Parallel Port CW Interface for N3FJP

Posted on Feb 5, 2014

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

Parallel Port CW Interface for N3FJP
Click here to download the full size of the above Circuit.

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: 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. ) 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...

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