On most Parallax boards this line is connected to a regulated 5V supply.
Do not connect a power source greater than 5V directly to pin 21.
When not connected this pin is pulled up to 3.3V by RP1 on the module.
When using MakeItC, this line is pulled low to download a C program, which
can be done automatically by connecting to an NPN transistor with the RTS line on the serial
On later revision ARMexpress and ARMexpress LITE a 1K pull-down has been added on the module between pins 3 and 4 (as pictured below. If your unit does not have this, then a 1K pull-down resistor is required, when there is no signal on pin 3.
A serial connection can be made with a USB breakout board. The suggested wiring should be done as follows. The inversion of RXD, TXD and DTR can be done by the FTDI chip using their MProg utility. Mprog programs ALL FTDI parts connected to the PC, so make sure only the one you want to change is connected. Also changes do not occur until the FTDI chip is powered up (so you must disconnect it and reconnect it).
Hints for debugging
Make sure you have both Power and GND connected.
When running BASICtools, the idle condition is
PIN 1 low
PIN 2 low
PIN 3 low
PIN 22 high
PIN 21 high
When RESET, either by pulling 3 high or 22 low, there will be some activity on pin 1 as the ARMexpress sends the Welcome message.
Reset and Boot for PRO boards
For the PRO, PROplus and SuperPRO boards when connecting a PC to a board that is running, the reset and boot control signals will be toggled by the PC. This is a function of Windows and the Drivers. This will reset the board or possibly put it into a load program state. To avoid this you can disconnect the Reset and Boot signals from the USB dongle, either by cutting pins or making an adapter using a 6 pin female header with long pins(available from SparkFun ).