computers suck

super-special serial port standards

The PC I put together a few years ago (well, Scott Sullivan told me which bits to get, I bought them and assembled it) is still working really well. It was quite spiffy in its day — i7-4790K, 32 GB DDR3, Asus H97M-E — and is quite fast enough for me.

One thing, though, has never worked. The hardware serial port (the old kind, not the USB kind) refused to do anything. Only in the last day or so did I work out why and managed to fix it.

PC serial ports for roughly the last 25 years connected to the motherboard like this:

motherboard pin 1 → RS232 pin 1; motherboard pin 2 → RS232 pin 6; motherboard pin 3 → RS232 pin 2; motherboard pin 4 → RS232 pin 7; motherboard pin 5 → RS232 pin 3; 
motherboard pin 6 → RS232 pin 8; 
motherboard pin 7 → RS232 pin 4; motherboard pin 8 → RS232 pin 9; motherboard pin 9 → RS232 pin 5; motherboard pin 10 not connected
ZF SystemCard – Data Book (1998)

This rather strange mapping makes sense as soon as you see an IDC ribbon-cable DB-9 connector:

serial cable for the SBC6120-RBC, unhelpfully the wrong way up

Going along the cable from left to right (reversed in the photo above), we have:

    1 2 3 4 5 6 7 8 9

    1   2   3   4   5
      6   7   8   9

This was good enough for everyone except ASUS, who decided that they needed their own way of arranging cables. Because of course they would:

ASUS wiring: motherboard pin 1 → RS232 pin 1; motherboard pin 2 → RS232 pin 2; motherboard pin 3 → RS232 pin 3; etc.
Oh ASUS, how could you?

With a bit of resoldering, I’ve got a working serial port. You can never have too many.

Leave a Reply

Your email address will not be published. Required fields are marked *