UART Communication

This page gives some tips and tricks regarding serial communication.

Gateworks boards often have connectors available for either RS232 or TTL level UART with and without hardware handshaking. Refer to the board specific hardware manual for pinout details.

Cables are available from Gateworks in the online shop:

Hardware Support

The following product families support UART:

  • Venice:
    • IMX8M UART
    • up to 4Mbd relative to a 1.875MHz clock with 7 or 8 bit data, 1 or 2 stop, programable parity
    • Note SP33E RS232 transceiver limit is 1Mbd
    • drivers/tty/serial/imx.c
  • Newport:
    • CN80XX UAA ARM PL011 r1p5 compatible
    • up to 10Mbd (Note SP33E RS232 transceiver limit is 1Mbd)
    • drivers/tty/serial/amba-pl011.c
  • Ventana:
    • IMX6 UART
    • up to 5Mbd (Note SP33E RS232 transceiver limit is 1Mbd and MAX3223E transceiver limit is 500Kbd)
    • drivers/tty/serial/imx.c

See below for more product-specific details

Linux serial port devices

Serial UARTS will be represented as /dev/tty* devices depending on the CPU architecture.

The following table describes the various on-board UARTs for standard Gateworks products. Please consult the board specific hardware user manual for additional details:

Family Board HW Dev SW Dev Function
Venice GW710x UART1 ttymxc0 GPS (optional)
UART2 ttymxc1 console: 3.3V TTL
UART3 ttymxc2 3.3V TTL J4
UART4 ttymxc3 3.3V TTL J41
GW720x-B UART1 ttymxc0 GPS (optional)
UART2 ttymxc1 console: 3.3V TTL and RS232/RS485(J11)
UART3 ttymxc2 3.3V TTL J5
UART4 ttymxc3 RS232 J11 1
GW720x-C+ UART1 ttymxc0 GPS (optional)
UART2 ttymxc1 console: 3.3V TTL and RS232 J11 1
UART3 ttymxc2 3.3V TTL J5
UART4 ttymxc3 RS232 J11 and RS232/RS485(J11) 1
GW730x UART1 ttymxc0 GPS (optional)
UART2 ttymxc1 console: 3.3V TTL and RS232/RS485(J15)
UART3 ttymxc2 Bluetooth (optional)
UART4 ttymxc3 RS232 J11 1
GW74xx UART1 ttymxc0 GPS (optional) and 3.3V TTL to J14.3 (RX in) J14.4 (TX out)
UART2 ttymxc1 console: 3.3V TTL and RS232/RS485(J15)
UART3 ttymxc2 Bluetooth (optional)
UART4 ttymxc3 RS232 (J15) 1
Newport GW610x UART0 ttyAMA0 console: 3.3V TTL J9
UART1 ttyAMA1 GPS (optional)
UART22 ttyAMA2 TTL J10
UART32 ttyAMA3 -
GW620x UART0 ttyAMA0 console: 3.3V TTL J12
UART1 ttyAMA1 GPS (optional)
UART22 ttyAMA2 RS232/RS485 J9 1
UART32 ttyAMA3 RS232 J9 1
GW630x UART0 ttyAMA0 console: 3.3V TTL J17
UART22 ttyAMA2 RS232/RS485 J12 1
UART32 ttyAMA3 RS232 J12 1
GW640x UART0 ttyAMA0 console: 3.3V TTL J18
UART22 ttyAMA2 RS232/RS485 J14 1
UART32 ttyAMA3 RS232 J14 1
Ventana GW51xx UART1 ttymxc0 GPS
UART2 ttymxc1 console: 3.3V TTL JTAG J10/J11
UART5 ttymxc4 TTL J11
GW52xx UART1 ttymxc0 RS485/CAN/TTL J12 / RS232 J10
UART2 ttymxc1 console: 3.3V TTL JTAG J14 / RS232 J10
UART5 ttymxc4 GPS
GW5910 UART1 ttymxc0 RS232 J12
UART2 ttymxc1 console: 3.3V TTL JTAG J14 / RS232 J12
UART3 ttymxc2 CC1352
UART4 ttymxc3 WiFi/BLE
UART5 ttymxc4 GPS
GW53xx UART1 ttymxc0 RS485/CAN/TTL J11 / RS232 J12
UART2 ttymxc1 console: 3.3V TTL JTAG J14 / RS232 J12
UART5 ttymxc4 GPS
GW54xx UART1 ttymxc0 RS485 J13 / RS232 J15
UART2 ttymxc1 console: 3.3V TTL JTAG J17 / RS232 J15
UART5 ttymxc4 GPS
GW551x UART2 ttymxc1 console: exp 3.3V TTL J3
UART3 ttymxc2 exp 3.3V TTL J3
GW552x UART2 ttymxc1 console: 3.3V TTL JTAG J7
UART3 ttymxc2 exp 3.3V TTL J5
UART5 ttymxc4 exp 3.3V TTL J5
GW553x UART2 ttymxc1 console: 3.3V TTL JTAG J11
UART3 ttymxc2 exp 3.3V TTL JTAG J10
UART4 ttymxc3 GPS
UART5 ttymxc4 exp 3.3V TTL JTAG J10
GW16111 UART2 ttymxc1 console: 3.3V TTL JTAG J8 / RS232 J16 / RS485 J20
UART3 ttymxc2 RS232 J16
  1. Depends on software configuration
  2. UART2/UART3 are configured in early boot firmware via the hwconfig variable (see [wiki:newport/bootloader#serialconfig here). Additionally CN80XX UART2 and UART3 can be steered via software PINSEL via software modification to any of the GPIO signals including the off-board DIO's - contact support@… for details


The maximum baudrate supported depends on the product family and sometimes whether or not you are using TTL or RS232 as at times a transceiver may limit the stream to below 250000 (consult the hardware manual). A safe and typical baudrate is 115200.

Flow control

Flow control refers to hardware or software handshaking that can tell a transmitter when its ok to send more data.

Hardware flow control is typically via RTS/CTS (request-to-send / clear-to-send) or via DTR/DSR (data-terminal-ready / data-set-ready). Depending on the board and port used you may be able to use hardware flow control.

Software flow control uses a specific character for start and stop and therefore cannot be used in binary data transfer.

Take care to set flow control properly. Typically this is done in Linux with the stty application. For example to disable the first UART's flow control on an IMX6 based Ventana product:

stty -crtscts -F /dev/ttymxc0

Serial Port Types (DCE vs DTE)

The Data Communication Equipment (DCE) pin assignments permit direct connection to a standard Data Terminal Equipment (DTE) PC running terminal emulation software. The inputs and outputs are swapped between DCE and DTE. Please pay careful attention to the connector pinout of any UART from the board's hardware user manual, specifically the TX/RX/RTS/CTS pin's direction (input vs output) to ensure proper interconnect with off-board equipment.

In certain scenarios, a null modem cable may be needed Null Modem Information

Please read more about the RS232 Specification here

Hardware Flow Control

A few Gateworks products that provide TTL level and/or RS232 serial can optionally provide hardware flow control via RTS# and CTS# pins. In this configuration. All Gateworks products present serial from the Data Terminal Equipment (DTE) perspective where TX and RTS are outputs, and RX and CTS are inputs with one exception:

  • Ventana UART's are in DCE mode by default however TX is still an output and RX is an input. Therefore Ventana UART's with flow control are pinned out as follows:
    • TX (output)
    • RX (input)
    • RTS (input)
    • CTS (output)


RS-232 is a standard for serial communication transmission of data. The most important aspects of the spec are:

  • defines electical signal characteristics such as voltage levels, signalling rate, timing and slew-rate of signals
  • mechanical characteristics such as pinouts

Many Gateworks boards have RS232 support (not to be confused with TTL level UART support which has different voltage and signalling specifications).

Example usage: RS232 connection to a PC

  1. Connect all hardware / cables

    Note: A NULL MODEM adapter / cable may need to be used between the Gateworks board and a PC!

  1. The serial port must be configured on the Gateworks board using the stty command for the baud rate:
    # stty --help
    BusyBox v1.19.4 (2013-06-26 04:34:22 PDT) multi-call binary.
    Usage: stty [-a|g] [-F DEVICE] [SETTING]...
    Without arguments, prints baud rate, line discipline,
    and deviations from stty sane
            -F DEVICE       Open device instead of stdin
            -a              Print all current settings in human-readable form
            -g              Print in stty-readable form
            [SETTING]       See manpage

Set the baud rate, example shown below for 115200:

root@OpenWrt:/# stty -F /dev/ttyS1 115200

Also Verify Flow Control: (adjust below example to exact application)

stty -crtscts -F /dev/ttyS1
  1. Configure PC:
    • On the receiving PC, a terminal program can be used. For example:
      • Windows: putty, hyperterm
      • Linux: screen, minicom
        • Example (adjust as necessary)
          screen /dev/ttyS1 115200,cs8
    • Be sure to select the same baud rate, format (ie 8 data bits, no parity, 1 stop bit - aka 8N1 or cs8) and flow control as configured on the Gateworks board.
  1. Test Connection:
    • A quick way to test this is to use an echo statement from the console of the Gateworks board like so:
      # echo "0" > /dev/ttyS1

The character '0' should appear on the serial console on the PC.


  • make sure you are either not using hardware/software flow control, or in the case that you do have hardware flow control (CTS/RTS or DTR/DSR) they are properly connected.See Link Here
  • use a null-modem if needed (DTE vs DCE)
  • make sure both ends are RS232 compliant (as opposed to TTL level logic)


Some Gateworks boards have RS485 transceivers connected to host CPU UART's. This often is an optional feature that must be loaded at the factory. Please contact support@… via email for more information.

RS485 uses a differential pair and is half-duplex such that the TX and RX share a differential pair on a multi-master bus and the TX and an enable that is controlled via one of:

  1. always enabled (only useful if there is only a single transmitter on the bus)
  2. connected to UART RTS line
  3. connected to host CPU GPIO

Note that RS485 is half-duplex and RS422 is full-duplex.

Because of the half-duplex nature, typically custom software needs to be written to:

  • control the enabling of the transmit driver (on more modern boards and kernels this is done for you by the driver via device-tree and/or the TIOCSRS485 ioctl)
  • receive the data you sent directly after sending (unless the driver does this for you)
  • implement a protocol such that multiple masters know when it is there turn to transmit

In a typical scenario you may have two devices on an RS485 half-duplex bus, NodeA and NodeB and a conversation would look like this:

  1. NodeA enables its transmitter, sends out a request packet, then disables its transmitter and waits for a response
  2. NodeB waits for a request packet and until it sees one its transmitter is not enabled. When it receives the request it enables its transmitter, sends a response, then disables its transmitter.
  3. NodeA knows the exact nature of the response so it can wait until the message is complete then knows that it can transmit again

The following table and sections below provides per-board details of RS485:

Family Board TXEN Transceiver Termination Fail-Safe Bias Notes
Venice GW720x / GW730x / GW740x RTS SP335E software selectable via 'rs485_term' gpio included TIOCSRS485 support
Newport GW630x / GW640x RTS SP335E software selectable via hwconfig included TIOCSRS485 support
Ventana GW52xx (optional) gpio193 MAX14840 optional optional TIOCSRS485 support
GW53xx (optional) gpio193 MAX14840 optional optional TIOCSRS485 support
GW54xx (optional) gpio193 MAX14840 optional optional TIOCSRS485 support
GW551x+GW16111 gpio19 MAX14840 optional 4.75k pull up/down see ventana RS485 below
GW5904 / GW5909 RTS MAX14840 software selectable via 'rs485_term' gpio included TIOCSRS485 support


RS485 Termination

As a general rule, termination resistors should be placed at both far ends of the RS485 network. Without termination resistors reflections of fast driver edges can cause data corruption. Termination resistors also reduce electrical noise sensitivity due to lower impedance. The value of each termination resistor should be equal to the cable characteristic impedance (typically 120 ohms for twisted pairs).

Some boards with RS485 capability may have transceivers with specific fail-safe features within the transceiver however optional on-board termination resistors typically exist as well and it is the responsibility of the system designer to determine where termination needs to go and what values should be used. In general, gateworks boards with RS485 transceivers have an optional resistor for termination that can be loaded with a customer specified value. Contact sales@… via e-mail for more information.

Additionally some boards have software selectable termination via a gpio (see the table above)


RS485 Failsafe Bias Resistors

When inputs are between -200mV and +200mV the receiver output is 'undefined'. There are four common fault conditions that result in this undefined receiver output that can cause erroneous data:

  • All transmitters in a system are not driving
  • The receiver is not connected to the cable
  • The cable has an open
  • The cable has a short

Fail-safe biasing is used to keep the receiver's output in a defined state when one of these conditions occur. The biasing consists of a pull-up resistor on the noninverting line and a pull-down resistor on the inverting line. With proper biasing, the receiver will output a valid high when any one of the fault conditions occur. These fail-safe bias resistors should be placed at the receiver end of the transmission line.

Some boards with RS485 capability may have on-board termination resistor options and it is the responsibility of the system designer to determine where termination needs to go and what values should be used. In general, gateworks boards with RS485 transceivers have an optional resistor for termination that can be loaded with a customer specified value. Contact sales@… via e-mail for more information.

on-board failsafe bias resistors 4.75Kohm pulls and an optional on-board 121ohm load termination resistor.



Software must enable RS485 at boot via device-tree.

See below


The GW630x / GW640x have 2 CN80XX UART's (UART2/UART3: /dev/ttyAMA2 and /dev/ttyAMA3) going to a SP335E transceiver which allows selecting a variety of RS232/RS485 configurations:

  • 2x RS232 w/o flow control
  • 1x RS232 w/ flow control
  • 1x RS485 half duplex
  • 1x RS485 full duplex

RS485 modes feature:

  • optional half-duplex, multi-drop RS485:
  • CN80XX UART2 (/dev/ttyAMA2)
  • optional on-board termination (enabled by gpio16)
  • in-chip fail-safe protection to default idle inputs to logic-high (no external bias resistors required)
  • TXEN connected to CN80XX UART2 RTS

The pl011 Linux driver supporting the CN80XX serial ports (SERIAL_AMBA_PL011 drivers/tty/serial/amba-pl011.c) supports RS485 via the TIOCSRS485 ioctl as of Linux 5.15 and this support has been backported to the Gateworks 5.10 kernel.

Software must enable RS485 at boot via device-tree or use the TIOCSRS485 ioctl to configure TXEN - see below).


The GW52xx/GW53xx/GW54xx support optional half-duplex, multi-drop RS485:

  • IMX6 UART1 (/dev/ttymxc0)
  • MAX14840 transceiver
  • TXEN connected to gpio193
  • optional on-board termination resistor
  • optional on-board fail-safe resistors (D+ pull-up and D- pull-down)

By 'optional' this means the baseboard design supports this, but it is not loaded on standard product therefore would be a Gateworks Special. Contact sales@… if interested to see if a configuration already exists.

Software must enable RS485 at boot via device-tree or use the TIOCSRS485 ioctl to configure TXEN - see below).

The GW551x + GW16111 breakout module support half-duplex, multi-drop RS485:

  • IMX6 UART2 (/dev/ttymxc1)
    • Note that a jumper must be placed on J10:2-3 to enable RS485 RX (routes UART2 RX to RS485 vs RS232 transceiver)
    • Note that a jumper must be placed on J10:1-2 to enable RS232 (routes UART2 to RS232 transceiver)
  • MAX14840 transceiver
  • TXEN connected to gpio19, or always enabled, or enable on transmit (selected via J10 jumper)
    • always drive mode - jumpers placed on J10:2-3 and J10:4-5 will cause the transceiver to always have its transmit enabled. This is useful for fast signal switching (fast/large bus) if you are using a single master and one or more receivers.
    • TXD drive mode - jumpers placed on J10:2-3 and J10:7-8 will cause the transceiver to be enabled only when TX is asserted. Because there are 4.75k pull's on D+/D- the bus is never 'idle'. This is useful for multi-master scenarios but could pose issues with fast/large busses if the 4.75pull's are not strong enough to switch the signals quick enough.
    • DIO-drive mode - jumpers placed on J10:2-3 and J10:7-8 will cause the transceiver to be enabled only when IMX_DIO1 (gpio19) is asserted high. This is useful for fast/large busses where the TXD drive mode doesn't provide fast enough switching. If using this mode you either need to manage the assertion/de-assertion of gpio19 in usersapce or modify the GW551x device-tree to configure rs485-txen for TIOCSRS485 support by adding fsl,rs485-gpio-txen = <&gpio1 19 GPIO_ACTIVE_HIGH>; to the uart2 device-tree node in arch/arm/boot/dts/imx6qdl-gw551x.dtsi
  • 121ohm termination (R38) is loaded and can be enabled by placing a jumper on J10:9-10
  • 4.75k pull-up on D+, 4.75k pull-down on D- fail-safe bias resistors are loaded (R37/R40) (bus defaults to logic 1 - never idle)

TIOCSRS485 ioctl

For UART's that have a built-in half-duplex mode capable of automatically controlling line direction via a transmit enable by toggling the UART's RTS signal the TIOCSRS485 ioctl can be used to configure RS485 TXEN.

Some UART drivers such as the IMX SoC's also allow generic GPIO's to be used for this by configuring the UART via the device-tree rts-gpios property.

Code Examples:

  • ANSI-C:
  • Note the below code is not required for Venice, as it is handled in the device tree
            #include <linux/serial.h>
            /* Include definition for RS485 ioctls: TIOCGRS485 and TIOCSRS485 */
            #include <sys/ioctl.h>
            /* Open your specific device (e.g., /dev/mydevice): */
            int fd = open ("/dev/mydevice", O_RDWR);
            if (fd < 0) {
                    /* Error handling. See errno. */
            struct serial_rs485 rs485conf;
            /* Enable RS485 mode: */
            rs485conf.flags |= SER_RS485_ENABLED;
            /* Set logical level for RTS pin equal to 1 when sending: */
            rs485conf.flags |= SER_RS485_RTS_ON_SEND;
            /* or, set logical level for RTS pin equal to 0 when sending: */
            rs485conf.flags &= ~(SER_RS485_RTS_ON_SEND);
            /* Set logical level for RTS pin equal to 1 after sending: */
            rs485conf.flags |= SER_RS485_RTS_AFTER_SEND;
            /* or, set logical level for RTS pin equal to 0 after sending: */
            rs485conf.flags &= ~(SER_RS485_RTS_AFTER_SEND);
            /* Set rts delay before send, if needed: */
            rs485conf.delay_rts_before_send = ...;
            /* Set rts delay after send, if needed: */
            rs485conf.delay_rts_after_send = ...;
            /* Set this flag if you want to receive data even while sending data */
            rs485conf.flags |= SER_RS485_RX_DURING_TX;
            if (ioctl (fd, TIOCSRS485, &rs485conf) < 0) {
                    /* Error handling. See errno. */
            /* Use read() and write() syscalls here... */
            /* Close the device when finished: */
            if (close (fd) < 0) {
                    /* Error handling. See errno. */

Some Linux UART drivers that call the 'uart_get_rs485_mode' function allow you to configure this behavior on boot via device-tree properties that can be added to the UART device-tree node:

  • linux,rs485-enabled-at-boot-time - enables the rs485 feature at boot time. It can be disabled later with proper ioctl
  • rs485-rts-delay - Delay between RTS signal and beginning of data sent in milliseconds corresponding to the delay before sending data
  • rs485-rx-during-tx - enables the receiving of data even while sending data
  • rs485-rts-active-low - drive RTS low when sending (default is high)
  • rs485-term-gpios - GPIO pin to enable RS485 bus termination (if defined will drive this low when the UART is configured). As there is no API to alter this behavior often its best to leave this up to userspace GPIO control (unless driving low to disable it on boot is what you want)

An example of using device-tree to enable RS485 TXEN configuration at boot can be seen here

See also:

Specific Product / Model Notes


Most Venice products have a flexible MaxLinear SP335E RS-232/RS-485/RS-422 transceiver which is software configurable. For these boards a device-tree fragment is used to configure the kernel appropriately for the mode you desire:

  • 2x RS232 TX/RX (default configuration requiring no dt overlay)
  • 1x RS232 TX/RX/RTS/CTS (uses rs232-rts dt overlay)
  • 1x RS485 (half duplex) (uses rs485 dt overlay)
  • 1x RS422 (full duplex) (uses rs422 dt overlay)

WARNING Some Venice models such as the GW73xx have the default Linux serial console being output to the same UART that a RS485 might be using. Thus, it is critical to disable the Linux serial console with the console variable in u-boot. Please note that when doing this, it is critical to first enable something like ssh to access the board over the network.

  • Enable ssh first Instructions for Ubuntu
  • Disable serial console in uboot prompt or move to a different port:
    • setenv console 'console=/dev/null'
      • Also see silenceconsole for details on silencing or changing the serial console for boot firmware, Linux kernel, and OS.

To use dt overlays you specify the appropriate overlay(s) in the U-Boot fdt_overlays environment variable in the U-Boot bootloader. This instructs U-Boot to load and apply those dt overlay fragments to the board dt before booting Linux. Note that you can have more than one dt overlay specified in the fdt_overlays separated by a space:

  • imx8mm-venice-gw730x-0x RS485:
    setenv fdt_overlays "$fdt_overlays imx8mm-venice-gw73xx-0x-rs485.dtbo"
  • imx8mm-venice-gw730x-0x RS422:
    setenv fdt_overlays "imx8mm-venice-gw73xx-0x-rs422.dtbo"
  • imx8mm-venice-gw730x-0x RS232-CTSRTS flow control:
    setenv fdt_overlays "imx8mm-venice-gw73xx-0x-rs232-rts.dtbo"
  • imx8mm-venice-gw730x-0x default 2x RS232 TX/RX:
    setenv fdt_overlays

For Venice when configured for RS485 or RS422 you do not need to use the TIOCSRS485 as the dt fragments will enable this on bootup as well as configure the UARTs transmit enable pin.

If you wish to enable the SP335E on-board RS485 termination (which is disabled by default) you can set the GPIO manually:

  • GW730x/GW720x:
    • U-Boot:
      u-boot=> gpio status GPIO1_0
      Bank GPIO1_:
      GPIO1_0: output: 0 [x] rs485_term.gpio-hog
      u-boot=> gpio set GPIO1_0
      gpio: pin GPIO1_0 (gpio 0) value is 1
    • Linux (sysfs) (deprecated API)
      root@focal-venice:~# grep rs485_term /sys/kernel/debug/gpio 
       gpio-0   (rs485_term          )
      root@focal-venice:~# echo 0 > /sys/class/gpio/export 
      root@focal-venice:~# echo out > /sys/class/gpio/gpio0/direction 
      root@focal-venice:~# echo 1 > /sys/class/gpio/gpio0/value
    • Linux (gpiolib)
      root@focal-venice:~# gpiofind "rs485_term"
      gpiochip0 0
      root@focal-venice:~# gpioset --mode=signal --background gpiochip0 0=1
      • Note we have to tell gpioset to remaining running until it receives a signal in order to keep the GPIO line from reverting back to its original state when the process exits. If you use the deprecated sysfs API or set it in U-Boot it may be easier

Note that if you want to be able to change the UART mode at runtime instead of at boot time you can use the 'rs232-rts' overlay which adds the RTS signal capability to the UART, configure the various gpios manually at runtime via sysfs or gpiod, and use the TIOCSRS485 ioctl to invoke RS485 mode with the polarity of the RTS pin handled by setting rs485conf.flags SER_RS485_RTS_AFTER_SEND


Most Newport products have a flexible MaxLinear SP335E RS-232/RS-485/RS-422 transceiver which is software configurable. For these boards the Bootloader hwconfig environment variable can be used to configure the functionality of the serial ports at power-up between dual RS232 without flow control (default), single RS232 with hardware flow control, and RS485 (full/half duplex and optional termination). See Newport bootloader hwconfig for more details.

If you wish to configure the SP335E transceiver yourself you can control it at runtime through the following GPIO's (which is what the Boot firmware will do for you via the hwconfig variable):

  • GPIO21: UART_HALF - selects full (low) or half (high) duplex (RS485 mode only)
  • GPIO22: UART_TERM - disables (low) or enables (high) on-chip termination (RS485 mode only)
  • GPIO23: UART_RS485 - selects between RS232 (low) and RS245 (high) modes

See gpio for details on using GPIO from Linux userspace or use the U-Boot bootloader gpio command.

The CN80XX / CN81XX has 4 TTL level UARTs with the following mapping:

UART device-tree alias Linux device
UART0 serial0 /dev/ttyAMA0
UART1 serial1 /dev/ttyAMA1
UART2 serial2 /dev/ttyAMA2
UART3 serial3 /dev/ttyAMA3

The Linux kernel uses the console parameter from the Kernel cmdline to specify the serial console. You can usually modify the 'console' U-boot env variable if you want to change the default console UART as it typically gets passed on to the kernel:

Newport > setenv console '/dev/ttyAMA2,115200n8'; saveenv
  • Specify that Linux and userspace use UART2

The U-Boot Bootloader (as well as kernel if the console cmdline is not specified) uses the device-tree 'chosen' node 'stdout-path' property to specify serial console. For example the default is specified in the cn81xx-linux.dtsi:

        chosen {
               stdout-path = "serial0:115200n8";

If you wish to change the serial console in the BDK or ATF, you will need to modify the source code which will take some digging.

Code Examples

General Linux UART use

Here is an example application that demonstrates how to configure the UART (including TIOCSRS485 ioctl to for RS485 transmit enable) as well as sending and receiving data:

#include <ctype.h>
#include <errno.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <termios.h>
#include <time.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <linux/serial.h>

#ifndef TIOCSRS485
#define TIOCSRS485        0x542F

/** main function
int main(int argc, char** argv)
        struct termios orig_ttystate, ttystate;
        int fd, sz, rz, bytes;
        speed_t speed;
        const char *baud, *mode, *dev;
        int timeout = 2; // time in seconds to wait for response
        char *msg = NULL;
        char buf[8192];
        time_t start;

        if (argc < 4) {
                        "usage: %s <device> <baud> <mode> [<message>]\n",

        dev = argv[1];
        baud = argv[2];
        mode = argv[3];
        if (argc > 4)
                msg = argv[4];

        // open device
        fd = open(dev, O_RDWR | O_NONBLOCK | O_NOCTTY);
        if (fd <= 0) {

        // get original ttystate
        tcgetattr(fd, &orig_ttystate);

        // create a sane TTY state (raw mode, no HW/SF flow control)
        memset(&ttystate, 0, sizeof(ttystate));

        // enable receiver and ignore modem status lines
        ttystate.c_cflag |= CREAD;
        ttystate.c_cflag |= CLOCAL;

        // data-size
        ttystate.c_cflag &= ~CSIZE;
        switch(mode[0]) {
        case '5': ttystate.c_cflag |= CS5; break;
        case '6': ttystate.c_cflag |= CS6; break;
        case '7': ttystate.c_cflag |= CS7; break;
        case '8': ttystate.c_cflag |= CS8; break;
        default: fprintf(stderr, "invaid character size in %s\n", mode); break;

        // parity
        ttystate.c_cflag &= ~PARODD;
        ttystate.c_cflag &= ~PARENB;
        switch(toupper(mode[1])) {
        case 'N': break; // no parity
        case 'O': ttystate.c_cflag |= (PARENB | PARODD); break; // odd 
        case 'E': ttystate.c_cflag |= PARENB; break; // even
        default: fprintf(stderr, "invaid parity in %s\n", mode); break;

        // stop bits
        switch(toupper(mode[2])) {
        case '1': ttystate.c_cflag &= ~CSTOPB; break;
        case '2': ttystate.c_cflag |= CSTOPB; break;
        default: fprintf(stderr, "invaid stop bit mode in %s\n", mode); break;

        // baudrate
        switch(atoi(baud)) {
        case 1200: speed = B1200; break;
        case 2400: speed = B2400; break;
        case 9600: speed = B9600; break;
        case 19200: speed = B19200; break;
        case 38400: speed = B38400; break;
        case 57600: speed = B57600; break;
        case 115200: speed = B115200; break;
        case 230400: speed = B230400; break;
        case 460800: speed = B460800; break;
        default: fprintf(stderr, "invalid baud rate %s\n", baud); break;
        if (cfsetispeed(&ttystate, speed))
        if (cfsetospeed(&ttystate, speed))

        // set tty state
        printf("setting ttystate\n");
        if (tcsetattr(fd, TCSANOW, &ttystate))

        // configure rs485
                struct serial_rs485 rs485;

                printf("enabling rs485 via TIOCSRS485\n");
                memset(&rs485, 0, sizeof(rs485));
                rs485.flags = SER_RS485_ENABLED | SER_RS485_RTS_ON_SEND;
                if (ioctl(fd, TIOCSRS485, &rs485))

        // flush in/out data
        tcflush(fd, TCIOFLUSH);

        // transmit data
        if (msg) {
                if (strcmp("STDIN", msg) == 0) {
                        while (1) {
                                sz = read(0, buf, sizeof(buf));
                                printf("transmitting %d bytes...\n", sz);
                                if (write(fd, buf, sz) != sz) {
                } else {
                        sz = strlen(msg) + 1;
                        printf("transmitting %d bytes...\n", sz);
                        if (write(fd, msg, sz) != sz)

        // receive data
        while (1) {
                memset(buf, 0, sizeof(buf));
                rz = read(fd, buf, sizeof(buf) - 1);
                if (rz < 0) {
                        perror("read failed");
                if (rz == 0)
                printf("rx %d(%d): '%s'\n", rz, strlen(buf), buf);

        // restore terminal state
        tcsetattr(fd, TCSANOW, &orig_ttystate);


        return 0;


./tx_enable <device> <baud> <mode> [<message>]
#EXAMPLE: ./tx_enable /dev/ttymxc0 115200 8N1 [hello_world]

Serial Console

serial Console

There are times when one may not care about serial Linux console because the serial port is desired for other uses.

See silenceconsole for details on silencing or changing the serial console for boot firmware, Linux kernel, and OS.

Serial Console Window Size

console applications running inside Linux virtual terminal applications (xterm, rxvt etc) will receive SIGWINCH after a resize operation has taken place and adjust accordingly.

When using a serial console there is no such mechanism and you can see the affect of this by connecting to a serial console and showing your size either via env vars or via the 'stty size' app:

24 80
~# stty size
24 80

No matter how you resize that virtual terminal window (ie xterm), the above commands will always report the same size.

It is possible for the application to actively ask for the current console window size. This can be achieved by sending a ANSI code to position the cursor to 999,999 then request the cursor position.

The resize app from the xterm package does this, but here is also a cleaner version with less dependencies:

gcc resize.c -o /usr/local/bin/resize

You can also do this with a shell script that do the same thing:

resize() {
  old=$(stty -g)
  stty raw -echo min 0 time 5
  printf '\0337\033[r\033[999;999H\033[6n\0338' > /dev/tty
  IFS='[;R' read -r _ rows cols _ < /dev/tty
  stty "$old"
  #echo "size:${cols}x${rows}"
  stty cols "$cols" rows "$rows"

you could add this and the following to your /etc/bash.bashrc so that the console size is detected and adjusted on login:

# IMX6 serial console
[[ "$(tty)" =~ "/dev/ttymxc" ]] && { resize; }
# Newport serail console
[[ "$(tty)" =~ "/dev/ttyAMA" ]] && { resize; }

Some people also opt to use the 'trap' shell function to execute it on every command in order to adjust continually:

# IMX6 serial console
[[ "$(tty)" =~ "/dev/ttymxc" ]] && { trap resize DEBUG; }
# Newport serail console
[[ "$(tty)" =~ "/dev/ttyAMA" ]] && { trap resize DEBUG; }


  • 'tty' shows the name of the terminal device (ie /dev/ttymxc1 if on a Ventana serial console)
  • 'stty size' outputs the current terminal size based on what the kernel thinks
  • this has nothing to do with terminal type (TERM)
  • when remotely connected to a board via ssh for example, this isn't an issue as that can use SIGWINCH

orts 24 80

  • the COLUMNS and LINES env variables reflect the size of the tty
  • resizable terminals are the result of NAWS 'Negotiate About Window Size' from RFC 1073 Telnet Window Size Option


    /* resize.c */
    #include <stdio.h>
    #include <ctype.h>
    #include <stdlib.h>
    #include <unistd.h>
    #include <sys/types.h>
    #include <string.h>
    #include <termio.h>
    #include <termios.h>
    #include <signal.h>
    #include <pwd.h>
    #define ESC "\033"
    #define TIMEOUT         10
    char *myname;
    char getsize[] = ESC "7"  ESC "[r" ESC "[999;999H" ESC "[6n";
    char restore[] = ESC "8";
    struct termios tioorig;
    char size[] = ESC "[%d;%dR";
    int tty;
    FILE *ttyfp;
    static void onintr (int sig);
    static void resize_timeout (int sig);
    static void Usage (void);
    static void readstring (FILE *fp, char *buf, char *str);
    char *
    x_basename(char *name)
       char *cp;
       cp = strrchr(name, '/');
       return (cp ? cp + 1 : name);
      tells tty driver to reflect current screen size
    main (int argc, char **argv)
           int rows, cols;
           struct termios tio;
           char buf[BUFSIZ];
           struct winsize ws;
           char *name_of_tty;
           myname = x_basename(argv[0]);
           if (argc > 1) Usage();
           name_of_tty = "/dev/tty";
           if ((ttyfp = fopen (name_of_tty, "r+")) == NULL) {
               fprintf (stderr, "%s:  can't open terminal %s\n",
                        myname, name_of_tty);
               exit (1);
           tty = fileno(ttyfp);
           tcgetattr(tty, &tioorig);
           tio = tioorig;
           tio.c_iflag &= ~ICRNL;
           tio.c_lflag &= ~(ICANON | ECHO);
           tio.c_cflag |= CS8;
           tio.c_cc[VMIN] = 6;
           tio.c_cc[VTIME] = 1;
           signal(SIGINT, onintr);
           signal(SIGQUIT, onintr);
           signal(SIGTERM, onintr);
           tcsetattr(tty, TCSADRAIN, &tio);
           write(tty, getsize, strlen(getsize));
           readstring(ttyfp, buf, size);
           if(sscanf (buf, size, &rows, &cols) != 2) {
                   fprintf(stderr, "%s: Can't get rows and columns\r\n", myname);
           write(tty, restore, strlen(restore));
           if (ioctl (tty, TIOCGWINSZ, &ws) != -1) {
               /* we don't have any way of directly finding out
                  the current height & width of the window in pixels.  We try
                  our best by computing the font height and width from the "old"
                  struct winsize values, and multiplying by these ratios...*/
               if (ws.ws_col != 0)
                   ws.ws_xpixel = cols * (ws.ws_xpixel / ws.ws_col);
               if (ws.ws_row != 0)
                   ws.ws_ypixel = rows * (ws.ws_ypixel / ws.ws_row);
               ws.ws_row = rows;
               ws.ws_col = cols;
               ioctl (tty, TIOCSWINSZ, &ws);
           tcsetattr(tty, TCSADRAIN, &tioorig);
           signal(SIGINT, SIG_DFL);
           signal(SIGQUIT, SIG_DFL);
           signal(SIGTERM, SIG_DFL);
    static void
    readstring(register FILE *fp, register char *buf, char *str)
           register int last, c;
           signal(SIGALRM, resize_timeout);
           alarm (TIMEOUT);
           if ((c = getc(fp)) == 0233) {   /* meta-escape, CSI */
                   *buf++ = c = ESC[0];
                   *buf++ = '[';
           } else {
                   *buf++ = c;
           if(c != *str) {
                   fprintf(stderr, "%s: unknown character, exiting.\r\n", myname);
           last = str[strlen(str) - 1];
           while((*buf++ = getc(fp)) != last)
           alarm (0);
           *buf = 0;
    static void
            "Usage: %s\n"
            "   sets size via ioctl\n", myname);
    static void
    resize_timeout(int sig)
           fprintf(stderr, "\n%s: timeout occurred\r\n", myname);
    /* ARGSUSED */
    static void
    onintr(int sig)
           tcsetattr (tty, TCSADRAIN, &tioorig);
Last modified 11 months ago Last modified on 07/12/2023 09:42:10 PM

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