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.