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I'm (still) putting an old Pentium PC together. I just found that the connectors of my serial port's ribbon cable have one less hole than needed. The socket on the mainboard has 10 pins, but the 10th hole of the connector is blocked. I have two serial ports, and both have the same connector.

The parallel port's connector is good. Later I found that the IDE cables also had a blocked hole. Fortunately I had a pair of "regular" ones, so that's not an issue. But I don't remember ever seeing connectors like this in the IDE era.

Can I simply drill the missing hole into the connector so it sits in the socket? To the best of my knowledge, a serial port has 9 pins, not 10.

Does the 10th pin do anything?

Will the port work if it isn't connected?Please admire my nail job

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    Guess: Your ribbon cables have one hole less because they are made for a socket which is missing that pin, to determine the orientation of the cable. OTOH, your motherboard doesn't have those pins, so clearly the cables are not made for your motherboard. To find out if you can use your cables at all, you need to find out the pinout of your motherboard's sockets (e.g. with a manual) and also the pinout of your connectors (e.g. with a multimeter/continuity tester). If you are lucky, the pinout matches, then you can drill the hole.
    – dirkt
    Jun 14, 2021 at 20:47
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    Ah yes, problem No. 2.: I don't have a manual for this motherboard, and it's nowhere to be found. I even opened a question about it.
    – Tamás Polgár
    Jun 14, 2021 at 20:48
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    Do you have a multimeter to check the pinout? Pin 5 is ground, and pins 3, 4 and 7 are outputs that should have either positive or negative voltage at levels between 5 and 12 volts.
    – Justme
    Jun 14, 2021 at 21:01
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    I do, and now i wonder why have I never thought of this... Thanks!
    – Tamás Polgár
    Jun 14, 2021 at 22:04

4 Answers 4

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In more recent designs, since the mid-2000s I think, it became common to include "key" pins like this to reduce assembly errors. In most cases the pin removed was non-essential in the first place, eg. an extra ground pin or a no-connect. I think in this case it is a no-connect.

In the case of COM headers, there are actually two different pinouts which require two different cables to interface. You should probably check your motherboard's manual to see if it describes which one it happens to use. Modern cables will probably be for the AT/EVEREX pinout, which maps the numbered header pins directly to the same numbered D-sub pins. That, at least, is what my current MSI boards use.

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  • ...and that's not counting the pinouts which aren't de facto standard. I have a hand-me-down dual-core HP small form factor PC from the DDR2 era that uses a completely non-standard connector with more than 10 pins for the COM2 header, where eBay photos reveal the matching part to have a small PCB with at least one IC on it hanging off the back of the expansion slot plate. (I didn't feel having COM2 on it was worth over $20 Canadian when I've got Chinese USB-Serial cables, so I can't say anything more.)
    – ssokolow
    Jun 14, 2021 at 21:21
  • Oh, now I remember why we always loved the PC industry. Thanks!
    – Tamás Polgár
    Jun 14, 2021 at 22:04
  • Yeah, gotta watch out for that. The older standard maps directly to IDC cable numbering, while the newer one (like on my Asus) doesn't.
    – scruss
    Jun 16, 2021 at 14:55
  • @ssokolow That must have been some relative of the CNR, AMR, and ACR slots. The latter actually had a legitimate purpose in moving various analogue components off the m/board onto a small, more easily certified daughterboard, but since nobody uses analogue modems any more it isn't needed.
    – Chromatix
    Jun 16, 2021 at 20:03
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In addition to the fact that some serial-port adapter cables have a blocked off pin, I've seen boards and cards use two different pinouts. On a DB-9 male, the pins are numbered

1   2   3   4   5
  6   7   8   9

but on the 10-pin connector the pins are numbered:

2  4  6  8 10
1  3  5  7  9

I've seen some cables which connect pin 1 to 1, 2 to 2, etc. but I've also seen cables that connect each row of pins on the DB-9 to a row of pins on the 10-pin connector. If you have an ohmmeter, you may be able to determine which kind of connector is required for a given board by measuring to see which pin has the lowest resistance to ground. That pin should connect to pin 5 of the DB9.

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Thank you for all the answers. Turns out that the 10th pin is indeed "fake", it's not connecting anywhere. So the answer is yes, I could drill a hole on the connector. However, there are two different types of serial connectors, and they're not interchangeable. A 9 pin cable will not work correctly. So much for standards, thank you, PC industry!

More info: https://pinoutguide.com/Motherboard/rs232_header_pinout.shtml

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Instead of drilling, you may be able to clip off the mating pin with small wire cutters. If the pinout is suitable, this will be easier. Of course it damages the connector on the motherboard if it's not.

For testing you can also bend the offending pin down and push the connector part way on.

A stackable header can be used between the cable and motherboard, with one pin removed. These are cheap and easy to modify.

The issue may be that even with a compatible pinout, some boards with keyed headers like that don't mark the orientation in any other way so you'd be left with 2 possibilities. The multimeter approach suggested in the comments will be needed then

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  • I'd rather damage a $2 port than a $200 motherboard...
    – Tamás Polgár
    Jun 15, 2021 at 20:55
  • @TamásPolgár the motherboard is easier to modify, if you can be sure of the pinout. Wrecking it would take a really ham-fisted attempt at modification (but I suppose I do quite a lot of electronics on that scale)
    – Chris H
    Jun 15, 2021 at 21:08
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    I'd rather chop the pin off of a 75¢ stackable header (or one of these ) if space & pitch permit
    – A C
    Jun 16, 2021 at 4:07
  • @AC even better
    – Chris H
    Jun 16, 2021 at 6:06

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