Modbus RTU Interface

The Modbus RTU interface offers the following features:

Supported over RS-485 physical interface.
Can function as either a Modbus/RTU Master or a Modbus/RTU Extension.
Supported Function Codes: #01, #02, #03, #04, #05, #06, #07, #0F, #10.
Maximum of 32 devices on the bus (1 master and 31 extensions).
RS-485 interface is not isolated.

Main RS-485 characteristics

Mode of operation: Differential.
Voltage at any bus terminal: -7 V to +12 V.
Receiver Input Sensitivity: +/-200 mV.
Sink/Source current: 60 mA.
Built-in asymmetrical protection against transient voltages (ESD, EFT, and lightning).
Non-isolated RS-485 interface.

Connection Types:	Point-to-point connections. Point-to-multipoint connections.
Trunk Cable Type:	Shielded cable with 1 twisted pair and at least a third conductor.
Maximum Bus Length:	1,000 m (3,280 ft) at 19,200 bit/s with the Telemecanique TSX CSA cable.
Maximum Devices (without repeater):	32 devices (1 master and 31 extensions).
Maximum Tap Link Length:	20 m (65 ft) for one tap link. Total of 40 m (131 ft) for all tap links on the bus.

Common ground wires

To maintain voltage within the allowed range (-7 V to +12 V) between drivers and receivers:

An additional third wire (in 2-wire systems) serves as a common circuit.
Connect this wire directly to protective ground at one point for the entire bus.
Choose the master device or its tap as the grounding point.

NOTE:

No terminal for cable shield.
In harsh environments, consider placing an additional shield clamp near the controller to drain EMC disturbances.
Earthed connectors from USB, RS232, LAN, and Modbus should be interconnected cautiously to avoid earth leakage currents affecting controller operation.

Maximum devices without repeater

An RS-485 network can handle up to 32 devices. If you need more, integrate a repeater.

Biasing the network

When there is no data activity on the Modbus bus – meaning all nodes are in receive mode and no active driver is available – the state of the line becomes uncertain. In such cases, external noise or interference can affect the line. To prevent receivers from adopting improper states, it is essential to bias the line. This involves maintaining a constant state by using an external pair of resistors connected to the RS-485 balanced pair.

RC termination

Use RC termination to minimize the loop current and the line reflections.
Choose 1 nF (10 V minimum) serial capacitors and 120 Ω (0.25 W) resistors as line termination.
Integrate these components at both ends of your Modbus SL communication line.

R termination only

If R = 150 Ohm termination is preferred (not RC):

Connect external polarization resistors (450 – 650 Ohm) at the master tap.

Modbus interface isolation

The controller Modbus interface is not isolated. Consider using optic-coupler separation for increased robustness and reduced noise in the Modbus network.

Termination, polarization, and separation using Schneider Electric devices

Schneider Electric provides devices for RS-485 termination, polarization, and separation. You can refer to the following product numbers:

TWD XCA ISO
TWD XCA T3J

These devices are designed to ensure the recommended RS-485 connection scheme (see the picture below). For more detailed information about TWD XCA ISO and TWD XCA T3J, please consult the product documentation on the Schneider Electric website.

Legend:

Master
Extension 1
Extension n
Polarization resistors
Line termination
Shield

Maximum cable length without termination

When a cable is not terminated, you need to adjust the transmission rate based on the cable length. Here are the relevant details:

Transmission rate:	9600	19,200	57,600	115,200
Bit time (μs):	104.1	52.08	17.36	8.68
Bit time/4 (μs):	26.0	13.0	4.3	2.2
Max. cable length without termination (meters):	859	430	143	72

Modbus TCP Interface

Supported over Ethernet interfaces (10Mb, 100Mb)
Can function as a Modbus/TCP-IP Client or Modbus/TCP-IP Server
Supports DHCP
Maximum opened TCP connections: 100
Supported Function Codes: #01, #02, #03, #04, #05, #06, #07, #0F, #10

Grounding-isolation

The metal cover of the RJ45 socket is connected to the device ground.