Saturday, October 13, 2012

USB RS-485 Adapter Surface Mount Boards

The surface mount PCB version of my RS-485 adapter recently arrived from overseas.  This one is cheaper and cleaner using the 20-pin PIC18F14K50, mostly surface mount parts, and fewer LEDs.  These require more patience to build than the through hole version.

I like the expansion ports on this one.  They are Arduino-esque, with power rails opposite data rails, allowing good stability of possible shield expandability.  Every data pin, including the ones used for RS-485 are drawn out to pads. 

Jumpers are provided to choose whether to connect the RS-485 read enable and drive enable lines from PORTB or PORTC.  This allows for varied resource allocation depending on the other I/O used.  If PORTC is completely free, it can be used as a full 8-bit I/O port.  Otherwise SPI can be used.
The first board I built out has the headers, and a Beau Interconnect (now Molex) terminal block socket with detachable screw terminals socket.  The second board built has dedicated screw terminals and no headers for a cleaner look.
If you look closely, you can see the green wire work done on the first board due to me stripping a pad of one of the capacitors while trying to replace an incorrect value capacitor.  And I originally built the first one with a 4MHz crystal before reading the datasheet and realizing a 12MHz or 48MHz crystal are the only ones that work for USB with this cpu.  I was able to remove the wrong crystal using two soldering irons, and then replaced it with the 12MHz crystal after another order from Digi-Key.
Next step is to load these with the Microchip USB HID bootloader, write up some documentation, and start selling some on eBay.


  1. Superbly written article, if only all bloggers offered the same content as you, the internet would be a far better place..
    torrent vpn

  2. Thanks for sharing the post.. parents are worlds best person in each lives of individual..they need or must succeed to sustain needs of the family.
    yowhatsapp apk download

  3. It seems that the more gadgets we have that were all touted as being able to de-stress us by keeping us up to date, the more stressed out we have become.Best cheap projector under 100