Microprocessors usually have an UART. When the application does not require an UART, the UART could be still very helpful to print out debugging information. However the reasons to print debug information does not justify to put a relatively expensive RS232 level shifter and a big Sub D 9 connector on the board (or a FT232 chip creating a USB interface). Therefore the UART can be wired to a 2.54mm pin header where a RS232 level shifter can be plugged in. The level shifter should operate at 5V and 3V3 to avoid having two versions. This RS232 level shifter should have the following pin out:

2.54mm header (one in a row)

From FTDI there are also USB to TTL cable available for the same reason. However the FTDI cable use at least 6 pin since RTS/CTS signal (or 8 pin on a 2mm keyed header connector) are also included to allow hardware handshaking. Additionally there is a strict separation between 3V3 and 5V versions.

If it would be desired to connect a FTDI cable to the ElecX, the RTS/CTS signals could make use of the regular digital input and output signals or simply be not connected. However the ElecX interface is targeted as low cost debug interface and not as full RS232. Full RS232 should contain all signals, that are more than just RTS/CTS and have a proper Sub D 9 male connector.

RJ plugs and cables can be taken, cross talking between SDA and SCL is minimized since the power lines are used to shield.

Avoid soldering cables, since the standard cables for crimping RJ connectors have not heat resistance insulation and will melt during soldering. Therefore try to use always sockets and plugs (or crimp the wires) or used the special heat resistant cable version.