Choosing a communication interface

1. Engineering problem

Every sensor and actuator needs a link to the controller. The wrong choice shows up as lost data, noise sensitivity or a bus that cannot be extended. A quick payload-rate calculation removes most wrong choices before any hardware is bought.

2. When is this needed?

• When selecting a new sensor, motor driver or controller board.
• When adding a device to an existing bus.
• When a link drops data and you need to know whether it is overloaded or noisy.

3. Selection and design decisions

General characteristics (the facts marked as sourced come from Wikipedia: classic CAN carries 8 bytes per frame and CAN FD up to 64; CAN uses a twisted pair terminated with 120 Ω at each end; common I²C speeds are 100 and 400 kbit/s with a 7-bit address and pull-up resistors; SPI is full-duplex with an active-low select line per device). The rest is general engineering knowledge; confirm on the datasheets.

4. Advantages and disadvantages

• Simplicity vs robustness: UART is trivial to wire and weak at noise and errors; CAN is differential with error detection but needs termination and configuration.
• Rate vs distance: faster signalling shortens the usable cable length.
• Shared vs dedicated links: a shared bus saves wiring and couples devices (one faulty node can disturb all).

5. Requirements and constraints

• Required payload rate ≤ 70 % of the link’s usable payload rate (a design margin, not a standard).
• Cable length and environment match the signalling (differential for noisy, long runs).
• The controller has the peripheral and the driver support for the interface.

6. Architecture

sensor/actuator ──link (UART | SPI | I²C | CAN | Ethernet | USB)──▶ controller or computer ──▶ ROS 2 driver node

7. Implementation

1. List each device with its required data rate in bit/s.
2. Compute the usable payload rate of each candidate link (below).
3. Keep only links where the need is ≤ 70 % of the payload rate.
4. Filter by distance, noise, number of nodes and driver support.
5. Write the choice and the numbers in the interface control document.

def uart_payload_bps(baud, data_bits=8, parity=0, stop=1):
    frame = 1 + data_bits + parity + stop          # start + data + parity + stop bits
    return baud * data_bits / frame
def i2c_payload_bps(clock_hz):
    return clock_hz * 8 / 9                         # 8 data bits + 1 ACK bit per byte; addressing overhead ignored
need = {"imu": 0.2e6, "gps": 0.04e6, "lidar": 90e6}   # required bit/s
links = {"UART 115200": uart_payload_bps(115200), "UART 921600": uart_payload_bps(921600), "I2C 400k": i2c_payload_bps(400e3)}
for name, rate in need.items():
    ok = [l for l, c in links.items() if rate <= 0.7 * c]
    print(name, "fits:", ok or "none of these links")

Printed (checked by the site’s tests):

imu fits: ['UART 921600', 'I2C 400k']
gps fits: ['UART 115200', 'UART 921600', 'I2C 400k']
lidar fits: none of these links

Reading it: a UART frame of 8 data bits with one start and one stop bit carries 8 payload bits per 10 line bits, so 115200 baud gives about 92 kbit/s of payload; I²C sends 9 clock cycles per byte (8 data bits and an acknowledge bit), so 400 kHz gives about 356 kbit/s before addressing overhead. A 0.2 Mbit/s IMU stream does not fit 115200 baud with margin, a 90 Mbit/s LiDAR fits none of these links (it needs Ethernet). Rates are illustrative.

8. Real examples

• Mobile robot: IMU over SPI to a microcontroller, motor drivers on CAN, LiDAR on Ethernet, GNSS on UART.

9. Failure scenarios

• An IMU configured at a data rate that exceeds a 115200 baud UART, so samples are silently dropped.
• Two I²C devices with the same address.
• A CAN bus without termination.

10. Troubleshooting

1. If data is lost, compare the measured rate with the payload rate and the 70 % limit.
2. If errors appear only when motors run, suspect noise and look at the cabling guide.
3. If a device does not answer, check address, mode, voltage level and pull-ups.

11. Validation

• The fit check above prints the lines shown (executed by the site tests).
• Measured message rate equals the configured rate with no gaps over the longest mission.

12. Engineering checklist

• Required data rate per device
• Payload rate per candidate link
• 70 % margin
• Distance and noise checked
• Driver support confirmed
• Choice recorded in the ICD

13. Further references

• Wikipedia, UART, SPI and I²C, read 2026-11-06 (secondary sources; the I²C specification itself was not read).
• Wikipedia, CAN bus and CAN FD, read 2026-11-06 (secondary source; the standards are ISO 11898, which was not read).

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