Null Printer

A Null what…? Printer? Why?

I had a problem.  I needed to measure the print spooler speed when printing to a remote location.  My tests required that I be able to send actual documents from a Windows desktop to a printer in the remote office, and see how long it takes to spool. 

My first idea was to get a cheap printer and locate it in the remote location.  But, after doing some quick calculations, I would kill the printer by printing a page every 5 minutes for a year.  Esp since I had nearly zero budget.  The consumables (paper, toner/ink) would have made the costs prohibitive.  I looked into using a dot matrix printer w/o a ribbon – but still, that is a lot of moving pieces subject to jams and breaks.

I needed a device that would appear as a standalone printer (no printing to files – long reason, not important to this discussion) that would not produce any physical output. 

My solution? The Null Printer.

I did some poking around on the net, and found a circuit that would do this.  It was fairly simple circuit – it would send the ACKs and handshake properly to the print spooler.  What to use as a print spooler? I ended up using a Netgear PS101.  A little device about 2x 3 inches that took Ethernet in one side and output to a Centronics 36 pin port.

A big help in this project came from Tomi Engdahl.  He published a circuit that did exactly what I needed – on a 25 pin parallel port.  I needed to convert it to the 36 pin Centronics.

After poking around a bit, I stumbled on the 25 pin to 36 pin conversation chart.  Since I already know the 25 pin circuit, it was a simple matter of mapping the circuit over to the 36 pin.

In the above diagram, the 25 pin circuit provided by Tomi Engdahl is colored.  Same color means the pins are to be wired together.  I mapped the circuit to the 36 pin Centronics (right side).  There are some pin differences, bit it mapped perfectly.

Wanting to make sure that I got it right the first time, I built a prototype.  I cut about 10 inches off an old Centronics cable and stripped the shielding back to expose the wires.  I opted to twist the wires together first. That way, if there were any mistakes, I could easily remap the wires.  Here is my prototype pigtail:

After testing the pigtail, it WORKED! PERFECTLY!  The Netgear thought there was a printer attached, I could send large printer jobs to the nullprinter and the print spooler was perfectly happy.  No errors.  It spooled and “printed” just fine.

Here is the result of my soldering job:

Now, I had to make the “production” socket (dongle). My goal was to make it clean as possible – no exposed wires and make it look professional.