What the interviewer wants
- Frame format and addressing
- Half-duplex timing and bus ownership
- Error handling, addressing new nodes, and throughput
Constraints
- Half-duplex RS-485, transceiver DE pin controlled by the MCU
- Every node must report about 40 bytes every 2 s
- Up to 600 m cable; electrically noisy (pumps, fans)
- Nodes are installed by farm staff who can't set DIP switches reliably
If you ask the interviewer
- Can we use Modbus RTU? Yes, if you justify it. You may also design your own.
- Do nodes ever need to talk without being asked? Alarms (e.g. a flooded sensor) should reach the controller within 1 s.
- What baud rate? Your choice. Consider the cable length.
How it's graded
The rubric a senior interviewer would use. Each item is scored 0 to 3 and weighted.
| Framing and integrity | weight 3 |
| Half-duplex timing | weight 3 |
| Throughput and polling budget | weight 2 |
| Addressing and commissioning | weight 2 |
| Errors and recovery | weight 2 |
| Validation | weight 1 |
How to structure your answer
- Requirements and assumptions: the numbers that drive the design.
- Architecture: blocks, tasks, data flow, memory.
- Key decisions and trade-offs, with the alternatives you rejected.
- Failure modes and recovery.
- Testing and validation.
More design rounds
Write your design in five sections and get it graded against this rubric, with a follow-up question from the AI interviewer.
Answer this round