Serial input device to kernel input

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Although USB is now probably the most popular way to connect input devices such as mice, serial input devices, such as older mice, and more exotic input devices such as 3Dconnexion Spaceballs are still quite usable, either with a serial port built into the computer, or via a USB to serial converter dongle.

The traditional way to support these serially attached devices was to configure each application with the details such as which serial port the input device was attached to, and what protocol the device used. As the most common application people used with a serial device was / XFree86, this wasn't too much of a problem. However, if you wanted to use the gpm utility to have mouse control at the console, as well as using / XFree86, you may encounter limitations as to which input protocol each application supported.

A better approach is to have the Linux kernel input subsystem manage the serially attached device, and then present the input signals the device generates in the same way that USB and PS/2 input device signals are presented, via the /dev/input/{mice|mouseX} device files.

This guide describes the simple steps necessary to "attach" a serial input device to the Linux kernel input subsystem.


Firstly, you'll need to install the inputattach utility. This utility tells the kernel input subsystem which serial port the input device is attached to, and what type of device is attached to the specified serial port. An AUR package for the utility is here, or you can search AUR for a package named inputattach.

Once you have compiled and installed the AUR package, you can view the inputattach help, to see the large list of serial input devices the Linux kernel input subsystem supports. Here is an example of the help output:

$ inputattach --help

Usage: inputattach [--daemon] <mode> <device>

  --sunkbd         -skb      Sun Type 4 and Type 5 keyboards
  --lkkbd          -lk       DEC LK201 / LK401 keyboards
  --vsxxx-aa       -vs       DEC VSXXX-AA / VSXXX-GA mouse and VSXXX-A tablet
  --spaceorb       -orb      SpaceOrb 360 / SpaceBall Avenger
  --spaceball      -sbl      SpaceBall 2003 / 3003 / 4000 FLX
  --magellan       -mag      Magellan / SpaceMouse
  --warrior        -war      WingMan Warrior
  --stinger        -sting    Gravis Stinger
  --mousesystems   -msc      3-button Mouse Systems mouse
  --sunmouse       -sun      3-button Sun mouse
  --microsoft      -bare     2-button Microsoft mouse
  --mshack         -ms       3-button mouse in Microsoft mode
  --mouseman       -mman     3-button Logitech / Genius mouse
  --intellimouse   -ms3      Microsoft IntelliMouse
  --mmwheel        -mmw      Logitech mouse with 4-5 buttons or a wheel
  --iforce         -ifor     I-Force joystick or wheel
  --newtonkbd      -newt     Newton keyboard
  --h3600ts        -ipaq     Ipaq h3600 touchscreen
  --stowawaykbd    -ipaqkbd  Stowaway keyboard
  --ps2serkbd      -ps2ser   PS/2 via serial keyboard
  --twiddler       -twid     Handykey Twiddler chording keyboard
  --twiddler-joy   -twidjoy  Handykey Twiddler used as a joystick
  --elotouch       -elo      ELO touchscreen, 10-byte mode
  --elo4002        -elo6b    ELO touchscreen, 6-byte mode
  --elo271-140     -elo4b    ELO touchscreen, 4-byte mode
  --elo261-280     -elo3b    ELO Touchscreen, 3-byte mode
  --mtouch         -mtouch   MicroTouch (3M) touchscreen
  --touchright     -tr       Touchright serial touchscreen
  --touchwin       -tw       Touchwindow serial touchscreen
  --penmount       -pm       Penmount touchscreen
  --fujitsu        -fjt      Fujitsu serial touchscreen
  --dump           -dump     Just enable device


For example, if you have a Logitech TrackMan Marble serial mouse, as I do, the device type you would specify would be either --mouseman or -mman.

The inputattach utility needs to run continuously. In additon to installing the inputattach utility, the AUR package above installs a /etc/rc.d/inputattach script that can be added to the DAEMONS option of /etc/rc.conf, making it automatically start at boot time. A configuration file for inputattach is installed as /etc/conf.d/inputattach.conf.

The default /etc/conf.d/inputattach.conf file assumes a Microsoft serial mouse, and assumes the mouse is attached to the first serial port of the computer. The IAMODE variable specifies the input device type, and the IADEV variable specifies the serial port the input device is attached to. These variables are directly passed to the inputattach utility, so they need to be formatted exactly as the inputattach utility expects them.

Here's an example of a /etc/conf.d/inputattach.conf file, modified to suit a Logitech TrackMan Marble serial mouse:

$ cat /etc/conf.d/inputattach.conf 
# Configuration for inputattach

# inputattach mode - see 'inputattach --help' for list e.g. "--microsoft" for
# 2 button Microsoft mouse

# inputattach device - /dev file entry where device is attached e.g.
# "/dev/ttyS0" for device attached to first system serial port

Once you have modified the /etc/conf.d/inputattach.conf file, you can then attempt to start the inputattach startup script, by running /etc/rc.d/inputattach start. It is likely to start up silently. If you happen to be within Xorg when you do this, and have an InputDevice mouse section that specifies /dev/input/mice as the input device file, your new input device is likely to now be another source for Xorg mouse pointer movements, in addition other input devices e.g., a USB mouse.

Another way to confirm that it worked is to check your system's kernel log using the dmesg utility. For a Logitech TrackMan Marble serial mouse, the kernel output is:

serio: Serial port ttyS0
input: Logitech M+ Mouse as /class/input/input6

And that is all you have to do. To have your serial input device work everytime you boot, make sure you add inputattach to the DAEMONS line of your /etc/rc.conf file.