If you want to take advantage of the great I/O power on the Adafruit Grand Central M4 Express or any other Arduino Mega-compatible board, this should be a great help.

This shield features:

  • Two Qwiic connectors from the main I2C bus
  • One additional Qwiic connector from the secondary I2C bus*
  • A shrouded connector for I2S
  • A Picoblade connector and a 4-pin connector for each of the RX/TX pin pairs

*Assuming this is a regular Mega, Duo, or similar board. The Grand Central ties both I2C buses on the shield together into one, so this extra Qwiic connector is just a third I/O for that.

While the 4-wire UART connector (currently represented by a 4-pin header) may be better for connecting to unpowered devices, the 3-wire Picoblade connectors are meant for communicating with other Megas with this board, or with Feathers with the GateWing (qv).


The Picoblade connectors are arranged so that, looking at the end, the TX is on the left and the RX is on the right, with Ground in the middle. The ideal way to connect these to other units would be to put an identical connector on them (possibly using a proto board), and then use a cable that gives a straight-through connection so that the TX on the remote device is connected to the Union Station's RX and vice versa.

Note that all connectors are either at the board's "west" end, or in the space that's left open with a conventional shield on top. Unless you're not using the latter set, this should be the uppermost Mega-sized board on the unit.

Remember that the RX/TX ports are not buses, but direct communications. Don't connect more than one unit to any given port, regardless of how many connectors you have. (That may seem obvious to some, but even the most intelligent and learned among us can miss the painfully obvious!)


Here are some potential future edits (which may be heavily influenced by others' feedback, if I get any):

  • There are a few items based on the double-row header that are blocked by Fritzing's inability (or my own inability to figure out how) to run wires between the pins.
    • A Parallel Capture connector (pins 26-43)
    • The pair of wires that currently run around the end for the I2S connector
    • An SPI connector (I'd rather add a new one that put a hole in the board for the one on the main board)