Notice the RS232 chip added on the lower left side

Adding RS-232:

For many applications the board described in the previous article will be adequate.  If you need to communicate with the SBC you can easily use one of the Ethernet or RF enabled Rabbit Core Modules instead of the RCM41xx.

However, there are many cases where RS232 connectivity is required.  The second board design, see schematic 2 and Fig. 2, adds a dual RS232 to CMOS convertor.  This circuit can be used as two independent 3-wire circuits or a single 5-wire circuit.  The board is the same size as the board described above and can still be implemented with two layers.

There is one additional 5 pin header for the RS232 I/O.  There are quite a few dual channel convertors available so I just picked the one that is typically used by Rabbit.  It is relatively inexpensive and easy to implement.  All it needs are the four external 0.1uf capacitors.

Updated Schematic (Click for high resolution)