ADSL-router as print server: TRENDnet instructions

Turning a TRENDnet TDM-C400 ADSL Router into a Print Server: A Step-by-Step Guide

An old TRENDnet TDM-C400 ADSL router can be repurposed as a print server for a USB printer like the Samsung SCX-3200 without Ethernet. This allows the printer to be networked through the router, extending the life of outdated hardware. It requires minimal components and basic soldering skills.

Required equipment:

• TRENDnet TDM-C400 (ADSL router without antennas).
• Samsung SCX-3200 (USB printer).
• USB-UART converter (costs around $5).
• Soldering kit: soldering iron, flux, solder, isopropyl alcohol.
• Dupont pin headers.
• PC with Windows 10.

Disassembly and Accessing the Board

Open the router case and remove the board. It features a custom TRENDnet microcontroller, likely running a lightweight Unix-like OS. The goal is to gain root access to configure the print server.

Identify the UART pins: RX (Receive), TX (Transmit), GND (Ground). These pins enable serial port connection and interaction with the device's console.

Solder Dupont pin headers to the corresponding contacts. Use flux for a reliable connection, and clean any residue with isopropyl alcohol after soldering.

Connecting UART and Launching the Console

Connect the USB-UART converter to the board:

• Connect GND to GND.
• Cross-connect: RX on the converter to TX on the board, TX on the converter to RX on the board.

Power on the router and connect the converter to a Windows 10 PC. Install UART drivers (typically CH340 or similar), and set the port speed (standard is 115200 baud, 8N1).

Open a terminal (PuTTY or Tera Term) to monitor the console. If successful, a boot log or shell will appear.

Gaining Root Access and Common Issues

During power-up, the router may become unresponsive. Possible causes:

• Short circuit from soldering.
• Incorrect UART connection (check the cross-connect scheme).
• Microcontroller protection against unauthorized access.
• Age-related board defects.

Diagnostics:

• Use a multimeter to check for short circuits.
• Reconnect power without the UART.
• Use an oscilloscope to check signals on RX/TX.

If access is gained, log into the shell (default login/password: root/root or admin/admin). Configure the USB host for the printer: mount /dev/usb0, start CUPS or an LPD server.

Example USB configuration in embedded Linux:

# lsusb
Bus 001 Device 002: ID 04e8:3260 Samsung SCX-3200

# modprobe usb-storage
# mount /dev/sda1 /mnt/printer

Setting Up the Print Server

After root access, set up the print server:

• Compile and load USB-Gadget drivers for printer emulation.
• Configure Samba or Avahi for network discovery.
• Add the printer to /etc/printcap:

scx3200|Samsung SCX-3200:\
    :sd=/var/spool/lpd/scx3200:\
    :mx#0:\
    :lp=/dev/usb/lp0:\
    :sh:

• Restart lpd: /etc/init.d/lpd restart.

Test printing from a client via LPR: lpr -P scx3200 file.ps.

Key Points

• Cross-connect UART is essential for stable communication.
• Soldering requires precision: avoid overheating the microcontroller.
• Root access provides a full shell, but check firmware for backdoors.
• Samsung SCX-3200 is recognized as USB Mass Storage; use vendor ID 04e8.
• This project saves money but requires debugging hardware defects.

— Editorial Team