While testing the FTDI serial connection, it can be quite useful to instantly see what’s going on with each of the pins. Here’s a little custom “plug-through” circuit we can use for that:

The idea is: 5V is reduced to approximately 3V via three diodes. These are common 1N4148 diodes, with each about 0.65V drop when connected in the forward (conducting) direction.

The rest is simple: an LED tied to a signal pin, with a 1 kΩ resistor in series. Some math to explain that resistor value: 5V – 3x 0.65 – 1.8V = 1.25V. So a 1 kΩ resistor will pass 1.25 mA, which is also the current through the LED: enough to light it up, but not too high to load down the signal line too much (it can probably handle somewhere from 3 to 10 mA).

Note that all signal lines use inverted logic, so we want the LED to light up when the signal is low. If you follow the path from 5V -> diodes => resistor => LED => signal, you’ll see that this will only pass current when the signal is a “0”. On a “1”, it’ll be almost 3.3V, and then there will not be enough voltage on the LED to light it up. Simple!

Here is the above circuit, placed on a little protoype board:

This is the complete setup, with an FTDI BUB and the LPC810 programmer hooked up:

And sure enough: this little tool was essential to solving some issues. Coming up next…

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