Serial Consoles with picocom and screen

Notes for talking to the VAX (and anything else on a serial line) from Linux. Verified on picocom v3.1 and GNU Screen 5.0.1.

# The serial port is owned by a group, not by you:

ls -l /dev/ttyS0

Typically crw-rw—- root uucp. On Debian/Ubuntu the group is dialout.

# Check you are in that group:

id

# Add yourself if not (log out and back in afterwards):

sudo usermod -aG uucp $USER

If you get “Permission denied” opening the port, this is why. Do not reach for sudo picocom — fix the group instead.

The simple choice. One terminal, one session, no multiplexing.

# Connect at 9600 baud:

picocom -b 9600 /dev/ttyS0

# Connect with explicit settings (8N1, no flow control):

picocom -b 9600 -d 8 -p 1 -y n -f n /dev/ttyS0

# Log the whole session to a file:

picocom -b 9600 --logfile session.log /dev/ttyS0

# Connect without taking an exclusive lock:

picocom -b 9600 --no-lock /dev/ttyS0

Every command is prefixed by Ctrl-A. Press Ctrl-A then the second key.

# To send a literal Ctrl-A to the far end, press it twice:

Ctrl-A Ctrl-A

# Change the escape key if Ctrl-A clashes (here, to Ctrl-O):

picocom -b 9600 --escape o /dev/ttyS0

Use this instead of picocom when you want any of:

• the session to survive closing the terminal window
• two people (or a person and a script) on the same session at once
• an automatic log of everything

# Start a detached, logging session called “vax”:

screen -dmS vax -L -Logfile ~/vax.log /dev/ttyS0 9600

# Attach to it (multi-attach: others can be attached too):

screen -x vax

# Attach exclusively, detaching anyone else:

screen -r vax

# Detach and leave it running:

Ctrl-A d

# Kill it for good, without attaching:

screen -S vax -X quit

# Send a command into the session without attaching:

screen -S vax -X stuff "SHOW TIME^M"

# List sessions:

screen -ls

# Remove “(Dead ???)” entries left by crashed sessions:

screen -wipe

# 1. The log file is buffered — 10 seconds by default.

Reading ~/vax.log right after sending a command shows nothing, and it looks like the command never ran. Turn buffering off:

screen -S vax -X logfile flush 0

Add it right after starting the session. Without it, anything scripted against the log will read stale data.

# 2. stuff expands shell-style variables.

This does not send what you think:

screen -S vax -X stuff "LYNX :== $DKA0:[LYNX293]LYNX.EXE^M"

$DKA0 is eaten and the VAX receives LYNX :== :[LYNX293]LYNX.EXE. Escape every dollar sign:

screen -S vax -X stuff "LYNX :== \$DKA0:[LYNX293]LYNX.EXE^M"

This bites constantly on VMS, where $ is everywhere — SYS$LOGIN, f$search, VMI$ROOT.

All prefixed by Ctrl-A.

picocom is built with USE_FLOCK, so it takes an exclusive lock. While it is running, nothing else can open the port — screen will simply fail.

# Find out what is holding it:

fuser -v /dev/ttyS0

lsof /dev/ttyS0

Output like this means picocom (PID 1996744) owns it:

                     USER        PID ACCESS COMMAND
/dev/ttyS0:          josephk 1996744 F.... picocom

Quit that program before starting another. If it is a screen session you have forgotten about, screen -ls will show it.

# Show current settings for the port:

stty -F /dev/ttyS0 -a

Look for speed 9600 baud, cs8, -parenb (no parity), -cstopb (1 stop bit), -crtscts (no hardware flow control).

# Set them by hand:

stty -F /dev/ttyS0 9600 cs8 -parenb -cstopb -crtscts clocal

Normally unnecessary — picocom and screen set the line themselves.

Slow. At 9600 baud you get roughly 1 KB per second, so about 17 minutes per megabyte. Use the network if the machine has one.

# Paste a small text file into a VMS session (works with no extra tools): On the VMS side:

CREATE MYFILE.COM

Then paste the text, and finish with Ctrl-Z.

# ZMODEM (needs lrzsz installed on Linux):

sudo pacman -S lrzsz

Then in picocom, Ctrl-A Ctrl-S to send and Ctrl-A Ctrl-R to receive. The far end needs matching ZMODEM software, which classic VMS does not have by default.

# For binary files on VMS, use FTP over the network instead. Always set binary mode first, and repair the record attributes on arrival:

SET FILE/ATTRIBUTES=(RFM:FIX,MRS:512,LRL:512,RAT:NONE) FILE.EXE

Rule of thumb: picocom to poke at something, screen for anything that must not die — a long build, an install, or a session you want a transcript of.