Serial 16550A Driver

Real-Time Serial Driver for 16550A-Compatible Devices

Driver version: 1.5 and later


  • decide which serial ports are to be managed by the real-time driver
  • identify their I/O addresses and IRQ numbers:

setserial /dev/ttySx

  • disable the Linux driver for all these devices:

setserial /dev/ttySx uart none


modprobe xeno_16550A io=<addr>[,...] irq=<no>[,...] baud_base=<val>[,...] tx_fifo=<val>[,...] start_index=<val>[,...]

Argument Description
I/O addresses of the devices, comma separated (e.g. “0x3f8,0x2f8”) Note: In the driver that comes with Xenomai 2.3 and earlier (driver versions < 1.5), this argument is called ioaddr.
Interrupt numbers of the devices, comma separated (e.g. “4,3”)
Maximum baud rates of the devices, comma separated, default is 115200
Transmitter FIFO sizes of the devices in bytes, comma separated, default is 16
First device instance number to be used, default is 0 (⇒ “rtser0”)

baud_base detection

If you are not sure about Baud Base of serial ports, try to use next values:

  • 115200
    • 16550A-compatible onboard RS-232 controllers
  • 921600
    • Moxa CP-132U
    • Moxa CP-118U
    • Moxa CP-134U V2

If your board is not listed here, you can try value of maximum speed of you board, that listed in “features” list in user’s manual, like … up to 921.6 Kbps … that mean 921600 baud is maximum tranfer speed of your board.

io, irq detection

For onboard controller io, irq parameter could be found in BIOS, standart values are “0x3f8,0x2f8” for io and “4,3” for irq. For multiport adapters you can use setserial to detect io, irq :

[root@dvplayout ~]# setserial -g /dev/ttyM2
/dev/ttyM2, UART: 16550A, Port: 0xc810, IRQ: 22

This works only if you have loaded specific board kernel module.

Another method is to use lspci to detect io port ranges and irq:

[root@tvdevctl ~]# lspci -v
--- skipped --
06:01.0 Serial controller: Moxa Technologies Co Ltd Unknown device 1180 (prog-if 02 [16550])
        Subsystem: Moxa Technologies Co Ltd Unknown device 1180
        Flags: medium devsel, IRQ 9
        I/O ports at 1040 [size=32]
        I/O ports at 1000 [size=64]
        I/O ports at 1060 [size=16]
--- skipped --

As you can see, multiport board occupied IRQ 9 and 3 I/O ports range. Moxa boards use two address regions for io: 0x1000 for the first UART, 0x1008 for the second one, etc.

Loading driver for all 8 ports:

/sbin/modprobe xeno_16550A \
               io=0x1000,0x1008,0x1010,0x1018,0x1020,0x1028,0x1030,0x1038 \
               irq=9,9,9,9,9,9,9,9 \

Full-featured example

If you want to use onboard serial controllers together with an additional multiport board, you just need to specify the full set of UART configurations with io, irq, baud_base parameters on load:

/sbin/modprobe xeno_16550A \
               io=0x3f8,0x2f8,0xdf00,0xdf08 \
               irq=4,3,11,11 \

In-Kernel Driver Setup

If you prefer to compile xeno_16550A into the kernel, the arguments listed above have to be provided as kernel parameter. Just prepend the prefix xeno_16550A., e.g.,0x2f8. To control the standard Linux 8250 serial driver, either disable it at compile time or set the number of managed ports via 8250.nr_uarts=N.


The device API is described in the Xenomai API documentation under Serial Device Profile.