NP5 Modbus RTU Online Manual

Connecting Modbus RTU

The following figure shows a reference circuit for connecting the NP5-02 Modbus RTU via RS485.

Note: For the standard assignment of the connections, see Pin assignment.
Figure:

PCI-specific pin assignment for Modbus RTU:

Pin

Name

Description

A13 ADC_ANALOG_2 The slave address is defined via the applied voltage, see Setting slave address and baud rate
A16 SLOT_SPI_MISO Modbus RX Data
A17 SLOT_SPI_SCK Modbus TX Data
A19 COMM_SPI_MOSI Receiver Enable
B2 U_REF_ANALOG Is used as a reference voltage of 3.3 V DC to define the slave address, see Setting slave address and baud rate
B22 COMM_RESET Driver Enable
Tip: If you do not use the analog input ADC_ANALOG_2 to set the address and baud rate, connect pin A13 to GND.

RS-485 line polarization

Note: The controller is not equipped with line polarization and expects the master device to have one.
If the master device on the bus does not have line polarization of its own, a pair of resistors must be attached to the RS-485 balanced cables:
  • A pull-up resistor to a 5V voltage on the RS-485+ (D1) cable
  • A pull-down resistor to earth (GND) on the RS-485- (D0) cable
The value of these resistors must be between 450 ohm and 650 ohm. A 650 ohm resistor permits a higher number of devices on the bus.

In this case, a line polarization must be attached at a location for the entire serial bus. In general, this location should be on the master device or its connection. All other devices then no longer need to implement line polarization.

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