DNP3 Control Operations — CROB, Direct Operate, and SBO

Quick Answer

DNP3 provides two control models: Direct Operate (single-step command execution) and Select-Before-Operate (two-step verification for safety-critical controls). CROB (Control Relay Output Block) commands control binary outputs; analog output commands write setpoint values.

Control Relay Output Block (CROB)

CROB commands control digital outputs with configurable operation types:

• Pulse On — Turn output on for a specified duration, then off
• Pulse Off — Turn output off for a specified duration, then on
• Latch On — Turn output on and hold
• Latch Off — Turn output off and hold
• Trip/Close — Paired commands for circuit breakers and valves

Direct Operate

Single-step execution — the master sends the command and the outstation executes it immediately. Faster but less safe for critical operations.

Select-Before-Operate (SBO)

Two-step execution: the master sends a Select request (reserving the output), verifies the response, then sends an Operate request. If the Select fails (wrong point, already selected), the Operate is not sent. SBO prevents accidental operations on wrong outputs.

Analog Output Commands

Analog output commands write values to setpoint registers on the outstation — PID setpoints, valve positions, speed references. These use DNP3 Analog Output objects (Group 40/41).

Safety Considerations

• Use SBO for safety-critical controls (ESD valves, circuit breakers)
• Configure output timeout on the outstation (revert to safe state if no subsequent command)
• Use Secure Authentication to prevent unauthorized control commands