Verify RS-485 Driver Enable Through the Last Stop Bit

A half-duplex RS-485 frame can contain correct bytes in the transmit buffer and still lose its final bits on the cable. The driver-enable signal must remain active through the required transmission and release before another node drives the bus. Verify those events at the physical interface rather than relying only on a software completion callback.

Identify what completion means

ST’s AN3070 examines driver-enable timing and compares ways of managing it with a USART. Its examples are device-specific, but the central review question applies broadly: which event proves that the final serial bit has actually left the peripheral?

Distinguish a DMA transfer completing, a transmit register becoming available and the serial shift operation finishing. Read the exact peripheral documentation for the selected controller. Do not assume that similarly named status flags on another processor have identical timing.

Draw the direction-change interval

Place UART transmit data, driver enable, differential bus voltage and receiver enable on one timing diagram. Include the transceiver’s enable and disable delays under applicable conditions. Mark the earliest valid response from the other node and the local receiver’s required readiness.

Check the start of transmission as carefully as its end. Asserting enable too late can truncate the start bit. Releasing it too early can truncate the final character; releasing it too late can interfere with the reply. Each is a different timing boundary in the same exchange.

Capture representative traffic

Measure short and long frames at the highest intended data rate, including back-to-back exchanges and the fastest permitted response. Capture the final character and turnaround at adequate time resolution. Record both firmware state and physical waveforms so that an apparently correct callback can be compared with bus activity.

Repeat under relevant processor load. An interrupt-based method can behave differently when other work delays service, while a hardware-controlled method still needs the correct configuration. A bench test with an otherwise idle processor may miss the operational case that consumes the timing margin.

Keep protocol and hardware limits separate

The protocol determines when a node may reply; the transceiver and controller determine how quickly the interface changes state. Preserve both in the budget. Avoid adding an arbitrary delay that fixes one partner but prevents compliance with another required response interval.

Document the completion event, enable strategy, measured margin and tested traffic cases. If the UART driver, DMA configuration or transceiver changes, repeat the relevant capture. This provides a reviewable reason that the final stop bit reaches the bus and that the next transmitter can take control without overlap.