Monthly Archives: March 2013

How to Get a Real Serial Port – PCI Express under Linux


I do a lot of hardware and software development on things with embedded processors, ranging from little 8-bit Atmel chips to big 32-bit ARM-based things like the Balloon Board. Often the systems they’re built in to don’t have much of a display or keyboard, so getting information in and out of them for testing and debugging purposes is tricky. One thing that really helps is a good old-fashioned serial port. Pretty much every embedded processor has some kind of serial port on it, and they’re generally very easy to use from software so can be got working very early in the development process.

In order to make use of a serial port, you need another one to connect it to. This used to be easy: they used to be ubiquitous on desktop and laptop PCs. However, in the last 10 years they’ve gradually died out and been replaced by USB to serial adapters. Though such adapters are cheap and readily available, I’ve had loads of problems with them. Since they’re easily detached, they often seem to go missing, or get left in toolboxes and attached to projects. Some of the ones I’ve used also sometimes seem to get themselves in a mess and start turning my data into junk until they’re unplugged and reinserted. This is not helpful.


It can also be hard to predict how the serial port will be identified when it’s plugged in: under Windows, it gets a COM port number, but which number it gets depends on how many other such devices have ever been plugged into that machine and even into which USB socket, so setting debugging software up to find the one you want is impossible. Under Linux the same problem exists but with an extra feature: if you have a terminal program watching a serial port, then accidentally unplug it and plug it back in again, the program carries on as if nothing has happened but you don’t see any output . That’s because the operating system thinks that the old port is still in use even though it’s not there any more, and allocates a new, available number when it’s plugged back in, so /dev/ttyUSB0 magically becomes /dev/ttyUSB1. Even restarting your terminal program doesn’t work, because it’s still going to look at the old number. You can reconfigure the terminal program to look at the new one, but then you’ll find that if you shut everything down and restart it, the USB serial port will have moved back to /dev/ttyUSB0. The only way out is to stop the program, unplug the adapter again, plug it back in and restart the program. This is a serious pain.

I recently had the chance to do something about this problem in my own workshop. I was configuring a new Linux PC for the workbench, and discovered, to my great joy, a little-documented feature on the shiny new Gigabyte GA-Z77N-WIFI motherboard: a serial port! Tucked away on a little white connector, there it was, forgotten and unloved. All it needed was a cable to bring it to the outside world, and it worked. It turns up as COM1 under Windows and /dev/ttyS0 under Linux, and it’s always there, can’t be unplugged, and never changes its number. Wonderful.

If one serial port is good, more must be better. The motherboard has a PCI Express slot on it, intended for a turbo-nutter graphics card which this machine didn’t need. I found a PCI Express serial port card, similar to the PEX4S553 from which looked like it would do the trick. Actually I had it lying around because I’d fitted it to a Mac Pro a couple of years ago, but it never worked properly in the Mac.


I put it in the machine and switched on. Would it work? Typing lspci revealed that it was there, and was working, so were the ports usable?

ls /dev/ttyS*
ttyS0    ttyS1    ttyS2    ttyS3

Not bad, but why only four? There’s the one on the motherboard, plus four on the card, so that should be five, right? Well, to cut a long story short, after a bit of web research, I found that the standard serial port driver in the Linux kernel only looks for four serial ports as standard, but responds to a kernel command line parameter to make it look for more (or indeed less). Experimentally rebooting and manually editing the kernel command line in grub to add


to the end of it worked! I had all five ports, and they all did what they should. To make the change permanent, I edited¬†/etc/default/grub to add 8250.nr_uarts=5 to the end of the GRUB_CMDLINE_LINUX_DEFAULT parameter, and ran update-grub. Now all five ports turn up every time I switch the machine on, and I’m a happy developer. By the way, I’m running Debian Linux 6.0 (squeezy) with kernel version 3.2.something from backports, but the kernel version won’t make any difference – this stuff is all ancient history.

It remained only to tidy up the wiring. The ports on the Startech card appear on 10-pin headers. The wiring on these is simple: pin 1 on the header goes to pin 1 on the 9-pin D connector, pin 2 to pin 2, and so on. Pin 10 isn’t used. However, the way the pins are numbered differs between the headers and the D connectors: the headers are numbered on alternate sides, like a street, so one side is 1/3/5/7/9 and the other is 2/4/6/8/10. The D connectors are numbered 1-5 on the top row and 6-9 on the bottom row.

I had a load of serial port breakout cables in the bottom of my PC parts box, left over from the early 90s. I found that many of them were wired straight-through, like this:


which is wrong for the Startech card. I had to rewire them to look like this:
:DSC_0362[1] DSC_0361[1]

according to the correct pin numbering, and then they worked. The card fitted neatly in the back of the machine:

DSC_0366[1]and, as a finishing touch, I did a ‘case mod’ to mount two of the serial ports on the unused 3.5″ floppy drive blanking plate:

DSC_0365[1]Notice how they’re labelled, so I always know which is which! It’s a very practical arrangement for the workbench, having reliable serial ports always at hand. The other two are round the back, and will probably be used less frequently.