Independent protocol reference
Learn Modbus
Understand the request and response model, then follow the transport-specific rules for serial RTU or TCP. These guides explain Modbus itself and do not require ModbusLab software.
Guides
Modbus RTU framing
Address, function, data and CRC fields, serial timing, and how to recognize a complete frame.
Read the RTU guideModbus TCP messages
MBAP header fields, transaction matching and message boundaries over a byte stream.
Read the TCP guideFunction codes and FC03
How a function selects an operation, plus a complete holding-register request and response example.
Read the function-code guideRegister addressing
Relate familiar 4xxxx references to the 16-bit address carried in a Modbus PDU.
Read the addressing guideModbus troubleshooting
Separate transport problems, protocol exceptions and application-map mismatches.
Read the troubleshooting guideProtocol knowledge and product help
These articles are vendor-neutral explanations based on published protocol documents. For steps that apply specifically to the current Windows application, see ModbusLab product documentation. The ModbusLab simulator implements RTU serial behavior; the app also contains an RTU-to-SNMP gateway. Modbus TCP and ASCII transport are not claimed as current app features.
Primary sources
Technical statements in the serial and application-protocol guides link to specifications from the Modbus Organization. TCP/IP behavior is described in its Modbus Messaging Implementation Guide.