USRobotics USR9108 serial console access

I had to debug my USRobotics router recently (a good old USR9108) as it ceased to respond completely to any network traffic. The CFE was working fine ( could be pinged), so I thought a tftp firmware flash would do the trick, but to no avail (I got timeouts all the time). Accessing the serial console seemed to be the only way left, so I had to practise some PCB soldering the hard way.

A good tutorial would have been really reassuring at the moment. There’s a lot of material on Linksys routers (due to its OpenWrt popularity), but I couldn’t find much about USR9108, so I had to pick and guess the proper pinout. Now I know, doing it with a properly working multimeter (mine is probably appropriate for battery testing only) would have been much easier.

The USR9108 has a 4-pin pad next to the Broadcom BCM6348 CPU. It’s 3.3V transistor-transistor logic (TTL), so a special USB converter (or some reworked USB Nokia data cable) is a must (mine is FT232R based, and works flawlessly). The proper connection sequence is TX, RX, NC, GND as seen below:

USR9108 UART USR9108 pinout

Router’s TX and RX have to be connected with RX and TX on the PC side correspondingly, and GND with GND (it’s data/signal ground, so just grounding it is not the best idea). Connection settings which worked in my case are as follows:

Baud rate: 115200
Data bits: 8
Stop bits: 1
Parity control: none
Flow control: software (XON/XOFF)

I was having some problems with RealTerm (though I don’t know why), but PuTTy worked a treat.

More technical details about the router can be found in the OpenWrt wiki.