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I'm upgrading an enterprise system to new hardware and have run into a problem fitting a serial connection into the COM2 serial header on the new motherboard.

I've checked the motherboard manuals and the headers seem to be wired identically, but the COM2 header on the new board is physically smaller than the header on the old board. Not by much, there's maybe a millimetre or two's difference between them, but the new board's header is very definitely incompatible with the plug I need to attach to it.

I've noticed that the plug is, however, exactly the size of the 9-pin USB header on the new board and plugs in to that perfectly (with the boards turned off of course - I'm not sure what sort of damage actually sending voltage down those lines would cause).

I should mention that the DB9 port on the back of the board is in use for something else, so I can't just wire the external devices into a female DB9 socket and attach it to that.

The old board is around ten years old, so is this a standard that has changed over time and I can get an adapter for or is this some custom-build enterprise-system weirdness and I should just re-wire the external devices into a smaller plug?

Edit: I've taken a picture of the plug next to the COM ports of the new motherboard. The old motherboard's COM header is bigger, physically it seems identical to the USB headers.

Image

Edit 2: I've taken pictures of both the old and new motherboard's COM headers. Old motherboard's COM2 header (the one that's the same as a USB header):

Old motherboard's COM2 header (the one that's the same as a USB header)

And the new motherboard's COM headers:

And the new motherboard's COM headers

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    Sounds like on is a USB header and the other is a COM port. How about screenshots as a tool to clarify your question – Ramhound Oct 18 '18 at 12:31
  • That looks like the front panel connection. It's usually a 9-pin connector with the missing one down-left – dmb Oct 18 '18 at 13:26
  • Please provide a clean picture of the both ports, and the label on the motherboard, next to each. – Ramhound Oct 18 '18 at 13:48
  • I've uploaded pictures of both headers, looking at it closer as I took pictures the old motherboard's COM2 header really does seem identical to a USB header (other than which pin is missing). Both the motherboard and the manual definitely label it as a serial COM header though, the manual even gives the usual table of serial pin In/Out configurations. – Scabadus Oct 18 '18 at 14:19
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the COM2 header on the new board is physically smaller than the header on the old board ...
so is this a standard that has changed over time

Yes, as boards and components have gotten denser and/or smaller, so have connectors such as these pin headers.
The ubiquitous pin headers that you're familiar with are based on a 0.10" (i.e. a tenth of an inch or 2.54 mm) pitch (i.e. pin separation).
The modern trend is to use denser connectors (and based on metric dimensions), such as headers with a 2.00 mm pitch.

Adapters do exist (e.g. such as this), but you might want to check with the mobo manufacturer first for availability of a proper cable (because there are two wiring schemes for mobo-to-DB9-connector, see How to wire the LPT and COM port headers on a motherboard?.

  • Thanks, those pitch numbers were exactly what I needed to be able to find an adapter. – Scabadus Oct 19 '18 at 15:10
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I've uploaded pictures of both headers, looking at it closer as I took pictures the old motherboard's COM2 header really does seem identical to a USB header (other than which pin is missing).

The header in question is clearly marked as COM2, which means it's NOT a USB header, otherwise it would be labeled differently.

I've checked the motherboard manuals and the headers seem to be wired identically, but the COM2 header on the new board is physically smaller than the header on the old board.

Other than size they are identical. They both appear to be internal Serial COM ports.

Not by much, there's maybe a millimeter or two's difference between them, but the new board's header is very definitely incompatible with the plug I need to attach to it.

it does not appear that the cable you are attempting to connect to it is even a COM cable. Both headers have 9-pins the cable has I assume 10-wires.

Old motherboard's COM2 header (the one that's the same as a USB header)

A USB header would have a different pin that is blank. COM and USB headers are not identical.

I've noticed that the plug is, however, exactly the size of the 9-pin USB header on the new board and plugs into that perfectly (with the boards turned off of course - I'm not sure what sort of damage actually sending voltage down those lines would cause).

If you believe the cable to be a COM cable you shouldn't connected it to a USB header.

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