On doorpi.mit.edu, symlink led.service to /etc/systemd/system and enable and start the service. The TCP server runs on port 41337.
echo message | nc doorpi.mit.edu 41337 -c
See https://wls.wwco.com/ledsigns/m-500/m-500-protocol.php for docs on how to change the text color and font and other useless fluff.
On doorpi.mit.edu, you can check journalctl -xeu led for this program's logs.
You can also use the sign interactively by SSHing into doorpi and running in a Python console:
import serial
ser = serial.Serial("/dev/ttyUSB0")
ser.write(b"~128~Hi")
Copied from Mattermost:
tl;dr: sign has 10 sequencer slots, each of which can be programmed with (a subset of days of week, a time span 0000-2359, a list of message slots).
sequencer slots are swept left to right, lower index takes higher priority; if date and time matches, it loops the contents of the message slots in sequence
if no slots match, default to last-set page (which is what the sign's currently been doing)
protocol string is S[slot][daysofweek][starttime][endtime][slot#*] where:
slot ::= 0-9, slot id
daysofweek = (0-1){7}, each number corresponds to one day
startime, endtime ::= 0000 thru 2359, obvious parse
each slot# ::= a 2-number ID that can be set through the f prefix