I recently picked up an HP 7475A off of craigslist. A while back I borrowed a friend’s 7550A, so I’m generally familiar with how these machines work. However, it still took me some tinkering and head-scratching to make it work, so I’m sharing this post in the hopes that future explorers find it helpful.

My model features the DB25 sized RS-232 serial port connector. The previous owner had installed a USB connection by attaching 4 jumpers to a Sparkfun FTDI usb breakout board. This approach works but has the drawback of forcing the user to enable XON/XOFF flow control when driving the plotter. Referencing the wiring diagram in this blog post, I was able to find a (cheap!) DB25 <-> DB9 cable for sale with the right configuration. From there, a common RS-232 to USB adapter should do the trick.

Another challenge: the geneva wheel mechanism to drive the pen carousel broke as soon as I plugged in the plotter. This seems to be very common, as there are many folks who have shared designs for 3d printable replacements. I ordered a print of the one found on this blog post on Shapeways.com, using the MJF GB material (nylon infused with glass beads). The plotter still happily does single color plots without this part.