One board, two rails

One cable in, three things out

The USB-C socket at the bottom is the only input. Out of it come three things: 9 V on the printer power socket, serial on the printer data socket, and a supply plus serial for your own board on one of the two headers along the top.

The 9 V rail is not one of your options. It leaves the board on its own socket, goes to the printer, and should never be anywhere near a microcontroller pin.

The two rows that look the same

Along the top edge are two five-pin rows printed with the same five names in the same order — DTR RX TX and then a supply pin and a ground. The only difference is the supply: one row is 5V MCU and the other is 3V3 MCU.

That similarity is the hazard. The rows are adjacent, the pin order is identical, and a header pushed onto the wrong one fits perfectly. What changes is what your board's RX pin is connected to — and a 5 V transmitter driving a 3.3 V input is the mistake that costs a development board rather than an evening.

• 3V3 MCU — ESP32, ESP32-S3, Raspberry Pi Pico, anything that runs at 3.3 V.
• 5V MCU — Arduino Uno, Nano, Mega, anything that runs at 5 V.

RX and TX cross over

The board's TX goes to your board's RX, and its RX to your board's TX. The labels on the header are from the base board's point of view, so wiring TX to TX produces two transmitters shouting at each other and a printer that never hears anything.

DTR is a flow-control line the printers can use to say "stop sending, my buffer is full". None of the sketches in this book use it, and leaving it unconnected is fine at these speeds and these message sizes.