A protocol exception is an answer
Exception 01 usually points to an unsupported function. Exception 02 points to an illegal address or range. Exception 03 points to an invalid quantity or value. Keep the request and exception response together; changing serial wiring after a clean exception is usually wasted effort.
A timeout starts at the transport boundary
On RTU, verify port ownership, framing, Unit ID, wiring, bias, and termination. On TCP, verify route, port, firewall, and gateway Unit ID. Use retries only after one request is known to be correct; otherwise they merely repeat the same mistake more slowly.
CRC errors mean the line produced a bad frame
Check baud and parity first, then grounding, cable topology, termination, noise, and adapter quality. Record the raw bytes and frequency. An isolated CRC failure under heavy traffic is a different problem from every response failing in exactly the same way.