What is this?
On this page I describe how I modified my Linksys WRT54GS v4 so that it provides two serial ports. There are dozens of instructions on how to do that on the Internet, so how is this description different? Well, most instructions I found use the MAX233 which appearently works with a 3.3V supply voltage - although the datasheet does not guarantee this (the Maxim sheet says on page 3 that the Operating Supply Voltage of the 233A is between 4.5V and 5.5V). Anyways, when testing the much cheaper and much readily availbable MAX232 worked up to a voltage of around 3.6V, below the charge pump stopped working. Sadly, this is just 300mV from the supply of the Linksys Router. So I made my own little circuit featuring a MAX232 and a voltage regulator. I didn't manufacture a PCB as many guys out there did who modified their Linksys. The circuit is really easy to solder and making a PCB was just too much effort for me.
Many pages out there warn that if you modify anything you will void your guarantee and if your house burns down or your dog dies because you modified your Linksys router, they are all not responsible. I will tell you no such thing. If you really think that you can solder stuff onto your Linksys router and drill holes into the enclosure and still not void your warranty -- that's absolutely fine with me. If you additionaly think that I am responsible when you break your Linksys router because I gave instructions on how I modified mine -- that's also quite fine with me. If your house burns down and you think you can sue me, well, that's where the fun starts! See, I live in Europe where courts expect from their plaintiffs to apply a strange little concept called "common sense". If you lack common sense and go to court in Europe, you will not get the millions of dollars you demand, but rather the public ridicule you deserve. That said, please try to sue me in case anything goes wrong, I would love to have another fine funny story to tell!
Basically, Rod Whitby did all the work already and from him I copied the pinout of the connector (which really is the most cumbersome part of it all). Thank you, Rod! The circuit I designed really is basically only wiring stuff. I used a 7806 because i had one spare. Oh, I used the 1A version because it comes in a TO-220 - I wasn't sure if the TO-92 variant 78L06 could have handled the dissipation. The supply voltage I took from some SMD pad for which only the footprint was on the board, no actual part. It was 15V with quite some ripple. I used the 7806 because I had some left and I rarely use them and wanted to get rid of them. A 7805 would work just as fine, although you then probably want to change the voltage dividers (change R? and R? from 1k to 1.5k). It really isn't exactly rocket science.
Yay, it works!
To test if everything works, connect a 1:1 cable to ttyS0 and a nullmodem cable to ttyS1. Then setup the ttyS0 terminal for 115200/8N1 and the ttyS1 to 9600/8N1 and you can do the following trick: