Update: The ModemManager rule should really only apply to the USB device, not the TTY. I updated the rule below to be a bit more specific.

Around the end of July, I ordered a Bus Pirate from Seeed Studio. It took about a week to ship, which is pretty quick, I think. I picked it up from the post office on Tuesday.

After opening the package and plugging it in, I was greeted with nice blinky lights, and my computer didn’t start smoking, so that was a good sign. However, I seemed to run into a bug, where every few lines or so, the display would be screwed up. I started with minicom, until I found out that I could talk to the bus using screen instead. But the output was still not working. Even if I did something simple like press Enter repeatedly, after 5 or so lines, the prompt would show incorrectly. That was kind of disappointing, since Seeed had put a sticker on it for passing QC, so I thought I had a bad cable or set up something wrong.

But today, I finally hit a stroke of luck. One time, upon connecting, I noticed something writing to the port. The text started with AT... and I of course recognized those as modem commands. I remembered a bug report or blog post about ModemManager messing with (things that look like) serial ports.

ModemManager works alongside NetworkManager to provide support for connecting through GSM and CDMA modems. The problem is that a lot of modems suck and ModemManager needs to probe serial ports to see if it’s really a modem. Since I’m on a desktop and I probably wouldn’t ever use a modem, I could just remove it, but then what would I do on the laptop?

So I set out to find how to blacklist a serial port from ModemManager. I couldn’t just get ModemManager to ignore all Bus Pirates, because it identifies itself as a generic FTDI USB-to-serial converter. While I can let Dangerous Prototypes go for that since it’s a little hacking board, I’m sure there are actual companies using an FTDI chip who don’t identify their modem devices uniquely. Thus, blacklisting the entire FTDI series was out.

All was not lost however, as ModemManager’s blacklist basically works through udev. Because USB device numbers are not static, I was already using a udev rule to give me a static device name for the Bus Pirate (from the Arch Linux wiki). After a quick look at the ModemManager blacklist rules, I found that it uses ENV{ID_MM_DEVICE_IGNORE}="1" to signal to ModemManager to ignore a device.

So, to sum it all up, you need to create a udev rule in /etc/udev/rules.d/98-bus-pirate.rules:

SUBSYSTEM=="tty", ATTRS{idVendor}=="0403",\
    ATTRS{idProduct}=="6001", ATTRS{serial}=="...",\
    GROUP="users", MODE="0660", SYMLINK+="buspirate"

ACTION=="add|change", SUBSYSTEM=="usb",\
    ENV{DEVTYPE}=="usb_device", ATTRS{idVendor}=="0403",\
    ATTRS{idProduct}=="6001", ATTRS{serial}=="...",\

Check the Arch wiki for how to get the proper serial number. The first rule sets up a /dev/buspirate device node link so you have a memorable name to use (instead of /dev/ttyUSB#). It also sets permissions so that it’s accessible to the users group. Make sure to add your user to that group and you won’t need to be root to use the Bus Pirate. The second rule sets a variable on the USB device so that ModemManager doesn’t attempt to probe it.

To connect, just use screen. I set up an alias to run screen /dev/buspirate 115200 8N1 and it works great. Now I just need to get something to hack on with it.