# Modbus Issues

Modbus is a simple Master/Slave protocol originally used for connectivity between Modicon products. Now, almost every vendor of industrial equipment supports a Modbus interface. In fact, it has become something of a de facto industry standard for open system interconnection.

Specific procedures for physical network connection are impossible to provide for this diverse equipment base. The section below describes the most common devices in some detail. Please refer to the Modicon Modbus Protocol Reference Guide, PI-MBUS-300, for further information.

Important restriction regarding use of Modbus protocol:

The Modbus protocol is configured on the Modbus port (NET 1). The serial communication settings of the RTU must match the network or other device exactly. The Catalyst RTU must be the network master.

The Modbus Port default settings are:

Note: A slow Modbus device can respond to a poll after the RTU has timed out and moved on to the next transaction. A late response may be matched up with the current pending transaction. The result is a mix-up of channel data. The solution is to increase the Modbus application timer.

A Second Note: PLCs running with configuration MEM/DEF switch in DEF position seem to have their node addresses equal to the MBPLUS node address. Attempts to poll registers using any other node address give error 602. When switch is in MEM position the configured MBSLAVE port address is active. When writing a '1' to a discrete PLC output (0x refs), firmware is reporting the value as 255.