Modbus protocol for SCADA in SCADA systems - Step-by-Step Execution

Process Flow - Modbus protocol for SCADA

Start: SCADA Master sends request

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Request: Read/Write command + Address + Data

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Modbus Slave receives request

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Slave processes command

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Slave sends response with data or status

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SCADA Master receives response

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Process data or handle error

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End

The SCADA master sends a Modbus request to a slave device, which processes it and replies with data or status, completing the communication cycle.

Execution Sample

SCADA systems

Master -> Slave: Read Holding Registers (Address 40001, Count 2)
Slave -> Master: Data [123, 456]

SCADA master requests two holding registers from slave; slave replies with the register values.

Process Table

Step	Action	Message Content	Device	Result
1	Master sends request	Function: Read Holding Registers, Address: 40001, Count: 2	Master	Request sent to Slave
2	Slave receives request	Same as above	Slave	Request received and parsed
3	Slave processes request	Reads registers 40001 and 40002	Slave	Registers read: [123, 456]
4	Slave sends response	Data: [123, 456]	Slave	Response sent to Master
5	Master receives response	Data: [123, 456]	Master	Data received and ready for processing
6	Master processes data	Values: 123, 456	Master	Data used for monitoring/control
7	End	-	-	Communication cycle complete

💡 Communication ends after master processes the slave's response.

Status Tracker

Variable	Start	After Step 1	After Step 3	After Step 5	Final
Request Message	None	Read Holding Registers (40001, 2)	Same	Same	None (sent)
Slave Registers	[Unknown]	[Unknown]	[123, 456]	[123, 456]	[123, 456]
Response Message	None	None	Data [123, 456]	Data [123, 456]	None (received)
Master Data	None	None	None	[123, 456]	[123, 456]

Key Moments - 3 Insights

Why does the master send an address like 40001 in the request?

Because in Modbus, addresses starting with 4xxxx refer to holding registers, which the master wants to read or write. This is shown in execution_table step 1 where the master specifies address 40001.

What happens if the slave cannot read the requested registers?

The slave would send an error response instead of data. This is not shown in the current table but would replace step 4's normal data response with an error code.

Why does the master process data only after receiving the response?

Because the master must wait for the slave's reply to get the requested data. This is clear in steps 5 and 6 where the master receives and then uses the data.

Visual Quiz - 3 Questions

Look at the execution table, what data does the slave send back at step 4?

A Request message

B Data: [123, 456]

C Error code

D No data

💡 Hint

Check the 'Message Content' column at step 4 in the execution_table.

At which step does the master receive the data from the slave?

A Step 5

B Step 3

C Step 2

D Step 6

💡 Hint

Look at the 'Action' column for when the master gets the response.

If the master requested 3 registers instead of 2, how would the slave's response change?

A Slave would send 2 register values

B Slave would send an error immediately

C Slave would send 3 register values

D Slave would ignore the request

💡 Hint

Refer to the 'Count' in the request message and the corresponding data sent by the slave.

Concept Snapshot

Modbus protocol in SCADA:
- Master sends request with function code, address, and count
- Slave receives and processes request
- Slave replies with data or error
- Master receives and processes response
- Communication uses simple request-response cycle
- Addresses like 40001 refer to holding registers

Full Transcript

In Modbus protocol for SCADA, the master device initiates communication by sending a request message to a slave device. This request includes a function code (like reading holding registers), the starting address (e.g., 40001), and how many registers to read. The slave receives this request, reads the requested registers, and sends back the data. The master then receives this response and processes the data for monitoring or control. This cycle repeats for each communication. If the slave cannot fulfill the request, it sends an error response instead. The key is the simple request-response pattern that allows SCADA systems to read or write data from field devices reliably.