Bringing a QVT terminal to life on Linux over USB serial

Vintage serial terminals are simple when everything matches, and maddening when one setting is off by a single bit.

Recently, I got a Qume QVT-119 Plus terminal working as a serial console on Linux. The terminal is connected to a Raspberry Pi via a USB-to-serial adapter, and the Pi is running agetty to present a login prompt on the terminal.

Here are some notes on how the configuration works, why it works, and what each part of the setup is doing. This is a good example of how the terminal’s own settings and the Linux side have to be in sync for everything to work correctly.

I bought the terminal from a wallapop seller, and looks well, later I should check the keyboard (spacebar is a bit sticky).

Mine is the amber one

I connected it to a Raspberry Pi via a USB-to-serial adapter, and configured the terminal and Linux side to match each other. The terminal is set to emulate a Wyse 50, and Linux is using agetty with the wy50 terminal type. The serial settings are 38400 baud, 8 data bits, no parity, 1 stop bit (38400 8N1), and CR/LF translations are disabled.

It’s require a null modem cable, because the terminal is DTE and the USB serial adapter is also DTE, so a straight-through cable would not work. A null modem cable crosses the transmit and receive lines, allowing the two DTE devices to communicate properly.

Here is the terminal configuration I used

terminal configuration:
set 1  FDX | CONV | 80 COL | EM WYSE50 | KB ON | BCOPY | MON OFF | GRAPH OFF
set 2  JUMP | REPEAT ON | CLICK ON | MARGIN BELL OFF | EDM US / CR
set 3  LINE WRAP ON | CR = CR | SCROLL ON | XON & DTR | LIMITED X MIT OFF
set 4  EIA=HOST | 8BITS | BIT8 0 | PAR OFF | PAR ODD | STOP 1 | RX 38.4 | TX 38.4
set 6  SPLIT MODE ON | STD VID | RET ENTER CR/CR | PROT=REV | DISPLAY PE ON
set 7  CURSOR BLICK STEADY | TIME 5 | KB SP | STATUS ON | FREQ 60 | RS232
set 8  STD | CLOCK OFF | ALARM OFF
set 9  SCREEN SIZE 24 | PAGE ATTR | CHAR ISO7

EM WYSE50

That means the terminal is pretending to be a Wyse 50. On the Linux side, the matching choice is exactly what you used: wy50 as the terminal type in agetty. That is the reason programs like vi, less, mc, top, or htop have a chance to behave correctly. Linux has a wy50 terminfo entry, and that entry describes an 80-column, 24-line terminal with the escape sequences expected for that personality.

The next important block is the electrical/serial one:

8BITS | PAR OFF | STOP 1 | RX 38.4 | TX 38.4 | RS232

That is the classic 38400 8N1 serial framing: 38,400 bps, 8 data bits, no parity, 1 stop bit. In Linux terms, cs8 means 8-bit characters, -parenb disables parity, and -cstopb means one stop bit rather than two. Those are exactly the same choices you set with stty.

A small odd-looking detail is this line:

PAR OFF | PAR ODD

That usually means the menu is showing both the parity enable state and the parity flavor. If parity is off, the odd/even choice is effectively dormant. In other words, PAR OFF wins, and PAR ODD is just the remembered subtype that would apply only if parity were enabled. That interpretation also matches your Linux side, where -parenb disables parity entirely.

CHAR ISO7

CHAR ISO7 belongs to the display/character interpretation layer: it tells the terminal which character repertoire to use once bytes arrive. So it is completely possible, and often correct, to run the wire as 8-bit clean while still using a 7-bit character set personality on the screen. That is what you are doing here: the terminal is receiving 8-bit bytes, but it is treating them as ISO-646 (7-bit ASCII) characters. That is a common setup for serial terminals, and it matches the wy50 terminfo entry, which also assumes a 7-bit character set.

Raspberry Pi

Raspberry side, I have a systemd service that starts agetty on the USB serial port with the correct settings:

First I set the serial port settings with stty:

sudo stty -F /dev/ttyUSB0 38400 cs8 -parenb -cstopb -ixon -ixoff -icrnl -inlcr -igncr -opost

• 38400 cs8 -parenb -cstopb sets the port to 38400 baud, 8 data bits, no parity, 1 stop bit, which matches the terminal’s own serial settings.

• -ixon -ixoff tells Linux not to interpret XON/XOFF flow control in software. In the stty documentation, ixon enables XON/XOFF flow control and ixoff enables sending start/stop characters; disabling both makes the host treat the line as ordinary data instead of control-by-byte.

• -icrnl -inlcr -igncr disables carriage-return/newline translations on input, and -opost disables output post-processing. That matters on old terminals because automatic CR/LF rewriting is a common source of double-spacing, missing carriage returns, or strange cursor movement. The stty and termios documentation both show these flags as part of the path toward a raw, translation-free connection.

One thing that may look contradictory is that the terminal menu says XON & DTR, while Linux has -ixon -ixoff. In practice, that is not unusual. The terminal may be capable of software flow control or may advertise a combined host-flow-control mode, but the link can still work fine with Linux treating the line as a plain local serial connection. In your case, the evidence is simple: the line works reliably as configured, so the working behavior is more important than the label.

agetty

[Service]
Environment=LANG=C
ExecStart=-/sbin/agetty -L -8 38400 %I wy50
Type=idle
Restart=always
UtmpIdentifier=%I
TTYPath=/dev/%I
TTYReset=yes
TTYVHangup=yes
KillMode=process
IgnoreSIGPIPE=no
SendSIGHUP=yes

agetty is the program that opens a tty, prompts for a login name, and then invokes /bin/login. It is specifically meant for local and serial login lines. It can also adapt tty settings when reading the login name, and it supports both parity-handling and 8-bit-clean serial lines.

Important here is ExecStart, which is the command line that starts the getty process.

/sbin/agetty -L -8 38400 %I wy50

• -L means local line. In agetty, that forces the line to behave as a local connection and not require carrier detect. That is often exactly what you want for a directly attached terminal on a USB-to-serial adapter, because many such setups do not present proper modem-control signaling.

• -8 tells agetty to assume the tty is 8-bit clean and to disable parity detection. That matches your terminal’s 8BITS and PAR OFF settings. 38400 sets the login speed. And the final wy50 is the value that becomes the session’s TERM, inherited by login and the shell.

What the systemd service is doing

Systemd service serial-getty@ttyUSB0.service, is the standard systemd pattern for serial login prompts. Systemd’s getty generator exists specifically to start serial-getty@.service on serial-capable consoles.

The [Unit] (not here) section is mostly about dependency logic. BindsTo=dev-%i.device makes the service strongly tied to the device unit, so if the USB serial adapter disappears, the service goes away with it. After=dev-%i.device means systemd waits until that device exists before trying to start the login prompt. Before= and Conflicts= are standard systemd ordering and exclusion controls: Before= defines startup order, while Conflicts= means two units should not be active together. The [Install] section is not used at runtime; it is used by systemctl enable to hook the unit into a target, which is what WantedBy=getty.target is doing here.

In the [Service] section, Type=idle is there mostly for presentation. Systemd says idle delays the actual execution until active jobs have been dispatched, which helps avoid ugly interleaving with boot messages on the console. Restart=always means the getty will be started again whether it exits cleanly or not, which is exactly what you want for a login prompt that should always come back after logout.

TTYPath=/dev/%I tells systemd which tty this service owns. TTYReset=yes resets the terminal before and after execution, and TTYVHangup=yes disconnects other clients that may still have the tty open. Those settings are useful with serial terminals because they help you return to a clean prompt after logouts, crashes, or half-broken sessions.

KillMode=process and SendSIGHUP=yes control shutdown behavior. Systemd documents KillMode=process as killing only the main process, and SendSIGHUP=yes as sending SIGHUP to remaining processes after the main kill signal, which is useful for shells and shell-like programs because it tells them the connection has gone away.

Reading your terminal menu, one block at a time

A few of these abbreviations are vendor-specific, and I could not find the exact setup manual page for these exact labels, so this section is partly interpretation from standard terminal terminology and from your working behavior.

Set 1 is the identity block. FDX means full duplex, which is what a normal UNIX login wants. 80 COL matches the standard wy50 layout. EM WYSE50 is the big one. KB ON is keyboard enabled. GRAPH OFF likely means special graphics mode is disabled, which is fine for plain text UNIX use.

Set 2 looks like usability behavior. JUMP probably means jump scrolling rather than smoother line-by-line scrolling. REPEAT ON is key repeat. CLICK ON is keyboard click. MARGIN BELL OFF disables the warning beep near the right margin. EDM US / CR looks like an editing or keyboard-country/return behavior setting.

Set 3 is screen and flow behavior. LINE WRAP ON is what most shells expect. CR = CR suggests carriage return is being handled literally. SCROLL ON is obvious. XON & DTR is the only line that looks looser than your Linux side, but clearly it is not preventing operation.

Set 4 is the real serial layer. EIA=HOST means the serial interface is attached to the host connection. 8BITS, PAR OFF, STOP 1, RX 38.4, TX 38.4, and RS232 are the settings that absolutely must match Linux, and they do.

Set 6–9 are mostly presentation and local terminal features. SPLIT MODE ON and STATUS ON fit the published QVT-119 Plus feature set, which includes split-screen/status-line style capabilities. SCREEN SIZE 24 and your wy50 terminal type are also consistent with the standard 80x24 expectation.

Why this is a good configuration

The reason this setup feels “right” is that it follows an old but very reliable rule:

wire format on the serial side, terminal personality on the application side

On the wire, I’m using a plain, conservative serial link: 38400 8N1, no translations, no output processing. In the session, I’m advertising a known terminal personality: Wyse 50. That keeps the serial driver honest and gives programs a real terminal description to work with. agetty is doing exactly what it was designed to do on such lines, and Linux has a real wy50 terminal description in terminfo.