#303 - Modbus RS485 tips

- Wiring:

Sensor 24V supply, Sensor 0V Ground, RS485 (+), RS485 (-).

PLC Modbus Comms card: connect the M input pin to the Ground, A to RS485 (-) and B to RS485 (+).

- CAS Modbus Scanner: Scan the network to discover the sensor and identify its communication parameters (address, baud rate, parity, etc).

- Siemens hardware configuration (after upgrading PLC and CM card to latest firmware available):

Operating mode: Half duplex (RS485)

Receive line initial state: none

Protocol: Freeport/Modbus

Port parameters: 19200 / even parity (sensor dependant but it may be different than what is found on the CAS Scanner) / data bits (8 bits per char), Stop bits (1).

IO addresses: ensure the block is called on the correct Organization block (OB1, OB30, etc) and the correct Process Image is assigned (see the properties of the OB to confirm the PIP number (General tab)).

Configuration of message transferred / received: some sensors may expect a specific char (e.g., 16#0D (CR), 16#0A (LF)).

- Siemens blocks:

Modbus_Comm_Load:

Modbus_Master (Mode 1 Write, Mode 0 Read):

Ensure these parameters are being passed and configured correctly: