A controller publishes its process value as one signed, scaled holding register at protocol address 200. The linked sandbox opens the exact unit, function, address, quantity and value so the reader can verify the PDU rather than copy an unexplained hex string.
Follow the workflow
This Panel temperature controller example uses Modbus function 03. Select the function from the documented object type and whether the operation is a read or write—not because two register names look similar.
Do this in the product
Open the preconfigured request and confirm the function-code label before running it.
Open the exerciseThe request transmits protocol address 200. Human-facing 0xxxx, 1xxxx, 3xxxx and 4xxxx references are notation, not values to paste blindly into the 16-bit address field.
Do this in the product
Compare the address field with the expected PDU 03 00 C8 00 01.
Open the exerciseThe request quantity is 1 and the training value is 0. Multiword data, signed values, enums and decimal scales remain application-map concerns beyond the core Modbus frame.
Do this in the product
Run the server grader and read each function-specific limit check before copying the request.
Open the exerciseA normal Modbus response proves that a server accepted the request format. It does not by itself prove that a motor moved, a valve reached position or a process value is correctly scaled.
Do this in the product
Save the known-good request, then use a paid project to retain later fault and commissioning evidence.
Open the exerciseCore concepts
The simulator creates a repeatable result; these concepts make that result transferable to real vendor software and supervised practical work.
The transmitted address is a zero-based 16-bit field unless the specific device manual states a different table convention.
The expected request begins 03 00 C8 00 01; TCP adds MBAP framing while RTU adds unit address, timing and CRC.
Function, address and response can be valid while units, signedness, scale or machine permissions remain wrong.
Continue in the workspace
Run the foundation exercise publicly, then use a subscription for advanced challenges, saved configurations, full attempt history, sharing, assigned paths and team reporting.
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