Step 1 : What and why.

I was running into EXTREME interference problems in several of my projects. Most notably with my new 595 controller. I had found that if I kept the lines from the printer port to my project under 2-3 feet, the interference wasn't that bad but I was still getting undesirable results. It looked like it was just ambient RF (florescent lighting, furnace motor, etc...)that was causing the problems. What I needed was a buffer/amplifier for the line but standard printer extenders wouldn't work because they're "smart" and attempt to communicate with the printer (that isn't there).

In all these steps, click on the diagram to get a clear image. The web site shrinks them a bit to make them fit the page and some of the thin lines in the diagrams may not show.

Step 2 : Research and findings


After a lot of research I discovered the DS9638 and DS9637A line driver and receiver pair. One is the transmitter and one is the receiver. They are RS-422 compatible, they operate off of a +5v supply, and they're TTL compatible so they're ideal for use on the printer port.

Step 3 : How it works


The DS9638 line driver is a Schottky, TTL compatible, dual differential line driver designed to meet RS-422 specifications. It is designed to provide unipolar differential drive to twisted pair or parallel wire transmission lines. In plain English:
It takes your input and provides two outputs. One, a buffered version of your input, the second, an inverted version of your input. You connect this directly to your twisted pair wire. CAT-V wire is the best, in my opinion. It's a dead on match for the chip specifications, it's got enough conductors for what we're doing here, and you can get a 1000' roll for $40.

Step 4 : Interferance


Now that we have our data flying down a cable, it's susceptible to any noise that may be in the air. Anything can cause unwanted signals on your wires: electric motors, nearby kitchen appliances, fluorescent lighting, radio transmitters, sun spots, even the 60Hz house wiring can cause problems. You name it and it could potentially cause noise on your lines.

Step 5 : Interference


As the unwanted signals are picked up by your pair of lines, it will appear the same on each line.