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. The supported Modbus RTU serial device count may be limited by the manufacturer, the cable, and the 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 protocol that indicates values using voltage differences (-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 can be connected to the cable screen (SG – Screen Ground) on 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 B sides of the RS-485 loop.

Why is 120 ohms used in RS-485?
In RS485, the twisted-pair cable is specified 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 attenuated by the resistor rather than reflected back onto the bus.