Update to SDI-12 USB adapter adds more protection

It’s been a while since my last post. So here is an update. Based on inputs of a number of users, I have made some updates to the basic SDI-12 USB adapter. Here are the new features:

Zener diode between SDI-12 bus line and ground.

This protects the microcontroller from voltage spikes due to power spikes or other interference, as well as accidentally short circuiting power with SDI-12 bus. Some SDI-12 sensors come with stereo plugs, they are notorious for short circuiting all three contacts when plugging in. This causes shocks to the SDI-12 bus if you are using external 12V power, which eventually causes it to die or at least its pin to degrade.

Inline 510 ohm resistor

This also helps with transient protection. I’ve made a few prototypes and plan to test them. Here is a photo:

This update shouldn’t affect the logging script or your existing loggers. I’ll consider adding these components to the SDI-12 USB + Analog and GPS adapters later after I identify space for them since there’s not a lot of space on those variants. I recommend the 1N4737 Zerner diode if you wish to add some transient protection yourself. Just make sure to place it correctly, with the black stripe side connected to the SDI-12 and the other side to ground, on a spare SDI-12 terminal or solder between the S and – pins on the middle 8-pin add-on header:

FT231X USB IC

This IC replaces the FT232RL that is a bit outdated. The new IC will not affect any existing logging scripts, which only identifies the adapter by its vendor ID of 0x0403. If you have modified my scripts to identify the adapter by its USB vendor ID:device ID, the old device ID for FT232RL is 0x6001 while the new device ID for FT231X is 0x6015.

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