FIELDBUSES
Modbus
A client requests data and a server returns a register value.
Who communicates with whom?
Modbus RTU: one client (formerly master) initiates a request containing a unique server (formerly slave) address, function code and data. Only the addressed server replies. For example, the client reads status register 100 from address 2; server 2 returns value 1, interpreted by this example as “all good”. Other servers do not reply. A normal write request receives a protocol response or exception; this does not prove that a motor physically started. Read a separate feedback signal. Serial broadcast to address 0 receives no replies.
Voltage and waveform
Modbus defines a protocol, not a single electrical voltage. RTU often uses RS-485 and the receiver reads the difference between A and B. A loaded RS-485 driver supplies at least about 1.5 V differential; receiver thresholds are around ±0.2 V. Absolute line voltages relative to ground depend on the transceiver and installation. Modbus TCP uses Ethernet.
Measuring with an oscilloscope
Use a suitable differential probe across the RS-485 pair or two safely referenced channels with a mathematical difference. Observe logic states, edges, reflections and noise. Verify termination, polarity and baud rate against device documentation. A multimeter averages the signal and cannot show individual bits.
The principle
Modbus defines messages and function codes. Modbus RTU commonly uses a serial connection such as RS-485; Modbus TCP runs over TCP/IP. A client reads or writes data at a server. The device documentation defines the meaning of registers.
An example
Example: an application reads one holding register with function code 03. The server returns its value. The address and value shown are illustrative.
Important
A reference such as 40001 may differ from the address transmitted on the wire. Check the device register map and byte order.