Modifying an Old HP7475a Plotter to Work Over USB

About a month ago I was at the local municipal dump when I came across this unusual looking printer on the sorting table for electronic waste about to be thrown into a container. I was curious, because I have never seen such an old device that had actual pens in it.

After taking it home, I found out that it was a plotter from the late 70s and that it was used to draw technical drawings, graphs, etc.

After turning it on, I was able to move the pen around with the keypad, meaning that it seemed to be functional.

My first thought was how on earth will I find drivers for this thing? and what kind of cable will I need? and what kind of software will I need?

The great news is that these types of plotters work on commands with HPGL (Hewlett Packard Graphical Language) which is literally just simple commands to do many graphical things and all over RS232 communication, meaning that it is really easy to modify these types of old plotters to work with modern computers over USB.

There is also a lot of original documentation available online, such as service manuals with circuit diagrams and part lists.

Important Note:

This modification is for the RS232 version only. If the plotter has a HP-IB port instead of the 25-pin RS232 port, then this modification will not work.

Step 1: Hardware Modification: Introduction

Even though the port has 25 pins, only 4 connections are needed. Since printers of the past operated with RS232 communication with +12 V and -12 V data signals, there is some additional circuitry on the plotter circuit board to achieve these voltages.

Looking at the part list in the service manual one can find the chip used for data communications. This chip operates on normal TTL communication, meaning that 0 V to +5 V are normal conditions for this chip.

Sparkfun has an RS232 module that runs over USB and has all the pins needed for successful data communication to this plotter.

Step 2: Hardware Modification: Soldering

By unscrewing the 3 screws on the rear side, one can open the top cover and expose the main circuit board (make sure the plotter is unplugged from the mains).

There are only 4 lines that will be needed: RX, TX, GND and RTS.

Since these plotters have very limited memory, it is very easy for data to be lost and have the plotter do unexpected things, therefore hardware handshaking will be required. This just means that the plotter will tell the computer over the RTS pin when it has enough memory available for new incoming data.

Solder 4 wires directly to pin 1, 2, 5 and 6 of the plotter's USART chip (GND, RX, RTS and TX of the chip respectively) and run them out the back to a header pin strip for the RS232 module.

I added a cable tie to prevent the wires accidentally being ripped out.

Gently close the top cover paying attention to the paper lever.

Step 3: Wiring

Connect as follows:

Plotter GND to RS232 GND

Plotter RX to RS232 TX

Plotter TX to RS232 RX

Plotter RTS to RS232 CTS

Adjust the DIP switches at the back as in the photo. The baud rate is set to 4800.

The plotter is now ready.

Step 4: Software: Inkscape

Inkscape is professional quality vector graphics software which runs on Linux, Mac OS X and Windows desktop computers. Most importantly, it's free and can create HPGL files that can be sent directly to the plotter.

To get started, a HPGL file is included. One might just need to change the page orientation to landscape.