Add a Serial port to the MIPS Linkstation

From NAS-Central Buffalo - The Linkstation Wiki

LS2 homemade serial level converter on top of J1 and a red jumper at J2 to enable the serial console. Electrical tape on the back of the level converter prevents short circuiting the main board. (Also on the picture installed JTAG header and R64.)

Serial port connector installed on the back of the LS2 below the red button.

Introduction

The console is the text output device for system administration messages. These messages come from the kernel, from the init system and from the system logger. On modern small computers the console is usually the computer's attached monitor and keyboard. The LinkStation, however, doesn't have monitor output; instead, it uses a serial connection for console output. Not only does the serial console provide valuable debugging output, it also allows root access!

Background

Most of the information on this page was originally only speculation, based on roughly deciphered information from a Japanes web page. At the end of 2006 the community bought a bricked LS2 and handed it over to Tampakuro aka Kuroguy for trying out various hardware modifications. While at it, kuroguy also verified the procedure for installing a serial console interface. This page has been updated with the information and photos he provided in Hardware Hacks for the LS2.

Prerequisites

• (1)Electronic-grade soldering iron (think really small)
• (1) 0.015", 1 oz solder
• (1) 0.1” series PCB header plug, 4-pin
• (1) Serial converter (or build your own - see step 4)

Access the Board

• Disassemble the LinkStation.
• Remove the main circuit board by removing the 4 mounting screws:
• Place the circuit board on a static-free work area:

Attach Header to the Board

• The console header needs to be attached to the J1 pad
• Remove the preexisting solder from J1

J1 Pinout

Enable Full Serial Tx/Rx

To enable write support across the serial port, simply bridge R186.

The Serial Converter

Buy One

The serial port signals from the processor are only 3.3V. For proper RS-232 12V signaling, an RS-232 level shifter needs to be added. These are very common in PDA serial cables also, but can be purchased from SuperDroid Robots (#MCU-026-172) or CompSys (#A232DBH3v).

Build One

As an alternative you can make the level shifter cable yourself:

Warning: The following image depicts 0.1mF (m = milli) capacitors, while in reality 0.1µF (µ = micro), alias 100nF (n = nano) capacitors are needed for the original MAX3232! The list of parts is correct.

Components can be found at e.g. Digikey

• If you want to build your own serial converter, you will need the following parts:
• (5) 0.1 µF tantalum capacitors
• (1) MAX3232CPE IC (Maxim), 3V type, 16 pin DIL
• (1) RS232 male socket
• (1) Some perf board (about 1" x 1")
• (1) Wire

You can etch a circuit board which makes building the interface and installing the interface really easy. Instructions are Here.

This should be attached as close as possible to the board. The signals are very weak and wires as short as 2" (5 cm) have been reported to cause data loss across the serial port.

• Some details about the MAX232 family
• Original manufacturer's MAX RS32xx Family Datasheet

Enabling the Serial Console

As the LS2 only has a single serial port which is used by the housekeeping microcontroller, it is necessary to enable the serial console. This is done by jumpering the pins of J2 and disabling the watchdog timer on startup (by pressing the red button and powereing it up). It is easiest to install J2 on the back of the board as it will be accessible by removing the cover and not the board from the chassis I used a red jumper so it can be easily located. Another option to removing the cover is to drill a 3/8" hole in the case directly above J2 and make a handle for your jumper from a piece of tape.

Using the Serial Console

Of course you need a serial cable to connect to the serial port. Depending on how you wired the plug on the LS, you will need a null modem, too, or a null modem cable.

Windows

You may access the serial console using Windows HyperTerminal (included with Windows) or TeraTerm Pro terminal emulator.

classic Unix

If you have a classic Unix you should either have tip(1) or cu(1) (or both). They are suitable to set up a serial connection. Linux typically doesn't have these.

Linux

If you are on Linux, consider the trusted and tried miniterm terminal emulator.

Terminal

If you are the proud owner of a real oldschool terminal, this is your chance to make use of the old dust catcher :-)

Serial Port Settings

External Links

• http://www.nslu2-linux.org/wiki/HowTo/AddASerialPort
• http://www.kurobox.com/mwiki/index.php/Add_Serial_Port
• http://www.kurobox.com
• http://www.tldp.org