate Class-Action legal mumbo-jumbo unless they take the initiative to send out their stupid adapter [free of charge] to all of their registered owners of that mobo 😉

    It’s doubtful anything will come of it- but it’s always good to make sure we keep our critical hardware supply chain folks true- and that means enforcing fucking IMPORTANT standards, such as the pinouts for DB9 FFS!!!

  2. Fantastic! Your post helped me a lot while figuring out why my Supermicro MB kept sending out shit from COM1.

  3. Not all heroes wear capes. I spend 2 hours swapping cables and changing all possible settings, and then found this blogpost 🙂

  4. Goddamit I wish I had seen this post earlier.
    I had the oscilloscope out and was seeing data come out of the ground pin (5).
    Thanks for the post.

  5. I am afraid to say how much time I spent debugging a standard cable before I found this page. In my defense (and another strike against Supermicro) the manual for my X11SAT-F has the pin definition chart but NOT the pin layout diagram!

    So for me I ordered the Supermicro cable on assumption they use the same non-srandard pin layout in this motherboard.

    Ugh.

  6. For anyone still stumbling across this, there are two Rs232 header pinout standards: Intel/DTK, also called crossover, and AT/Everex, also called straight. Supermicro uses the Intel/DTK standard. You can find a cable from any number of other manufacturers. Still, it would be nice of Supermicro to specify their standard in the manual by name as well as pinout.

  7. Thanks! You save me a day.

    I would like to add a GPS to the serial port. In this way, I can have a stratum 1 ntp server. It waste me a day to play around with all different settings, in BIOS, in operating system. In the end, I decided to do some search via Google. And I found your blog.

    I think I will end up with hack a cable.

    Thanks again.

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