HP provided a manual (scan attached) that described the printer's operation in great detail including full schematics and parts list. The thing that struck me while building this interface was the parallelism between the old service manuals and our current idea of Open Source Hardware. One could also build this circuit in an enclosure and use driver chips to interface with a Centronics interface driven over a cable. I used a ribbon cable to connect to printer port pins + an internal 5V power line in the printer so my board could be powered by the printer through a schottky diode (allowing power from either USB or the printer). A cheap ebay board or one hand-built using any one of a number of converter chips can be substituted. I used a TTL-RS232 converter board I found at my hackerspace originally designed for some other development board. I put the Arduino and RS232 driver circuitry on a proto-board that fit inside the printer (with some slight modifications to the back panel to allow access to the RS232 DB-9 and USB connectors). Coupled with sensing the paper status signal on the Centronics interface, the Arduino can communicate back to a host system the printer status (powered and paper status). A resistor-divider provides a sense for the Arduino to know when the printer is powered. The Arduino can be powered by either an external computer (to load code, for example) or the printer (when using the RS232 interface) without a USB-connected computer back-powering the printer. We use a schottky diode to isolate the Arduino (and RS232 interface circuitry) from the printer circuitry.

Since the Centronics parallel port is TTL-compatible, we connect the Arduino directly to the 8-bit data bus, the active-low Strobe signal, the active-high BUSY signal, the active-high PAPER EMTPY signal and the active-low RESET/INIT signal.

Tri-states Arduino signals when printer is not powered but Arduino is powered via USB.

Notification of printer paper status (paper available/paper out).

Command to reset printer and flush buffers.

Command to request power and printer paper status.

XON/XOFF flow control on two serial interfaces (USB Serial or TTL serial converted to RS232).

In addition to the ability to send character data to the printer, this project adds a few extra features: The Arduino connects directly to the Centronics Parallel port signals as well as taking +5 volts from the printer. This project shows you how to use an Arduino Leonardo (I used an Arduino Pro-Micro clone) to provide a printer with two serial interfaces (CDC-class serial over USB and RS232 over the Arduino's TTL serial port). The Centronics parallel port interface is fairly simple to control in its most basic form - which is useful if you want to drive an old parallel-port printer.

Parallel serial converter