RS-232 to USB serial adapters

That port is disabled by default and you cannot change this parameter unless you have a normal rs-232 to actually see the BIOS setup interface, so it's kind of useless imho.

Anyway it's in the connector called "COM2", that looks like a large fan header on the top of the board. Pinout is not mentioned in official docs but it just says it has 3.3V and TX/RX pins so I guess you just have to guess (once you found what is the 3.3v and ground with a multimeter) or you have to ask PCEngines directly.

Good tip, got the PID and tracked it down to the vendors site and found a tech note that says the SN of my device is not supported after Windows 7.

I could make this work with other OS's, but it appears that the CableCreations products have pins 2 and 3 reversed on the female devices, so I think this should work directly without a null modem cable like the PC-Engiusbcom1a.

Amazon has a PL2302 adapter and cable, as well as a FTDI adapter and cable. I think I will try the PL2302 adapter as I can just leave it plugged into the APU (and not loose it). USB extensions are a "Stock Item".

There are also null modem dongles like the following:

Please forgive me if this is a silly question. I have some cheap serial adapters left over from my previous work. Some ch340g, some pl2303 and few ftdi32 based with red pcb.
I'm asking for pc engines apu2. Can I use a DIY cable with db9 female connector like in the picture? (assuming the pinout of the connection is correct.) GND-GND, RX-TX, TX-RX

Or are these adapters I have useless in this case? I mean is rs232 a completely different protocol from them? Thank you.
PS: I'm sorry for thread hijacking and of course my terrible paint skills.


rs232c is the same protocol, but using different voltage levels. The typical UART found on routers are running directly on SOC voltage levels, with no safeguards in place (they're not meant to be user accessible), which typically means 3.3V (in very old devices sometimes 5V, in newer devices sometimes 1.8V; very rarely 2.5V).

rs232c as typically used by various devices since the 1960s defines +/- 15 volts (and devices must be tolerant to well over +/- 25 volts), which would immediately fry modern equipment (as in your wiring above).

Your wiring diagram above would instantly damage the usb2serial adapter (and probably the connected computer as well), but there are dedicated usb2serial adapters with db9 plugs connected, which add the voltage level shifters/ and overload protection circuitry - you need to use those for devices exposing db9/ db25 sockets.


Thanks for the incredibly helpful information. You literally saved my a** before killing my computer. I'll order the proper adapter ASAP. Thank you again.


For EU users:

  1. cp2105

  2. ftdi

get this kind of stuff on ebay, or Amazon, it's dirty cheap, 5€ or so.
the China sites work too, but are usually slower - DX, AX, GB, wish, etc ...

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Not the cheapest but truly universal with proper FTDI chip:

Yes, sorry, this is a TTL one which works with even tricky stuff like the R7800 or newer 1.8 volt only things like the AX3600.

That is not RS-232. You need one with a 9 pin connector, not loose wires.

And I'm skeptical of anything that an Ali seller claims about the provenance of their chips.


And no old school voltage support...

Yes, sorry, this is a TTL one that works with even tricky stuff like the R7800 or newer 1.8 volt only things like the AX3600/AX9000.

Similar FTDI ones with 9-pin Sub-D which even have the null-modem cable conveniently integrated are e.g. the following:

I know, rather pricey but professionally used at work without ever having gotten failed once.

Well, that's the only reason I posted it. I am using that one daily and so far it has not failed me ever.

This thread was split from this post on September 5. The focus of the post was a RS232 cable for use with a PC-Engines APU2, but I expect will work for other hardware.

I have recently received a CD0491 adapter as noted in post 4 above. This is based upon the Prolifics PL2302 (RS) chipset.

These links are relevant:

I have now tested this device on both Windows 10 and Windows 7. Both OS's found the device and install appropriate new or recent drivers. The device worked as expected on both OS's with no noted issues. There are MAC drivers, but Linux support is limited (see the Cable Creations driver packages)

Prior to purchase I sent a note to PC-Engines on sourcing product in the US. The reply I received, after receiving my product, strongly advised against Prolific. I am not clear if this is pretty much go\no go or if I may have some harder to recognize issues processing commands.

This is a female DB9 connector with RX and TX pins reversed (manual page 2)so no null modem accessory is needed. I choose the adapter and have tested with a USB extension of 3ft. I plan to store the adapter with the router. Cables are easier for me to loose.

Cable Creations also sells a similar item with a FTDI chipset. The Adapter is CD0492 and cables CD0486 or CD0487

All of the items are available from Amazon for between $10 and $14 as of this writing.

@sumo, yes, thank you, I have seen these, but it was more of an issue that the device I have is not supported on Windows 10.

@RangerZ, this allowed me to get serial console to an APU2 from a Win10 laptop.

I didn't need to install any specific drivers.. it just worked for me using putty.

Linux doesn't need third party drivers for that chipset, the driver has been upstreamed a long time ago so that adapter is plug-and-play in most Linux distros

In OpenWrt where kernel is kept thin and light there is a kernel module package for it

To be fair I think most if not all serial and TTL devices you can find are plug and play on Linux, even your old IOGear will probably still work fine on Linux (you can try doing a USB passthrough to your Linux VM).

As a side note, also a lot of ancient scanners and printers still all work fine on Linux, as long as their drivers are opensource and upstreamed to the relevant projects, there is no obsolescence because of driver incompatibilities.

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Same here, drivers were included with Windows and I did not need to download anything. Did not mean to imply I did.

Do any of you get garbled output over the serial adapter? It seems ok in the BIOS tuneable selection but not for install USB drives.

This can happen if your serial speed is correct for BIOS but the OS installed in the USB drive is running at a different speed.

What is the serial speed (baud)? What is the device? What is the OS in the USB drive?

I also get some garbled output for the first few seconds of boot with OpenWrt. Then it clears up and is usable again.

Also, on some devices the serial was garbled but it was just because the serial port pins are old and dirty, just disconnect and ri-connect the adapter 5 or 10 times will clean them and make good electrical contact.

115200, APU2, IPFire If I try to hit install or what looks garbled like install it goes to video mode or continue. Continue freezes. Video selection shows one option and freeze after selection.

It should not show garbled text, the configuration should be the same also in IPFire. Does OpenWrt work? Maybe it is a problem of IPFire.

Did you use the iso or the "flash" image? Try with both