Local Public Modbus Slave Device Search Methodology; Alias Device Id And Device Id Offset Functionality - Pepperl+Fuchs ICDM-RX/MOD Manual

Modbus router user

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ICDM-RX/MOD User Manual

Overview

Allows the ICDM-RX/MOD to search for lost devices on all Modbus To-Slaves ports with the Lost

Device Search Enable option turned on.

This will cause timeout delays on all Modbus To-Slaves ports with the Device Search Enable option

turned on until the device is found.

Note: This can be useful for locating devices if a device has been moved onto another port by moving the

serial cable or, perhaps, by moving the device onto a different Modbus To-Slaves serial bus.

One of the most common challenges people face when setting up Modbus systems are the problems caused

by the limited device ID range. The Alias Device ID and Device ID Offset functionality have been developed to

help solve those problems.

The Modbus specification has the following limitations:

•

Requires all public devices attached to gateway to be addressed by a device ID.

•

Allows only 256 device IDs with a range of 0 to 255.

•

Not all device IDs can be used for addressing devices.

Device ID 0 is reserved for broadcast messages

1-247 are for device addressing

248 to 255 are reserved for such things as gateway functions. Depending on your environment, these

device IDs may or may not be available for assignment to devices.

The following are common problems that can occur as a result of the device ID limitations:

•

A gateway must route Modbus messages based on the device ID. Therefore, it cannot route to multiple

Modbus devices with the same device ID.

•

It is not always possible or practical to change the device ID of serial Modbus slave devices.

•

Serial and Ethernet TCP/IP Modbus RTU/ASCII masters with one connection may need to access multiple

devices with the same device ID. Furthermore, these devices may be located locally or remotely.

•

It is not always possible or practical to modify the device IDs on existing Modbus master programs. This is

often true when adding a SCADA system to an existing PLC controlled system.

The Alias Modbus Device ID and Device ID Offset functionality has been developed to solve these problems.

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Troubleshooting