Serial Console

Most routers come with an UART integrated into the System-on-chip and its pins are routed on the Printed Circuit Board (PCB) to allow debugging, firmware replacement or serial device connection (like modems). The bootloader is responsible for listening on it.

So in contrast to the JTAG-Port, the Serial Ports needs some software running on the CPU so we can use it! In case the bootloader is damaged, or does not offer such a feature, the port is useless.

Finding an UART on a router is fairly easy since it only needs 3 signals (without modem signaling) to work : GND, TX and RX (often accompanied by VCC). Since your router is very likely to have its I/O pins working at 3.3V (TTL level voltage), you will need a level shifter such as a Maxim MAX232 to change the level from 3.3V to your computer level which is usually at 12V.

To find out the serial console pins on the PCB, you will be looking for a populated or unpopulated 4-pin header, which can be far from the SoC (signals are relatively slow) and usually with tracks on the top or bottom layer of the PCB, and connected to the TX and RX.

Once found, you can easily check where is GND, which is connected to the same ground layer than the power connector. VCC should be fixed at 3.3V and connected to the supply layer, TX is also at 3.3V level but using a multimeter as an ohm-meter and showing an infinite value between TX and VCC pins will tell you about them being different signals (or not). RX and GND are by default at 0V, so using the same technique you can determine the remaining pins like this.

If you do not have a multimeter a simple trick that usually works is using a speaker or a LED to determine the 3.3V signals. Additionally most PCB designer will draw a square pad to indicate ping number 1.

Once found, just interface your level shifter with the device and the serial port on the PC on the other side. Most common baud rates for the off-the-shelf devices are 9600, 38400 and 115200 with 8-bits data, no parity, 1-bit stop.

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Many people get along without a serial console for their device because they are able to flash working firmware the first time or are able to apply various recovery methods and do all their communicating with the device over a network. However, due to characteristics of their bootloaders or because they are not yet fully supported, for some devices it can be quite handy to have a compatible serial console available.

Most devices that are supported by OpenWrt include a serial port. These serial ports typically provide a console to the bootloader and, when the firmware has booted, a console to the running system. Typically, a console to the bootloader will allow you to configure a network, fetch and flash a new firmware, which can be a life-saver when the firmware is broken. A console to the running system will let you correct a misconfigured network.


All socs have some sort of UART, often in combination with a RS-232-Interface connected with the PCB. Now depending on the model, the device's serial port could be available

  • as a 9-pin D connector accessible from the exterior of the case,
  • as pin headers on the PCB,
  • as unpopulated holes in the PCB.

For help with the latter two, see soldering. But even if there is 9-pin D connector, beware the Voltage levels!

If the serial port is not readily accessible from the exterior of the device enclosure, you have some choices:

  • modify the enclosure, either to allow passage of a cable or to attach a convenient connector, e.g.:
  • a 9-pin D connector (this is a good choice if you are going to build a level-shifter into the interior of the enclosure, so as to provide a standard +/- 12V interface externally);
  • a 1/8-inch stereo headphone jack (this is a good choice if you are simply bringing the lines to the exterior)
  • open the case to attach to header pins or holes, as needed, this is a good choice if:
  • opening the enclosure is easy;
  • access to the serial port is needed only very occasionally; and/or
  • you have many devices you would rather not modify.

Voltage levels

This section descripes actions that might damage your device or firmware. Proceed with care!

Caution: Very few devices have standard RS-232 +/- 12V serial ports, but in many OpenWrt-supported devices the serial ports operate at TTL voltage (sometimes 5V, most often 3.3V) levels, meaning you cannot use a standard serial or USB to serial cable: it will fry your board.

In order for the serial console to work, the logic levels on the wires should match those expected by your device. The first step therefore is to determine which voltage levels are required. Often, you can find this documented on the OpenWrt wiki or elsewhere.

Check your device's voltage levels with this chart. For detailed description look here. In addition these levels are averages for the type of devices shown.

Talking and Listening

In order to interact with your device over its serial port, you need a minimum of three wires connected: a ground (GND); transmit (TX); and receive (RX). It is possible to get useful information about what is happening with only GND and RX, but in order to fix a problem you will usually also need TX. Your computer's TX should be connected to the device's RX, and your computer's RX should be connected to the device's TX. The computer's GND should connect the the device's GND. That way, what you say will get heard by the device and what the device says will get heard by your computer. This is often called a "null-modem" configuration.

You will also need a terminal emulation program on your computer, such as minicom, hyperterminal, etc. The terminal emulation program needs to be configured to be compatible with your device, in particular, with regard to baud rate and flow control. If you are using only three wires (GND, TX, and RX) then hardware flow control should be turned off; you aren't using the pins (RTS and CTS) necessary for it to work. Rarely, the baud rate that the device expects might be different in the bootloader and the running firmware; if so, you'll need to modify the baud rate settings in your terminal emulator after the firmware boots up.


Some things to consider:

  • If your computer has a serial port, you can use a level-shifter (as necessary) and a "null-modem cable".
  • If your computer has a USB port, then:
  • if your device uses standard RS232 logic levels, you can use a standard USB-serial converter along with a standard "null-modem cable"
  • if your device uses TTL logic levels, you can use a USB-serial cable with a serial to TTL adaptor or a USB to TTL adaptor (that use the right TTL voltage) and a connector suitably wired to connect to your device.
  • If your computer has neither a serial port or a USB port, you are in trouble!

These days, computer manufacturers are dropping RS232 serial ports, while USB ports are increasingly ubiquitous. Particularly if you need to TTL logic levels, USB is probably the way to go since you can get the right logic levels (the voltage) integrated in the USB-TTL converter.


USB enabled routers

For routers with a built in USB-connector (such as WNDR3700 for example), simply plugging in a USB-serial converter and installing the appropriate software will provide a serial console to the router.

  • Install the appropriate packages, e.g. kmod-usb-serial and kmod-usb-serial-ftdi for FTDI based converters or kmod-usb-serial-pl2303 for prolific based converters
  • Add the new serial port to /etc/inittab, (if you have multiple adaptors find the right one in /proc/tty/driver/usbserial):

ttyUSB0::askfirst:/bin/ash –login

Prebuilt Cables

Standard RS232 levels, for example:

TTL 5V, for example:

TTL 3.3V, for example:

You may need to rewire the terminals of the TTL cables to match your device pinout.

USB-serial parts

Cellphone Data Cables

A USB based data cable for a mobile cell phone is another possibility.

Ebay clone cables:

  • Datacable for Nokia 6210, 6250, 6310, 6310i, 7110


Build your own Serial to DB9F RS232 Cable

I found an easy way to get a cable working, it just require 7 components:

  1. 4 Resistances 1,5K 3,9K 4,7K 22K all by 1/4W
  2. 2 Transistors BC337
  3. 1 DB9F Terminal

The image shows how to make the cable

This cost minus than 2 Dolars in Chile, i hope it be the same for you! I tested it on a Zyxel 600 Series, it works!


Level conversion

TTL-RS-232 level conversion is a fairly common problem, so there are a number of ICs on the market that convert between these voltage levels. Maxim IC has made a few handy little ICs for us to use. The best (IMHO) is the MAX3232, which can operate at different voltages depending on the capacitors in the circuit. This guide will tell you how to solder everything together to get a pc-compatible serial port on your OpenWrt router.

From scratch

First, you need an "RS232-TTL level converter chip." RS232 refers to the standard defining what plugs into your computer, and TTL is a family of chips that use 0V and 0.8V as low and 2.2V and 5V as high. They can be purchased new (the Maxim IC MAX233x line is popular). Most vendors have large minimums, but some (e.g. Mouser Electronics) sell components in small quantities.

The wiring is fairly simple, but it depends on the chip. Generally, it involves connecting Vcc from the router to the chip's Vcc pin, both router and rs-232 grounds to the ground pin, and the TX and RX wires to the chip. Remember that the router's TX will "connect" to the same level conversion bank as the computer's RX. Additionally, some of these level converters require external capacitors, while some have them built in. Much of this varies, so consult the chip's spec.

From a PDA or cell phone serial cable

Another great source for RS232-TTL converters is in cell phone serial cables. Most cell phones need this same circuit to level-up for connection to a PC's serial port. Many people already have such a cable laying around, or can buy one fairly cheap. Using an existing cable is much easier than building one. If you open up the cell phone cable's serial port casing and see a MAX### chip, it's probably the cable you need. One known chip is a MAX323 (yes, 323, the original MAX232 is a 5V device and we need 3.3V here).

If you've found a good cell phone cable to use, you merely need to determine which wires are the VCC, GND, TX, and RX connections. Usually the VCC is red and the GND is black, but the other colors may vary (though blue and orange are common). There should be no need to modify the PCB embedded in the cable.

One type of the "Made in China" ones, not mentioned at ist the "S30880-S5601-A802-1"; its WHITE wire is data out (TX) and the ORANGE one is data in (RX), VCC and GND are red and black. Its a 3.3V converter built with the MAX3386E chip.

MAX232 Kits

Model-specific guides

These guides are somewhat model specific, but if you're struggling to build your own cable, they're filled with information that applies to that part of the process.

Use your old PDA as a console

Since many older PDAs (e.g. Palm series) have TTL serial connections already, you can use them to get a direct serial connection to the router.

Solder the RX, TX, and ground (but never Vcc) TTL-level connectors on the OpenWrt box to the PDA's TTL level serial connectors.

Example: Palm IIIc,

Terminal software

  • Hyperterm (comes with many versions of MS Windows)
  • Minicom (for POSIX systems)
  • cu(1) (part of the Taylor UUCP package, for POSIX systems)
  • Putty v0.59 or newer (now with serial console support!)
  • Pocketterm (for Palm PDAs)
  • Vandyke SecureCRT (commercial but very good)

Serial port pinouts

Pinouts for your model can often be found on your model's page in CategoryModel.

Finding Serial Console

First, check the OpenWrt wiki page describing your hardware and do a Google search. Most of the time, the serial port(s), if they exist, have already been documented by others.

Piezoelectric buzzer method

  1. Use a Piezoelectric buzzer and attach its ground (usually black) wire to a ground point on the router; the back of the power regulators are usually good candidates, but check this with a multimeter/voltmeter.
  2. Use the other wire to probe any of the header pins which may be pre-installed, or any of the component holes which look like they could have header pins installed into (typically in a row of 4 pins for a serial port). Reset the router. The bootloader/linux bootup messages will only happen for a few seconds, and after that, the serial console will be silent - so even if you have the right pin you will not hear anything.
  3. Once you get the right pin, the Piezoelectric buzzer should make a screeching sound much like that of a 56kbps connection.

Digital multimeter method

Typically there are four pins to identify: GND - Ground, Vcc - 3.3VDC or 5VDC, TXD - Transmit data, and RXD - Receive data. There may be additional/extra pins. Every router is different.

  1. Locate the set of four/five/more pins that are most likely to be the serial console.
  2. Set the multimeter to measure resistance/continuity. Place the black probe on a known ground point and use the red probe to check each of the pins. Whenever you see zero or nearly zero ohms resistance, that pin should be the GND connection.
  3. Switch the multimeter to measure DC voltage on a scale greater than 10 but less than 100 volts. Meters vary, but you should be able to select a range greater than five volts. Place the black probe on the known ground point again, and with the router powered on, use the red probe to check the remaining pins of the port for steady 3.3V or 5V DC. When you find it, that pin should be the Vcc connection.
  4. This leaves two pins to identify: RXD and TXD. It's easiest to find the router's TXD pin first, because all the console output from the boot process appears there. Connect the RXD pin of your level shifter to one of the remaining console pins and re-start the router. You should have a terminal window connected to the serial port at the correct bitrate and parity, and you've connected the proper pin, you should see output data the router's startup process. If not, try another pin, restarting the router until you receive valid output. Now you've located the serial port TXD connection.
  5. The only pin remaining is RXD, where the serial port receives data from your terminal session. Connect the TXD pin of your level shifter to the remaining pin (or multiple pins) until you find the one that correctly echoes characters you type in your terminal session.

Logic analyzer/oscilloscope

A more accurate method would be to use either a logic analyzer or an oscilloscope, but these are expensive and for the basic task of locating a serial pin a little overkill. ;-)

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doc/hardware/port.serial.txt · Last modified: 2011/07/10 13:12 by werner