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What is the maximum number of nodes and points allowed while using a Modbus TCP/IP or RTU xdriver on a BCX?

Issue

Definition of number of nodes and points allowed using Modbus TCP/IP or RTU xdriver.

Environment

Modbus Protocol

Cause

Definition of number of nodes and points allowed using Modbus TCP/IP or RTU xdriver according to the modbus protocol.

Resolution

The maximum number of nodes is 254.

The address range for points is 1 through to 65535. The number of addresses is limited by the controller memory. We have had ~6000 points but please note the Modbus is a polled protocol so the more points you have the longer the poll rate.

 

Specific Modbus RTU Details

Our Modbus RTU XDriver supports the node range from 0-255. According to the Modbus specification, the node range is from 0 – 255, with 0 being the master, 1-247 being the slaves, and 248-255 being reserved addresses.

After 32 nodes (1 master + 31 slaves), you may need RS-485 repeaters to ensure proper communications. This depends on a number of different items, including if the RS485 transceivers are full or quarter load.

You can find the Modbus serial standard at the link below for further details.

http://www.modbus.org/docs/Modbus_over_serial_line_V1.pdf

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