The present invention relates to a test device for a Modbus RT system, which communicates with a slave device to be tested according to the RS-485 communication protocol, and tests whether the slave device complies with the Modbus RTU protocol. A transmitter that transmits data, a converter that converts the test data of the transmitter to conform to the RS-485 communication protocol and transmits it to the slave device through a bus, and receives the response data of the slave device converted through the converter A receiving unit, a test controller comparing the received bit or frame of the response data with a reference bit or frame to determine whether the slave device complies with the Modbus RTU protocol, and detecting the voltage waveform of the bus and providing it to the test controller Thus, it may include a waveform detection unit to check whether the characteristics of the voltage waveform conform to the RS-485 communication protocol.
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