Capture Screenshots on Old Test Equipment with This Parallel Port Hack

Engineering test equipment seems to have a very long product lifecycle. Equipment from past decades is still in use in many tech companies’ labs and is also beginning to work its way into home test bench setups. This is due in part to the excellent construction of these devices but many times they still provide all the functionality needed to design and test state of the art circuits. One of the common tasks a user may come across when doing design or test is the need to capture screenshots from these devices for documentation purposes.

For older equipment generally only two methods will exist: a user can dig up or purchase an old floppy drive to insert into the device and save an image, or the user can take a picture with their phones. While both will suffice, they have their drawbacks as well — not to mention, using a floppy disk will almost certainly require a USB to floppy drive to be handy. One engineer by the name of Tom Verbeure decided to try and capture the screen data by hacking the parallel printer port on these older devices.

The parallel printing port was the “USB port” of early computing devices. It used a handful of control signals and eight data paths to send information in parallel across cables. To hack this port, three key components are required. This includes a DB-25 connector to connect to the instrument of interest, a USB connection to send the data out to a PC, and a microcontroller to handle the data capturing and handling. For the microcontroller, a Raspberry Pi Pico was chosen due to its availability, low cost, and ease of use. In addition, a level shifter also had to be included to interface the 5.0V parallel port voltage levels to the Pico’s lower 3.3V tolerant GPIO pins.

The schematic and PCB were created using KiCAD and are relatively straightforward. Test pins were also included in the design to support capturing bus signals to assist with debugging and testing. While the user mentions everything works fine with the first revision of the device, a second one was made to make some optimizations. These included reducing PCB layer count from four to two layers, which helps reduce overall cost and reducing the form factor to save space when in use. Both of the Gerber files for the PCB revision can be found here.

Firmware is available for the microcontroller programming, which consists of device configuration and an interrupt routine to capture and send the printer data through the USB port. Furthermore, Python code has been written to assist with the data capturing events. Although it is possible to capture the raw data using the device alone, the Python code offers a more simple means of doing this by automatically splitting print captures into different files and providing feedback on the data being received.

Overall, not many solutions exists for a convenient means to capture screenshots on old test equipment. Currently there are some devices that exist such as the Retro Printer Raspberry Pi HAT. However, this device costs over $100 and the Raspberry Pi that is also needed for it may not even be available in today’s market. Consequently, why not try and build your own? For around $30 and a day’s worth of time, you can be capturing screen shots on old test equipment with no need for USB flash drives.