Carsten Groen's (OZ9AAR) personal place

Centronics to USB interface

I have a number of instruments that was produced "some years ago", as probably many Radio Amateurs have. Some of these instruments have 3 1/2 inch floppy drives, these are typically used for storing screendumps etc. You can then move the floppydisk to another computer and load the screendumps and save these. Most PC's today does no longer have a floppy drive, so usually you will need an external drive with USB or something similar.

(Another possibility is to use a GPIB interface if your instrument has such an interface).

Instruments with floppy drives often have a simple Centronics printer port. Back in the days, you would connect your printer to this port and make direct screendumps to paper.

It would be nice if you could somehow direct the data/printout from this Centronics port directly into a computer, grabbing the printout directly.

Seems I'm not the only one that would like that. 

Below is a screendump of a small and simple PC application I made for the interface. The application as well as the complete project for this are available for download at the end of this page. Feel free to modify according to your own requirements.

If you don't want to assemble the adapter yourself, I have a few spare ones, price is Euro 35,- plus shipping incl 3 meter long USB cable (same as shown on the pictures below).

Hardware design

Brian K. White did a very nice and well done "Centronics to USB interface" and published his work on GitHub!

He calls his project "LPT Capture". 

The interface simply emulates a Centronics interface, the instrument "thinks" that a normal printer is connected to it. The data are sampled by the interface and sent as a normal "USB Serial stream" of data via the USB port.

On his GitHub page, you will find schematics, Gerber files etc. for the interface. I had a number of PCBs made and assembled a few of them for my instruments.

Below is a couple of pictures of the ones I made, I also did a BOM file (for Mouser) with the parts I used for the build. The adapter is very affordable to build, and Brian did an excellent job on the electronics and the mechanical aspect of it. He uses a standard Sub D25 shell as housing. I found some 3 meter long micro USB cables that fitted directly into the plastic shell. The cable I used is this one Goobay USB 2,0 Hi-Speed kabel, 3 m

You just need to make a small groove on both sides of the plastic at the connector, then it fits perfectly in the Sub D25 shell and is locked in place.

BOM file

I found all the components needed at Mouser. You can probably find them many other places like DigiKey etc.

1 pcs USB connector, center mounted: Link to Mouser.

1 pcs ferrite bead, Link to Mouser.

1 pcs D-Sub 25 backshell, Link to Mouser

3 pcs 100 Ohm resistor network, Link to Mouser, alternatively use 10 Ohm networks, Link to Mouser.

1 pcs FTDI USB interface chip (be careful to get the correct type -RNL), Link to Mouser.

1 pcs D-Sub 25 male connector, Link to Mouser.

2 pcs 100 nF 0805 capacitor, Link to Mouser.

1 pcs 10 nF 0805 capacitor, Link to Mouser.

1 pcs 4.7 uF/10V 0805 capacitor, Link to Mouser.

1 pcs 47 KOhm 0805 resistor, Link to Mouser.

Software

I made a super simple and somewhat rude PC application for grabbing the printouts. The application support HP PCL printers, on my FSIQ26 Spectrum Analyzer I installed a HP Deskjet 855C printer and selected that for hardcopy printouts.

The PC application grabs the data from the Centronics adapter, stores them in a temporary file, and then calls the "Ghost PCL" application which then handles the PCL to PNG conversion. This removes the need for doing all the PCL decoding in the application, leaving only the "house keeping" to it.

The PC application is written in C#, the complete project can be downloaded here. Please notice that the Ghost PCL EXE and DLL files needs to be present in the folder where the PC applications EXE file are located.

There is no installation required, you simply copy the 3 files to a folder and execute the application.

When the application runs and grabs screendumps from an instrument, it will show the grabbed image, this is also stored with a timestamp in the same folder as the EXE file is present.

The PC application, ZIP file

The complete C# project, ZIP file

Below a few dumps from my FSIQ26 Spectrum Analyzer.

Color image from FSQI26:

Black/white image from FSQI26:

Alternative software

My friend Joshua, DL3JOP, sent me a link for a Python application that also works with the Centronics/USB adapter. The project originally uses an Arduino connected to the Centronics port, but Joshua reports that the USB interface described here also works with the Python code. The Arduino project just samples the data from the Centronics ports and forwards the data using a serial port on the Arduino, that's why the Centronics/USB adapter shown here will also work with the same Python code.

You can find the project here.