RS-485 Communication Through Slip Rings: Configuration and Data Buffer Settings

RS-485 communication through a slip ring can be interrupted by contact movement, wear or signal distortion. With Slow fill selected, the controller repeats the last valid measurement during brief interruptions and applies the configured fill pattern after more than two seconds. Repeated values are held data, not new measurements. 

Why slip rings can disrupt RS-485 communication

Slip rings transfer electrical signals between stationary and rotating parts through sliding contacts. Although suitable for power transmission, they must also preserve signal integrity to support reliable RS-485 communication.

RS-485 transmits data using the voltage difference between two conductors. A slip ring can disturb these signals through:

• Contact interruptions: Vibration or contact bounce can briefly break the connection and corrupt data.

• Changing contact resistance: Uneven resistance between the signal paths can reduce noise immunity.

• Signal reflections: Changes in impedance at the slip ring can distort signal transitions.

• Electrical interference: Nearby power contacts, motors or switching equipment can introduce noise.

• Wear and contamination: Contact deterioration can cause errors to develop over time.

These disturbances can produce rejected messages or communication timeouts, particularly at higher communication rates. A slip ring suitable for power transmission is therefore not automatically suitable for RS-485.

Slow fill controls how interruptions appear in the measurement buffer. It does not correct the physical communication fault.

Configure Slow fill for communication through slip rings

When connecting the RS-485 bus through a slip ring, we recommend setting the data buffer pre-initialisation to Slow fill (Controller Settings → Settings → Buffer pre-init type).

During a brief communication interruption, Slow fill writes the last valid measurement value to the controller’s data buffer. These repeated values are held data, not newly acquired measurements, and may conceal changes in the measured signal during the interruption.

If the communication error persists for more than two seconds, the controller fills the buffer with the predefined fill pattern.

Buffer behaviour during RS-485 communication errors 

The Buffer Pre-Initialization setting defines how measurement values are handled in the data buffer when an RS-485 communication error occurs between the controller and the connected measurement modules.

The following Buffer Pre-Initialization options are available:

• Deactivated: During a communication error, the buffer continues to use the last valid measured value.

• Fast Fill: As soon as a communication error occurs, the buffer is filled with the predefined fill pattern. The default fill pattern is typically NaN, making missing or invalid measurement data clearly identifiable.

• Slow Fill: During short communication interruptions, the last valid measured value is retained. If the communication error continues for more than 2 seconds, the buffer is filled with the predefined fill pattern. This prevents brief communication interruptions from producing NaN values while still making longer communication failures identifiable.

How the buffer fill pattern works