Siemens S7-1200 Modbus RTU CB 1241 Communication Timeout Error 8281

Error `8281` on Siemens CB 1241 almost always indicates RS485 biasing issues or improper buffer polling cycle. 1. Ensure you pull-up (`TxD/RxD+` to +5V via 390 ohm) and pull-down (`TxD/RxD-` to GND via 390 ohm) to bias the bus when idle. 2. Do NOT trigger `MB_MASTER` repeatedly on a timer interrupt. Use the `DONE` or `ERROR` bit of the previous MB_MASTER call to step to the next slave ID in your state machine.