Serial console in Fritzbox 4040


Bought new router and promptly locked me out :slight_smile:

Of course I'll try first failsafe and then perhaps just reflash if that doesn't solve it but... in my previous routers I often had a serial port installed and I'd like to add one here as well, just for convenience.

I had a look at the HW page but there is no pinout for serial port. Digging around on the Internet, I came across a series of photos of the internals.

By the looks of it there are 2 serial ports. Has anyone tried using them yet?
Which one is the console and what is the correct pinout?



WARNING: The serial port needs a TTL/RS-232 v3.3 level converter!
The Serial setting is 115200-8-N-1. The SoC's serial port is right
next to the MXIC FLASH chip. The board has a unpopulated 1x4 0.1"
header for it. Use a multimeter to figure out the pinout!

Once you have figured out the correct pinout, please update accordingly, so other users can benefit from your findings.


Yes the other place with four empty holes appears to be connected to the switch chip. It's not going to be anything useful for OpenWrt development.

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@chunkeey, that wording is a bit confusing. Does that mean the 4040's serial port is actually on 5V RS232 levels? Or the usual "don't connect your computer's RS232 serial to the TTL level serial of the device"?

It would be an odd router that has a 5V port so I'm going to play it safe and use the 3.3V converter I already have. I don't know when I'll get around to do it, though.

Voltage is 3.3V. Baudrate is 115200-8-N-1. Pinout is "standard AVM":

VCC - RX - TX - GND (Square Pad is VCC)


Ah, so it's just the regular "use TTL, not serial" situation. Thanks for clearing that up.

The term TTL is confusing since it refers to 5V UART serial interface.
I prefer to be more specific "3.3V UART" Serial Inteface.
which you use a USB or RS232 converter with to communicate with your PC

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Hmm, anyone knows what type of screws the 4040 has?
I thought it was a simple hex nut type but the tools I have keep slipping at the slightest twist.

If it's like its mips based predecessors, it's probably plain torx (T8).


@slh thanks, I'll try asking at work. I have a couple of torx bits but they slip / do not engage properly.

as slh said you need a T8 Torx (no security) bit. There are two hidden torx screws behind the two front feets.

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@chunkeey then I guess the bits I have are not T8.
Thanks for confirming it is not the security variant.
I did in fact discover the hidden screws by looking at the internal pictures I posted earlier.

Would be nice if you could update once you have figured out the procedure to open the 4040.

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@tmomas added the instructions now for the case, the serial port will follow soon.
TLDR my Torx T8 slipped because... what I really needed was a T7.

Should I add my own internal pictures? Reuse those I linked in my first post? Nothing?

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Add pictures as appropriate, i.e. if useful for others to accomplish the task easily.

See for how to the obvious :slight_smile:

Take care to follow this:

When the media manager opens,
your very first action should be to change to the media namespace, and possibly further down until you have reached the namespace to which you want to upload the picture.

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@tmomas I believe no pictures are really needed for the opening, but I've made an annotated version using information provided by @blocktrron for the serial port wiring. Thanks for the coaching, it was appreciated.

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