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Parallel Port CW Rig Keying Interface for N3FJP Software and Elecraft K2

I've been doing some CW contesting with my K2 and N3FJP's Logging Software and since I now have this modern rig with computer control and everything I wanted to connect it up to my old Thinkpad and use the serial port for rig control and the parallel port for keying.

Now, I've heard talk that you can run both rig control and keying off the same serial port and I've also heard that the parallel port is a crummy, RF-sensitive interface, but I really want to keep my K2 serial line "pure" and do the keying on a separate circuit. (In practice, the final circuit has worked fine and I haven't had any RF problems.)

N3FJP's web site had a serial schematic, and he has since added a transistor driven parallel schematic, but I looked all over the web for an opto-coupled, parallel port circuit and couldn't find one, so after I finally built my own interface I decided to make this page and maybe save someone else some time.

I bought a stereo headphone cable extension cable at Radio Shack and cut it, then mounted the interface components in a 25 pin D-shell. The final product looks like this:

External view of cable

The schematic looks like this (it was easier to just draw it on paper and scan it than fiddle with a drawing program, so please excuse the somewhat rough quality.)

Interface Schematic

Now, I am a Big Hack when it comes to electronics, so I had to fiddle a bit to get this to work. Some notes:

  • I did a lot of math to figure out the right value for the current limiting resistor that is in the line that drives the LED in the opto-coupler. I even got a lot of help from the Elecraft Reflector, in particular Dale, KK6RK was very patient over several e-mails and figured a 390 ohm resistor would provide the needed 8-9ma to the LED. But, in the end, it was very hard to figure out how much current my crazy Thinkpad parallel port could source and nothing seemed to work. That's when I became a complete hack and dropped a 1K ohm pot into the circuit and twiddled it while sending from the computer until things started working. I discovered I could start to get reliable keying around 130 ohms and 100 ohms worked well. I think this is a good way to get a workable value for a particular situation and I recommend it -- though I do intend to swap out the pot for a-- -- fixed value resistor since the pot is "twitchy" and can be bumped out of value easily.
  • A 4N25 opto-coupler would also work, maybe better.
  • The 1N5817 diodes in the keyline that activate the K2's autodetect keying circuit work very well and I've seen some traffic on the Elecraft reflector that indicate these Schottky Rectifier diodes have good timing characteristics for this application (they turn on together and don't confuse the K2).

Here are the parts I ordered from Mouser:

MERCHANDISE:

STOCK NUMBER        ORDERED AVAILABILITY             PRICE  Ext. Price
======================================================================
78-4N35             1       1 Ships Immediately      0.290  0.290
583-1N5817          2       2 Ships Immediately      0.280  0.560

Finally, some pictures:

Internal view of D-Shell 1

Internal view of D-Shell 2

Internal view of D-Shell 3