DerbyLink USB to RS232 over CAT5

The DerbyLink system was put to the test last weekend - powering the Micro Wizard finish gate, converting RS-232 to RS-422, then RS-422 to USB, and simplifying the wiring down to a single lowly CAT5 cable. A complete success and nothing out of the ordinary to report during the pinewood derby of Cub Scout Pack 45 of Farmington, Michigan. I really like the whole PoE power setup, and I think I'll use it again in any projects that require cabling and power and perhaps not being convenient to an AC outlet.

I whipped up some end panels using Inkscape and sent off to Ponoko for fab. I chose 1.5mm thick black acrylic. Black wasn't my first choice, but Ponoko didn't have many options in acrylic less than 3mm thick. The nice, cheap, extruded enclosures from Adafruit didn't come with dimensioned drawings, so it was a bit of guesswork to get the panel overall size and mounting features, but after a few iterations with paper printouts, I managed.

I used crayon as the infill on the panels to allow the engraved text to show up. My teenaged son referred to it as "ghetto". I call it "industrious".

Now, just need to wait until next January to unveil the DerbyLink to the "pit crew"...

I was able to put together an ugly cable between the DB15 and the DC barrel connector and the DB9 serial con on the finish gate. Success! I can power the finish gate through 100 feet of CAT5 cable and the communication is working just fine between the PC and the finish gate.

All that's left to do now is to draw up some end panels with cutouts and fab those - ponoko maybe?

Then, all I have to do is wait for next January! :-)

I finally received my PCB's today. The DirtyPCB service thoughtfully bundled all three board types I ordered (two for this project, one for the Rubidium Oscillator controller project). The board quality seems good. A slight registration error in the silkscreen on one of the boards, but really nothing worth complaining about. It's been a while since I've assembled HASL'd boards - I got spoiled by the ENIG finish that OSHpark provides.

I put together both boards in less than an hour, and other than having to diddle a resistor value in each of the 48V and 9V voltage comparator circuits, no issues. I think I simulated the comparator circuit with an incorrect value of reference voltage (derived from the 5V power LED) so I had to adjust the divider input R to achieve the correct threshold voltages (40V and 8V). I'll have to look into this to be sure.

I threw together a quick loopback test setup with the two DerbyLink boards, a 100-foot CAT5 patch cord, a 48V supply, and a 30-ohm load (300mA at 9V) to simulate the draw of the MicroWizard finish line. It's presently undergoing 'burn-in' in my basement lab. I found a nice COM port loopback test program, so I'll let that run overnight checking the USB -> RS422 -> RS232 -> RS232 -> RS422 -> USB link at bitrates between 2400...115200 bps.

The complete fileset for proto1 of DerbyLink can be found on GitHub
https://github.com/tomcircuit/derbylink

Note: not tested yet!

I've completed the schematic capture and PCB layout for both sides of the link. I submitted them via the dirtypcbs.com service last week, and waiting on them to arrive. There's a bit of a delay because of Chinese New Year. No hurry, I've got until January 2016 :-)