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Wednesday, July 18, 2018
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BC7300 Modbus Configuration
Configuring Beckhoff Modbus RTU


My name is Taho. I have a BC7300 and 1 module KL3214 + end bus here. I'm trying to read data via Modbus from my laptop to BC using Modbus scan software. I had some issues with handshaking signal, which address of slave (BC7300) I should assign? Which modbus address of KL3214 i can read? Currently, RX led on BC7300 is ON, TX is OFF, do I need to program for BC7300 to send data via fieldbus?

Thank you so much, I'm looking forward to hear from you.
Best regards,

Hello Taho. Welcome to

Modbus is a master/slave protocol. A Modbus slave is silent, unless spoken to, at which point it responds with a reply message. A Modbus master always initiates communications. Unless the master is actively 'polling' a slave, the serial communications indicating LEDs will not show activity.

Your Modbus scan PC program is most likely a Modbus Master.

The master must be configured to:

- poll or query a specific slave (node) address

- use a Modbus Function Code to read data

- specify the register address where the data resides

- specify an address range or the number of bytes if the Function Code is capable reading multiple registers

On the Slave device, you need to know:

- that the device actually has Modbus slave functionality. Modbus is rarely referred to as a Fieldbus. Having either an RS-232 or RD-485 port does not mean that the device supports Modbus communications. If the documentation for the device does not mention Modbus, then it doesn't have Modbus functionality.

- which serial port Modbus uses

- which data is in which register on the slave device. For instrumentation, the manufacturer publishes a table or map of which data is where. Typically on a PLC, the PLC programmer moves data into specific registers.

- how to configure the Modbus slave address for the device

The serial settings need to be identical on both the master and the slave.

I can not help you on Beckhoff specifics, it's outside my experience.

Hi David,

It's glad to hear from you. Thanks for your experience. My device have beening handshaking to PC via Modbus Scan software at the moment. But I have some doubt with Modbus address. I will check gain.

Once again, thank you so much.

best regards,