Emulating a USB Printer

The Emulated USB Printer is based on the Linux USB Gadget API. It allows the emulation of a USB 2.0 High Speed Printer using only a bus-powered Raspberry Pi Zero!

As the USB Gadget API allows us to set low level USB attributes, we expect that using this method anyone can emulate almost any printer... BUT more complex/modern printers often include scanning or copying functions. Besides that, they offer more Interfaces in addition to the USB. You might run into more challenges trying to emulate those printers because either the emulated USB interface does not fit the driver's needs or you will need a more complex firmware running on the Raspberry Pi Zero to be able to respond to the Host's requests.

Purpose

The main goal behind this program is to offer a solution for transferring data from devices without any interface except the USB interface for printing. Instead of connecting the USB printer to print the reports and later scanning them to somehow store them electronically, you can now use a cheap Raspberry Pi Zero which is able to receive all print jobs and store them electronically without wasting paper and time.

Because the reports are now stored either raw as print jobs or converted as PDFs you will no longer need to run any text detection algorithms to hope finding at least somewhat correct letters or even words on potentially terribly scanned reports. Of course this will always require customized code to extract the needed data from the provided reports!

Even if you don't need to or already can extract the data of a system you can use this solution to trigger arbitrarily complex processes. The Pi might receive the report and:

• send a mail
• save it to a shared folder
• analyze print job and reports
• store the data in a SQL database
• trigger an ERP process
• activate electronics using the Pi's GPIO pins
• ...

Requirements

• Raspberry Pi Zero W
  • Alternatives: Raspberry Pi Zero, Raspberry Pi 4 Model B
• USB Micro B to USB A Cable
  • Alternatives: USB Dongle Adapter (Example), USB Mirco B to needed USB Port Adapter
• Micro SD card
• Micro SD card Reader
• WiFi Connection to Pi

Note that to use the Raspberry Pi 4 Model B different power and USB cables might be needed.

Installation

1. Install Raspberry Pi OS onto the Micro SD card (Latest tested version: Linux raspberrypi 5.4.83+ armv6l)
2. Change settings in files on Mirco SD card
   • Add dtoverlay=dwc2,dr_mode=peripheral to the end of config.txt
   • Add new and empty file to boot called SSH to auto enable SSH
   • Add wifi settings in wpa_supplicant.conf (Tutorial)
3. Plug in SD card and connect Raspberry Pi to PC using USB Cable. Make sure you are using the USB "OTG" Port which is closer to the mini HDMI port.
4. Connect to Pi using SSH
5. Change default password, update the Pi, ..., add any stuff you might need!
6. Find g_printer device descriptor
   • Add g_printer module by running modprobe g_printer
   • Find the device descriptor in /dev/ (E.g. /dev/g_printer or /dev/g_printer0)
   • Unload g_printer module by running rmmod g_printer and rmmod usb_f_printer
7. Optional: Install GhostPDL (Latest tested version: GhostPDL 9.53.3)
   • Download source here
   • Build GhostPDL
   • Confirm path/to/yourghostpdl/bin/gpdl at least shows the help message gpdl -h
   • Remember path to gpdl
8. Build emulatedprinter
   • Clone this repository to wherever you want
   • Edit printer.c
     • Change your device descriptor in PRINTER_FILE
     • Change path to gpdl in GPDL_BIN_FILE (optional)
     • Change sDEVICE method for gpdl in GDPL_SDEVICE_METHOD (optional)
     • Change file extension for files created through GPDL in GPDL_FILE_EXTENSION (optional)
     • Change folder for received print jobs in FILE_OUTPUT_PATH (optional)
   • Build printer.c by running make build
9. Check printer status
   • Add g_printer module by running modprobe g_printer
   • Run emulatedprinter -get_status and confirm Printer is Selected, Paper is Loaded and Printer OK.
   • If your status differs from the one above, use emulatedprinter -needed_option to set the correct status
   • Unload modules using (make stop) or (rmmod g_printer and rmmod usb_f_printer)

Emulate the Printer

Before you can start emulating your printer you will need to find out some information about your exact printer model. This includes the manufacturer, the model and the IEEE1284/PNP string. These are the minimum requirements for Microsoft Windows to load the correct driver.

Manufacturer & Model:
Typically you already know this. Manufacturer might be something like HP, Brother, Xerox, ... and Model might be DeskJet, DocuPrint, ... Now you will need look up both the vendorID and the productID assigned be the USB-IF. You will find multiple lists online listing both vendorIDs and productIDs. Here is an example.

IEEE1284 / PNP string:
This is where it will get a little trickier. The IEEE1284 string is send by your printer to your PC whenever you are connecting the printer to your PC. You need to use a USB traffic analyzer to capture the string. The USBlyzer Team offers a great solution called USBlyzer. You will need to start the traffic analyzer, capture hot-plugged devices and then connect your printer. During the enumeration the printer will send the PNP string. When you are already here, you can confirm that you looked up the correct vendorID and productID. Just search through the descriptors. You can find a more detailed description here

Once you have the requiered data you can load the g_printer module: modprobe g_printer idVendor=0x1234 idProduct=0x1234 pnp_string="MFG:;CMD:;CLS:PRINTER;" where idVendor, idProduct and pnp_string have your values. You can find all parameters including a great explanation here. If you want to, you can now add the modprobe command to run at boot time.

If you choose to emulate a different printer than your own, you might run into problems getting the information about your exact device. Luckily there is an awesome website called www.openprinting.org where you will find ~ 4000 (!) printers listed together with their IEEE1284 string. You can find a aggregated list of these PNP string here. Once you have a printer that fits your needs and has a pnp string, you can simply use the same online list as before to get the vendorID and productID. As before load the g_printer module and confirm that your PC recognizes the Pi correctly as the printer you selected.

Now simply call emulatedprinter -read_data to capture all print jobs and save them to the folder you specified earlier.

---

NOTE

To help others, it would awesome to start collecting vendor ids, device ids and PNP string here in this project. All working combinations for emulating a printer will be added to the Makefile and can be called using make start[Model].

---

Acknowledgements

This program is made possible due to the work of David Brownell and Craig W. Nadler who added the USB printer gadget to the Linux USB Gadget API. Here you can find a good explanation of how the g_printer module can be used! We also used these examples as a basis for this program.