sparkfun FTDI SmartBasic Hook-Up Manual page 3

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used for loading code unless the other device is removed. Here's a diagram
showing how to connect the FTDI SmartBasic in a way that removes that
problem.
You can see that the connections from the Bluetooth Mate and the Arduino
Pro Mini are straight-through; no need to cross wires, so you can plug them
right in. Also note the connection of digital I/O pin 2 to the SEL line on the
FTDI basic. This is what allows the multiplexing of the serial data: when the
board is in bootloader mode, that pin will be a digital input and the SEL line
will be pulled low by a pull down resistor on the SmartBasic. That will route
the data to the FTDI chip to be sent to the PC, and data from the FTDI chip
will be routed to the Pro Mini board, and bootloading of a sketch can occur
normally.
Note, as well, that the CTS and DTR pins between the SmartBasic and the
Bluetooth Mate are not connected. Since the multiplexer on the SmartBasic
only has two channels, only the data channels can be swapped. That's
important, though, because DTR is needed to reset the Arduino at bootload
time. If it were being re-routed, that would defeat the purpose of this board.
After the application sketch has loaded, the user can switch between the
two data endpoints (the PC and the Bluetooth Mate) by asserting pin 2 high
(for the Bluetooth Mate) or low (for the PC). Here's a simple Arduino sketch
showing that in action.