Modbus registers on their own are not a dashboard. Node-RED is the fastest way to turn the register map from lesson 3 into something an operator can look at, without writing a SCADA application from scratch.

What you will learn

• Place Node-RED in the data pipeline between the PLC and a browser.
• Build a flow that reads the tank level and pump state and shows them.
• Send a start command back to the PLC from a dashboard button.

You need: the Modbus TCP server from lesson 3 running on your PLC, and Node-RED with the Modbus and Dashboard nodes installed on a PC or on a Raspberry PLC.

Where Node-RED fits

Node-RED polls your PLC over Modbus using the holding registers you exposed, and feeds a browser dashboard. Your Arduino code does not need to know Node-RED exists, it only has to keep answering Modbus requests.

Build the flow, step by step

1. Add a Modbus client configuration with the PLC IP address, port 502 and unit id 1.
2. Add a Modbus read node for holding registers 0 to 1, polling every second.
3. Add a function node that converts the raw level to percent, using the scaling you learned in the foundations course.
4. Connect a gauge node for the level and a text node for the state (0 IDLE, 1 RUNNING, 2 FAULT).
5. Deploy and open the dashboard address in your browser.

Expected result: the gauge follows the tank level and the text changes when the machine moves between states.

Writing back to the PLC

Add a button node connected to a Modbus write node for holding register 2, sending the value 1. Pressing it asks the PLC to start. The state machine remains in charge: the PLC decides whether starting is allowed, and the dashboard only makes the request. Never let a dashboard be the only layer that protects a machine.

If the gauge shows nothing: check that the PLC answers a Modbus client from the same PC first, then the register index and the IP address in the node.

A starting point, not a finished SCADA

A Node-RED dashboard is a fast, real way to visualize and interact with a PLC, but it does not replace the alarm management, user access control and historian of a dedicated SCADA package on a larger installation. Treat it as the right tool for a first dashboard, a single machine or a proof of concept, and reassess as the project grows.

Go deeper

• Store data and display it on a Node-RED dashboard with Raspberry PLC
• Node-RED: how to make a custom design gauge for your dashboard
• Node-RED vs CODESYS vs Arduino IDE: which to pick
• Raspberry PLC

Next lesson: debugging on the shop floor. Take the quiz below first.