Modify the Linksys hardware

Summary

This will walk through adding hardware to a Linksys WRT54GL wireless router that will take advantage of existing leads on the PCB for two UART connections, which will be exposed as DB9 serial port connectors mounted to the faceplate of the router. These connections can be used to communicate with the serial console for XINU and also to interact with the Common Firmware Environment's command line interface. Gaining direct access to CFE is a key step towards being able to run XINU on the router.

Before Starting

Parts List

Quantity	Part Name	Details	Part / Model Number	Price
1	LinkSys WRT54GL Router	802.11b/g wireless broadband router	Linksys WRT54GL	~$65.00
1	Ribbon cable	28 AWG, 10 conductor, 25'	Jameco 643508CM	$4.99
2	IDC socket connector	0.1”, 10 conductor	Jameco 32491CM	$0.25
2	IDC shrouded double header	0.1”, 10 conductor	Jameco 67811CM	$0.33
1	ADM202 Transceiver Chip	?????????????	?????????????	$?.??
2	Capacitor 220 nF	?????????????	?????????????	$?.??
3	Capacitor 100 nF	?????????????	?????????????	$?.??
1	Transceiver Board	?????????????	?????????????	$?.??
1	DB9 Female	22AWG,SOLDER CUP	Jameco 15771CM	$0.59
1	DB9 Male	22AWG,SOLDER CUP	Jameco 15747CM	$0.59

(We provide this parts list as a data point; we offer no guarantees about current prices, and it is not our intent to endorse any particular vendor.)

Tools List

• Soldering Iron
• Dremel tool (for cutting holes in plastic case)
• Multimeter, or some other way of checking for proper connections

Steps to Modify the Hardware

Task One: Open the Router

It's really very easy.

There are no screws or tools needed to open the router, just pop open the front with your thumbs as shown in the picture. Some nice illustrated opening instructions can be found for a more detailed explanation of this step

DO NOTE: This is where the warranty on the router is voided!

Task Two: Attach the Serial Header

An overhead view to get your bearings. The serial header is (D) here.

Once the PCB has been removed from the case, locate the serial header holes provided by LinkSys. This would be a grid of 10 holes (5x2) located on the bottom-right corner of the board when the antennae stubs are on top (see the top-down photo for clarification). These ten holes hold all of the input and output for the two serial interfaces--UART0, and UART1--on the device.

A closer look at our attached serial header.

Now, we could just solder wires right onto these holes, but a by placing a nice 10-pin header on the board we can easily attach and detach a 10 connection cable. Here you will use your soldering iron to solder the IDC shrouded double header onto the board.

Task Three: Create the ADM202 Transceiver Circuit Board

Schematic showing the connections between the components of the ADM202 transceiver circuit board and between the board and the DB9 serial ports.

Blank transceiver board before adding components.

Transceiver board with all soldered components in place except ribbon cables.

The next step is to actually create the circuit board used to handle and bridge the input and output from the router and the two DB9 serial ports used to communicate between the backend router and frontend machine. Again, you'll need your soldering iron to secure the different pieces in the transceiver board.

Using the diagram on the left, solder the shrouded double header, the socket, and the capacitors into the board in the appropriate places. NOTE: the positions of the components on the physical board are not represented in the diagram, but the underside of the board shows the connections between each component.

The pictures to the right show the transceiver board in various stages of completion. The top one shows the blank board on which the other components will be added. The bottom one shows the shrouded double header, the socket, and the capcitors soldered in place. The next step is to insert the actual ADM202 chip into the socket. After that six wires from a chopped up piece of the ribbon cable (or any six spare wires) need to be soldered into place in the six holes (two sets of three) at the bottom of the transceiver board. The resulting completed transceiver board should look something like the picture below the diagram.

Complete transceiver board with all components in place.

Task Four: Attach Tranceiver Board to the Router

Transceiver board attached via ribbon cable to the serial header soldered into the router's board.

First take a piece of ribbon cable and attach each end to one of your IDC socket connectors. Then attach one connector to the header we soldered into the router's board and the other connector to the header we soldered into the transceiver board. The result should look something like the picture to the left.

Task Five: Attach Transceiver Board to DB9 Serial Ports

Before soldering the wires from the transceiver board to the serial ports, it is a good idea to drill two holes in the front of the router's casing to use for the serial ports. The three pictures to the right show how to take off the front sticker and what the case should look like after you've drilled two holes in the front for the serial ports. It is also a good idea to cut the sticker and place the left part back on to cover up the remaining holes in the casing.

Diagram of wiring to connect the two DB9 serial ports to the transceiver board.

Next, feed the six wires coming from the transceiver board through the two holes you just drilled. Make sure that the three wires soldered into the holes marked J1 on the transceiver board go through the hole on the left of the front of the router and the three wires soldered into the holes marked J2 on the transceiver board go through the hole on the right of the front of the router.

Following the diagram to the left, solder some spare wires or chopped pieces of ribbon cable into the two DB9 serial ports. Notice that solder cups 1, 4, and 6 are connected to each other and solder cups 7 and 8 are connected to each other on both the DB9 Female and DB9 Male serial ports. Then, again following the diagram, solder in the wires coming from J1 into the appropriate solder cups of the DB9 Female and the wires coming from J2 into the appropriate solder cups of the DB9 Male.

Notice that in the case of the DB9 Female the T1OUT pin of the ADM202 transceiver chip needs to be connected to solder cup 2, the R1IN pin of the ADM202 transceiver chip needs to be connected to solder cup 3, and the ground needs to be connected to solder cup 5.

Also, notice that in the case of the DB9 Male the R2IN pin of the ADM202 transceiver chip needs to be connected to solder cup 2, the T2OUT pin of the ADM202 transceiver chip needs to be connected to solder cup 3, and the ground needs to be connected to solder cup 5.

Feeding the wires through the holes and connecting the serial ports in this way ensures that the router's primary serial device will be connected to the DB9 Female serial port and will be located on the left side while the router's secondary serial device will be connected to the DB9 Male serial port and will be located on the right side of the front of the router.

After all the soldering is done it is a good idea to use a continuity tester to make sure that all the connections are good and no wires or solder cups may be accidentally touching. Your final result should look something like the pictures below.

Overhead picture of what the router will look like after the DB9 serial ports are correctly soldered into place, but before they have been screwed and secured into the router's case.

Front view of what the router will look like after the DB9 serial ports are correctly soldered into place, but before they have been screwed and secured into the router's case.

Task Six: Close the Router

This final task is best described in photos:

Now that everything is connected we can re-assemble it. First you put on the back/top half. Keyed ribbon cable is plugged in to serial port header on circuit board.

Next you can carefully install the front half (making sure not to break any of the wires we have).

Task Seven: Rejoice

Now you have a WRT54GL with two serial ports installed and ready to run your own operating system.

What to do next?

Connect UART0 (the DB9 Female serial port) to a computer and follow the next HOWTO on using a PC to connect to a modified router.