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I chose the BUK9Y40-55B, but there are lots of MOSFETs that'll work here - they need to be have more than 24 V of drain-source voltage (VDS), and more than 1 A of drain current (ID) with gate voltage (VGS) of 5 V. The BUK9Y40-55B has a VDS of 55 V and an ID of 26 A at 5 V VGS - more than enough! If you are trying to get two ancient pieces of equipment (both of which use true RS-232 drivers and receivers) to work with a long cable, the capacitance limit in RS-232 may apply, but RS-232 does not take bit rate into account. There are sophisticated gate driver chips that get around this, but I'm actually already using a part that can act as a reasonable driver in this situation: the 74HC2G34 buffer. Typically I'd include ESD protection on every output pin connected to the external connectors (J401, J402), but in this case the 74HC2G34 has built-in ESD protection.

This utilizes a 74HC2G34 dual buffer (U401) as a low-to-high level translator. C401 is a bypass capacitor for U401. The drawback to using literally every I/O pin is that the bypass capacitors are a little further away from the RP2040 than they could be, but, the RP2040 seems more than happy with this layout in practice. Keep all of the bypass capacitors as close as possible to their respective pins with short traces. Keep the QSPI traces to the flash chip (U302) relatively short and avoid crossing layers. Keep the connections to the crystal oscillator (Y301) short and well spaced from other traces. It's also well suited for this project: there's a lot of digital peripherals to talk to but no analog stuff to deal with which pairs well with the RP2040's abundance of digital I/O and near lack of analog I/O. The driver's ENABLE signal is driven by one of the RP2040's GPIO pins.

One circuit (Signal Common) is required for all interface types. RS-232 typically has a transmit wire, receive wire and signal common wire. This signal is used for sensorless homing- the TMC2209 will pull this signal high when it detects a stall which can be used to detect when the gantry collides with its physical endstop. Also note R305 which connects TX and RX for the TMC2209 motor controllers together. These motor controllers use a single-wire serial bus for communication and that resistor is required to tie the separate TX and RX lines together. R1401 is a pull-up resistor that keeps the motor turned off during power up. R1001 is a pull-down resistor to make sure the MOSFET is off while the board is powering up. The RP2040 is pretty flexible in terms of pin assignment for serial I/O, so most of the assignments here were made in order to make the layout easier.

Perhaps the most striking aspect of this layout is the fan-out of the signal lines. Now that the schematic is laid out, I can show you the board layout. These valves are used to control the vacuum on the pick up head's nozzle - when the valve is open the nozzle can pick up parts using the vacuum, and when closed the nozzle is vented to atmospheric pressure so that it can let go of the parts. Starfish uses a set of two pumps, valves, and sensors to control the vacuum system used to let the two nozzles pick up small components. USB to serial converters come with ADDC (Automatic Data Direction Control) to automatically sense and control data direction, making the handshaking control method obsolete. While this is incorrect, many to this day still believe that the RS-232 standard includes things such as baud rate and the bit stream protocol used in the IBM serial port. Despite appearing antiquated in our modern USB C world, USB B remains the most mechanically sound USB connector- that's why it's still prevalent in professional audio, printers, and industrial applications. The USB connector (J407) is a USB B jack because of its robustness.

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