Interfaces — how blocks talk to each other

Blocks have to talk to each other. Almost all of that happens over three "buses" — shared sets of wires with agreed rules. Knowing which is which lets you read any design at a glance.

• I2C — two wires (SDA + SCL), shared by many devices, each with its own address. Slower, but wonderfully tidy. Library blocks on I2C: the SSD1306 OLED display, the INA219 current monitor, the PCA9685 servo driver.
• SPI — four wires (data in, data out, clock, and a chip-select per device). Fast. Used by the microSD logger, the RC522 RFID reader, the LoRa SX1278 radio, the NRF24L01 link.
• UART — two wires (TX + RX), a simple point-to-point serial link between two chips. Used by the HC-05 Bluetooth module, the DFPlayer MP3 module, and the SIM800L GSM module.

A rough rule: many small devices → I2C; fast data → SPI; a single module chatting to the controller → UART. Once you can name the bus, you understand how a block connects.

Two small things trip up beginners when blocks connect — and the library handles both.

Pull-ups on I2C

An I2C bus needs two small pull-up resistors on the SDA and SCL lines to work at all. It's easy to forget; the I2C bus with pull-ups subcircuit bakes them in so the bus is reliable.

Voltage mismatch

Not every block runs at the same logic voltage. A 5 V sensor wired straight to a 3.3 V controller can damage it (or just not read). The fix is a bidirectional level shifter — it translates signals between 3.3 V and 5 V so both sides are safe.

You'll rarely place these yourself, but when Forge adds a pull-up block or a level shifter to a design, this is why. Spotting them tells you the parts are being connected safely, not just wired together.