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Connecting to Raspberry Pi

There are several ways to physically connect a Teleytpe to a Raspberry Pi:

  • Directly to the GPIO serial lines, or
  • Via the Teensy USB interface, or
  • Via a RS-232 adapter, and a RS232-USB cable.

For this I'm going via Teensy, because of the various line-discipline things implemented there in firmware (case conversion, and so on). But the direct GPIO serial connection works fine. There are many other hardware options too.

The port name given to the Teletype by Linux will change depending on the connection method. With a Teensy it will be "/dev/ttyACM0". GPIO will be "/dev/ttyAMA0". With a RS232-to-USB cable it'll usually be "/dev/ttyUSB0", but might vary. You can see all the ports using ls /dev/tty*. Adjust the port name below accordingly.

getty on systemd

The "getty" is the program that sets up the terminal connection to the port, and sends it to the login program so that you can start an interactive shell.

To request a getty on the 'ttyACM0' device (using systemd which is standard in recent versions of raspbian OS),

sudo systemctl start [email protected]

To have this start automatically at boot,

sudo systemctl enable [email protected]

Now a reboot should immediately print the login message at the console connected to the ttyACM0 device.

Note though, we didn't tell the getty to use 110 baud connection. So this probably isn't enough yet.

Auto-login and other parameters

Create a directory,

sudo mkdir /etc/systemd/system/[email protected]

Then create a file /etc/systemd/system/[email protected]/override.conf with contents below, replacing username with the desired linux user that should automatically be logged in, and making sure that ttyACM0 matches the port you're using.

[Service]
Type=simple
ExecStart=
ExecStart=-/sbin/agetty --nohostname --autologin username --noclear ttyACM0 110 tty33

(Yes, there are two ExecStart= lines).

You can omit --autologin if you want to log in from the terminal. But be careful, the getty doesn't really handle lowercase usernames or passwords. So auto-login is just easier (although obviously less secure. Someone might use the Teletype to exfil all your secrets on paper tape).

The tty33 is the terminal type (defined in /lib/terminfo/) and gives you a plain terminal without escape sequences for colors.

The 110 baud rate isn't needed if you're going via a Teensy or Arduino that can present a full-speed interface to the USB port.

Customizing settings after login

After login, you can use stty to set custom terminal settings if needed. For example, you may want to set stty brkint for BREAK to send Ctrl+C to the host.
With the custom kernel (below), you'll probably want to add to your .profile something like this:

stty ispeed 110 ospeed 110 icrnl xcase iexten ofill cr1

Using a custom Linux kernel

Linux originated in the 1990s and was never built with Teletype support in mind. So it's missing some of the features from original Unix that you will want for a natural Teletype experience, including upper-case escape characters and carriage-return delay. You can build a custom kernel that includes these features.

Using custom terminfo with a Teensy or Arduino interface

My Teensy firmware implements wordwrap, automatic CR for NL, delays for NL and CR, and other features, controlled by escape sequences. If you're using that firmware, the best settings are

  • Compile the terminfo file: sudo tic terminfo.txt
  • Use tty33-amx instead of tty33 in the getty override.conf ExecStart=... line

Using udev rules for more flexibility

With udev rules, you can control the name of the tty device. This is especially useful if you have several USB devices, and you want some of them to have a 'getty' but not others.

For example, one configuration I'm playing with is to connect the Teletype via RS-232 (using the DeRamp adapter and a StarTech USB cable), but I also want to connect a DECwriter III to the same host, using a FTDI cable.

With a file /etc/udev/rules.d/50-terminals.rules, here are some rules that distinguish between the terminals based on which cable is plugged in, regardless which USB port it's connected to:

# The PL2303-based serial-to-USB cable is for the Teletype
SUBSYSTEM=="tty", DRIVERS=="pl2303", SYMLINK+="ttyASR33"

# The FTDI-based serial-to-USB cable is for the DECwriter
SUBSYSTEM=="tty", DRIVERS=="ftdi_sio", SYMLINK+="ttyLA120"

Then the Teletype appears on /dev/ttyASR33 and has [email protected] to automatically log in; and the DECwriter always connects on /dev/ttyLA120 and has [email protected] to automatically log in to its own Unix account.