Remote Commands (DCMD)

Send write commands to Modbus devices via Sparkplug DCMD messages.

What is DCMD?

DCMD (Device Command) is a Sparkplug B message that allows SCADA systems to write values to devices remotely.

Topic Format:

spBv1.0/{group_id}/DCMD/{edge_node_id}/{device_id}

How It Works

1. SCADA publishes DCMD message to MQTT
2. Gateway receives command
3. Validates tag is writable
4. Writes value to Modbus register
5. Logs result

Configuring Writable Tags

1. Edit tag
2. Enable Is Writable
3. Tag must use HR (Holding Register)
4. IR/IS are read-only
5. Save tag

Command Logging

All commands are logged with: - Timestamp - User/source - Tag name - Old value - New value - Success/failure

Security

Commands require: - MQTT authentication - Proper topic permissions - Tag configured as writable

Next Steps

• Tag Configuration
• Sparkplug Protocol