Terminal-C Connector for Compact IO Modules

Terminal-C set: – 1 pcs female connector. – 1 pcs male connector. – 1 pcs 120 Ω resistor. T-Bus DIN-rail connector.

You need only one connector per group of IO modules.
So if you have 1 or 32 IO modules (The typical maximum Modbus device count between repeaters is 32. Supported Modbus RTU serial device count may be limited by manufacturer, cable, and communication speed. Check with the manufacturer for more information), only 1 terminal C is required.

A (DATA +) and B (DATA -) = Modbus RS485 RTU (Remote Terminal Unit) Connection.
Modbus RS-485 is a bidirectional, half-duplex and indicates values using differences in voltage (-7V to +12V).
RS-485 simply stands for Recommended Standard #485, and it’s been around since 1983. It is used for Serial communication.
+ and – = 24VDC Power, from DC power supply.
The blank terminal you can connect to the cable screen (SG – Screen Ground) from the Modbus network if required.

Modbus loop termination: On the last module, the Modbus loop must be closed by connecting a 120 Ω resistor between the A and the B side of the RS-485 loop.

Why 120 ohm is used in RS-485?
In RS485, the twisted pair cable used is defined to have a characteristic impedance of 120 Ohm. By adding a 120-ohm resistor at the end of an RS485 transmission line, the signal will be dampened by the resistor instead of reflected into the bus.

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