I tried to get a “homebrew” infrared receiver attached to the DCD line of a serial port working on Ubuntu Bionic. It seems that things have changed since Ubuntu 12.04, when I last had it working.

The changes ended up being:

  • The kernel modules are in the mainline kernel, but aren’t supplied with Ubuntu
  • The LIRC driver’s name has changed from “serial” to “default”

My first problem is that the receiver wasn’t working in the first place. I attached a DSO Nano to the data line, and noticed that the signal didn’t have a high enough voltage to trigger the serial port. The data sheet shows the receiver’s output being a pull-up resistor with a transistor pulling the output low; maybe this serial port draws a particularly large amount of current. I wired a 2.2k resistor between the data line and VCC (which should be within the limits of the receiver), and everything works.

I tested it with this program, which displays a time when the DCD line changes:

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <termios.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <time.h>
int die(const char* msg) {
  printf("%s\n", strerror(errno));

int main(void) {
  int fd = open("/dev/ttyS0", 0);
  if (fd == -1)
  while (1) {
    struct timespec tm;
    if (clock_gettime(CLOCK_MONOTONIC, &tm) == -1)
    printf("%ld\n", tm.tv_nsec);
  return 0;

(So why does LIRC need the kernel driver, if this works in userspace? I suspect it’s because the LIRC API requires a timeout, which the ioctl doesn’t support.)

LIRC’s serial port support works by using a kernel module to read the hardware, and sends the output to /dev/lirc0. LIRC connects to this device to read the input. The LIRC userspace driver that does this is called “default” (it used to be called “serial”).

It seems a few releases ago the drivers were upstreamed, and aren’t supplied with Ubuntu. I got the driver installed with DKMS:

  1. Create a working directory
  2. Put the driver in it. (Choose the appropriate version for your kernel.)
  3. Add this Makefile:

     obj-m += serial_ir.o
     	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
     	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean
  4. Add this dkms.conf:

     CLEAN="rm -f *.*o"
  5. Run sudo dkms add .
  6. Run sudo dkms install lirc_serial_ir/4.15.

Now run sudo modprobe lirc_serial. Run dmesg, and you should see:

[ 1627.908509] serial_ir serial_ir.0: port 03f8 already in use
[ 1627.908515] serial_ir serial_ir.0: use 'setserial /dev/ttySX uart none'
[ 1627.908516] serial_ir serial_ir.0: or compile the serial port driver as module and
[ 1627.908517] serial_ir serial_ir.0: make sure this module is loaded first
[ 1627.908532] serial_ir: probe of serial_ir.0 failed with error -16

To fix this, install the “setserial” package, and disable the serial port as the instructions say.

Try running mode2, and press some buttons on the remote:

$ sudo mode2 --driver default
Using driver default on device auto
Trying device: /dev/lirc0
Using device: /dev/lirc0
Running as regular user
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pulse 9015
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