Printer to CDC

Example source: examples/device/printer_to_cdc

This example demonstrates a USB composite device with a Printer class interface and a CDC serial interface. Data flows bidirectionally between the two:

  • Data sent to the Printer (from host) is forwarded to the CDC serial port

  • Data sent to the CDC serial port (from host) is forwarded to the Printer IN endpoint

This is useful for debugging printer class communication or as a reference for implementing printer class devices.

USB Descriptors

Interface

Class

Description

0

CDC ACM

Virtual serial port

2

Printer

USB Printer (bidirectional, protocol 2)

Configuration

Notable tusb_config.h settings:

#define CFG_TUD_CDC                1
#define CFG_TUD_PRINTER            1
#define CFG_TUD_CDC_RX_BUFSIZE     (TUD_OPT_HIGH_SPEED ? 512 : 64)
#define CFG_TUD_CDC_TX_BUFSIZE     (TUD_OPT_HIGH_SPEED ? 512 : 64)
#define CFG_TUD_PRINTER_RX_BUFSIZE (TUD_OPT_HIGH_SPEED ? 512 : 64)
#define CFG_TUD_PRINTER_TX_BUFSIZE (TUD_OPT_HIGH_SPEED ? 512 : 64)

How to Test

The device exposes two endpoints on the host:

  • /dev/ttyACM0 (CDC serial port)

  • /dev/usb/lp0 (USB printer)

Note: the actual device numbers may vary depending on your system.

Prerequisites (Linux):

# Load the USB printer kernel module if not already loaded
sudo modprobe usblp

# Check devices exist
ls /dev/ttyACM* /dev/usb/lp*

Test Printer to CDC (host writes to printer, reads from CDC):

# Terminal 1: read from CDC
cat /dev/ttyACM0

# Terminal 2: write to printer
echo "hello from printer" > /dev/usb/lp0
# "hello from printer" appears in Terminal 1

Test CDC to Printer (host writes to CDC, reads from printer):

# Terminal 1: read from printer IN endpoint
cat /dev/usb/lp0

# Terminal 2: write to CDC
echo "hello from cdc" > /dev/ttyACM0
# "hello from cdc" appears in Terminal 1

Interactive bidirectional test:

# Terminal 1: open CDC serial port
minicom -D /dev/ttyACM0

# Terminal 2: send to printer
echo "tinyusb print example" > /dev/usb/lp0
# Text appears in minicom. Type in minicom to send data back through printer TX.

IEEE 1284 Device ID

The device responds to GET_DEVICE_ID requests with:

MFG:TinyUSB;MDL:Printer to CDC;CMD:PS;CLS:PRINTER;

Verify with:

cat /sys/class/usbmisc/lp0/device/ieee1284_id