Comtrol Modbus TCP User Guide - Manual (Page 75)

DeviceMaster UP Modbus/TCP User Guide: 2000447 Rev. I Chapter 5. Alias Device ID Functionality - 75
Chapter 5. Alias Device ID Functionality
5.1. Overview
One of the most common challenges people face when setting up Modbus systems are problems caused by the
limited device ID range. The Alias Device ID functionality has been developed to help solve those problems.
The Modbus specification has the following limitations:
• Requires all devices attached to the 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. The Modbus/TCP firmware reserves
device ID 254 for Ethernet TCP/IP raw/ASCII devices and 255 for serial raw/ASCII 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 not always possible or practical to change the device ID of serial Modbus slave devices.
• 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.
• A Modbus master with one connection, such as serial PLC, requires connectivity to multiple Modbus slave
devices with the same device ID and one or more of the slave devices are connected remotely to different
gateways.
Note: The Modbus Router firmware has been designed to provide network-wide Modbus connectivity for serial
Modbus masters.
The Alias Modbus Device ID functionality allows modification of device IDs only when messages are received
from Modbus masters. When configured, a Modbus message from a master with the specified device ID is
converted to the alias device ID, the message is then routed internally using the alias device ID. All responses
are returned to the master with the original received message device ID.
The following table demonstrates several device ID aliasing examples:
Received
Device ID
Alias
Device ID
Routed Message
Device ID
Description
1 10 10
Convert messages with received device ID 1 to
10. Route message with device ID 10.
50 5 5
Convert messages with received device ID 50 to
5. Route message with device ID 5.
100 254 254
Convert messages with received device ID 100
to 254. Route message with device ID 254.
10 10 10
Invalid configuration attempt. No change to
device ID is performed.