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When I log into my serial console using screen /dev/ttyUSB0 115200, I see some garbage in the output:

��
Arch Linux 3.6.11-10-ARCH+ (ttyAMA0)

raspberry login: root
�Password: 
Login incorrect

raspberry login: root
Password:

These question marks are the FD bytes (hexadecimal) according to the hardcopy I made using Ctrl + A, H and F8 according to the screen log (screen -L). Observe that only the first Password line has the question mark, subsequent password prompts do not have that byte.

How can I stop these question marks from appearing? Do I need to change the tty settings or specify other screen parameters?

The getty process that is started by systemd has the following command:

/sbin/agetty --noclear ttyAMA0 115200 linux

I am using a USB device for hooking up the serial line:

067b:2303 Prolific Technology, Inc. PL2303 Serial Port

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3
  • Prolific is well known for buggy hardware, did you try any other USB-Serial adapter?
    – lenik
    May 15, 2013 at 12:05
  • @lenik I haven't got another. This adapter works fine with an other device (router), it only shows issues with this configuration. It looks like a software issue to me. Have you tried the serial console yourself? (btw, I tested with other baud rates too, it makes no difference)
    – Lekensteyn
    May 15, 2013 at 22:25
  • I would say that is normal. Have seen a lot of network devices giving out weird characters.
    – Harijs Krūtainis
    Oct 9, 2014 at 6:26

1 Answer 1

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This is probably the well known bug of the spurious character when the serial port is opened. I don't remember the details, something to do with the TX line dropping for an instant so signalling a spurious start bit. From memory it is a bug in the Raspberry Pi Linux driver. I don't know why it hasn't been fixed, it has been commented on for several years.

As I say the character happens on port open, e.g. 

fd = open("/dev/ttyAMA0", O_RDWR);

will cause the error.

The work around is to ignore the first byte and not to needlessly close/open the serial port.

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6
  • What about Password? The behavior is so deterministic that it looks like a special control character which somehow does not get handled?
    – Lekensteyn
    Oct 9, 2014 at 8:05
  • I'd have to connect a serial cable and have a look while a Pi is booting. Of course serial data can be corrupted for all sorts of additional reasons. In my case it's usually because I've forgotten to connect the grounds. Give me an hour or so and I'll have a look.
    – joan
    Oct 9, 2014 at 8:14
  • @Lekensteyn A link to a boot up webm. I only see spurious characters pretty much where I'd expect, the times the port is opened, apart from one character early on which was probably just corrupt.
    – joan
    Oct 9, 2014 at 8:42
  • Must be a bug in the RPi driver, the wires were already connected so any bit flipping happens because the driver is in a wrong state. No time to look into the driver source code, but thanks for your answer!
    – Lekensteyn
    Oct 9, 2014 at 9:37
  • 2
    "ignore the first byte" means ignoring one character, Yet the OP defines clearly this goes beyond the first byte...
    – Tyson
    Oct 9, 2014 at 11:35

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