cups-lpd - A way to generate PDF documents with older equipment

What I mean by this title is, that my Tektronix TDS3012B oscilloscope (With floppy drive and Ethernet) is now able to produce nice PDF documents. Before you had to use the floppy drive to get the pictures out of the oscilloscope.

I'm sure that this description is helpful for other people with older equipment or legacy software. The requirement is, that the equipment has a network interface and can somehow print to a standard Unix printer (LPD).

The central program in this setup is called cups-lpd. This program acts as a LPD server and forwards the print-jobs as CUPS client to the already running CUPS server.

This guide is divided into three parts, the common stuff, then the integration into the boot process on your computer (which is different for Sys-V/init and systemd) and then the configuration of the oscilloscope as an example use case.

I assume here that you already have a working CUPS printing server on your computer, that a virtual PDF printer exist and that the printer is named PDF (To install a CUPS-PDF printer you can follow this how-to).

We use a shell script here to pass additional options to cups-lpd and to do some post-processing on the PDFs (just copy and rename in this case). The CUPS-PDF back-end has also a mechanism to achieve this, but I wanted to limit the post-processing only to LPD print-jobs.

1. Create the destination folder and adjust the access rights (The cups server is running as user lp on debian. Change this if your distribution handle this differently):

sudo mkdir /var/spool/PDF
sudo chown lp /var/spool/PDF && chmod 777 /var/spool/PDF

2. Copy the following shell script (cups-lpd.sh) to the path /usr/local/bin and make it executable:

sudo cp <path to your file>cups-lpd.sh /usr/local/bin/
sudo chmod +x /usr/local/bin/cups-lpd.sh

cups-lpd.sh

#! /bin/sh

# Receive the print data and let it forward it to the PDF printer:
/usr/lib/cups/daemon/cups-lpd -o job-sheets=none,none -o document-format=application/octet-stream

cupspdffolder=/var/spool/cups-pdf/ANONYMOUS
outputfolder=/var/spool/PDF


# Wait until the print-job is finished and do some post-processing
logger -s "Wait until cups-lpd is finished"
while pgrep --exact cups-lpd >/dev/null 2>&1
do
     sleep 5
done 

sleep 5

logger -s "Cups-lpd finished, copy file to destination folder"

newfiles=$(find $cupspdffolder -atime -1 -type f)

for f in $newfiles 
do
	destname=$(basename $f .pdf)-$(date +%H-%M-%S_%d-%m-%Y).pdf
	/bin/cp $f $outputfolder/$destname
        /bin/chmod 666 $outputfolder/$destname
done

If your system is using the traditional boot process using init and rc scripts. If you are unsure you can check it by running the following command. It will tell you the name of the program with the PID 0:

ps -p 1 -o "comm="

If it is init then follow with this part, if it is systemd use the description in the next chapter.

The program cups-lpd will be started only when it is needed. This is done by inetd, an internet super-server (sometimes xined is used). To enable our script you have to add this line to the file /etc/inetd.conf:

printer                stream tcp      nowait lp	/usr/local/bin/cups-lpd.sh cups-lpd.sh

If there is no such configuration file, it is highly possible that you first have to install inetd with the help of your package management.

After that, restart the inetd server:

sudo /etc/init.d/inetd restart

Now it's time to configure your older equipment or legacy software and test the cups-lpd setup.

Newer Linux installations use systemd to manage the boot process and a lot of other things. Systemd replaces the traditional System-V init system and the init scripts (anything that was in /etc/rc*.d/ and /etc/init.d/). If you are unsure you can check it by running the following command. It will tell you the name of the program with the PID 0:

ps -p 1 -o "comm="

If it is systemd then follow with this part, if it is init use the description in the chapter above.

With systemd there is no need for an additional internet super-server like inetd, systemd can handle that directly! Here is a detailed description: Socket unit configuration

In our case we have to create two files: cups-lpd.socket and cups-lpd@.service (ListenStream means a TCP socket and 515 is the TCP port used by LPD):

cups-lpd.socket

[Unit]
Description=cups-lpd - receive print jobs and report printer status to lpd clients 

[Socket]
ListenStream=515
BindIPv6Only=both
Accept=true

[Install]
WantedBy=sockets.target

cups-lpd@.service

[Unit]
Description=cups-lpd - receive print jobs and report printer status to lpd clients 
After=network.target

[Service]
User=lp
ExecStart=/usr/local/bin/cups-lpd.sh
StandardInput=socket

1. Copy this two files to the path /etc/systemd/system/:

cd <path to your files>
sudo cp cups-lpd.socket cups-lpd@.service /etc/systemd/system/

2. Activate this service so that it is started during boot (Like “Add to runlevel” in the older days). For this you have to enable it:

sudo systemctl enable cups-lpd.socket

3. To start it right away to test your setup, type this:

sudo systemctl start cups-lpd.socket

Or if something went wrong or you changed your configuration file you can also restart your new systemd service:

sudo systemctl restart cups-lpd.socket

Now it's time to configure your older equipment or legacy software and test the cups-lpd setup.

I own a TDS3012B but this description should be true for the whole series (TDS3054B, TDS3014B and so on).

1. Go to the menu: Utility → System I/O → Ethernet Printer Settings

2. Press Add Printer and enter the needed information. Printer Name is essential, it is the name of the desired printer from the CUPS server! (My virtual PDF printer is named PDF)

3. Go to the menu: Utility → System Hard Copy Select as hard copy format EPS Color and for port Ethernet

4. Restart your oscilloscope and test your new PDF printing feature. No more fiddling around with storage media. Your measurements are already on your PC!

Links:
Description of the line printer daemon (LPD) protocol
Manual page for the cups-lpd deamon
Details about systemd socket configuration files for network sockets or file system FIFOs controlled and supervised by systemd