Arch Linux

ham430

Member

Registered: 2021-04-25

Posts: 1

Trying to get a TTY on a VT52-emulating serial terminal over USB

Hi there, hoping someone can give me a nudge in the right direction as I've kind of hit a wall trying to work this out myself.

I recently purchased a beautiful (in my opinion) serial terminal from 1984. I am only recently out of university, and am really curious about the way these things work, and the similarities/differences with the virtual terminals we use today. I also just think it's neat.

I am trying to configure my arch laptop so I can log in from the terminal and have a working bash prompt to run some simple CLI programs from. I found this post from 2014 which is very similar to what I want to do, but it diverges from my experience toward the end unfortunately.

My hardware setup:
- A Micro-term Ergo 201 (labelled as a Case Ergo 201 here in Australia)
  - Emulates 925, 1410, ACT-5A, ADM3A, and VT52
  - Here's a photo of it if anyone's interested
  - I have not been able to find a user manual for it anywhere, despite reading in a geekhack thread that it was once on the internet. I did also find a "refurbished" copy of it here, but would rather not pay for a physical copy if I don't need to.
- Thinkpad P14S (AMD version) running Arch. I also have a desktop running arch I can try but haven't yet
- Startech ICUSB232DB25 USB to RS232 serial cable (with a Prolific PL2302HXD under the hood)
- A null modem cable between the USB to serial adapter and the terminal

I have been able to "chat" with it using minicom, and by so doing I can say with some confidence that it uses 7 data bits, but I can't say for sure yet whether it expects odd, even or no parity, or how many stop bits it uses.

As for getty, I have created /etc/systemd/system/serial-getty@ttyUSB0.service, with:

ExecStart=-/sbin/agetty -o '-p -- \\u' --keep-baud --local-line 9600 %I vt52

(everything else is default)

After starting this service, I get a login prompt on the terminal, but after typing my username, the return key only returns the cursor to the left. No linefeed and no password prompt.

Running

sudo stty --file /dev/ttyUSB0 --all

shows that ttyUSB0 is configured to use 8 data bits (I can paste the full output if that helps too). I tried running

sudo stty --file /dev/ttyUSB0 9600 cs7 -cstopb -parenb

from a separate terminal on the laptop, which allowed me to enter my username and get a password prompt, but trampled my tty settings. I read on the TLDP text terminal HOWTO that I should expect this behaviour.

By running the above stty command on the laptop before pressing return on the terminal every time, I can log into the system, but from there it is even more abundantly clear that I have not configured everything correctly. Lots of garbled characters and if I mash the keyboard enough, the "kb lock" light eventually turns on on the terminal.

I have also tried adding

console=tty0 console=ttyUSB0,9600n7

to my kernel command line (I figured it was safest to add tty0 as well as ttyUSB0). I use systemd-boot, so this is in the options field of /boot/loader/entries/arch.conf. I expected that this would automatically trigger systemd to start getty on ttyUSB0, but this doesn't seem to happen. I do get some systemd messages during boot and shutdown though. In full, my kernel options are

root=/dev/nvme0n1p3 rw console=tty0 console=ttyUSB0,9600n7

I enjoy diving into stuff like this and figuring out how it works, but I am a bit stuck here, so even just some advice on where to go for documentation would be great.

Thanks!

edit: Had an incomplete stty command

Last edited by ham430 (2021-05-05 09:01:46)