ModSnmp Manual v3.07 - 7.1. Defining devices
The Add Device page enables you to define Modbus devices. To add a Modbus device, enter the following data and then click Apply.
Name. Enter a name for the device. This will be used to generate the descriptors for the device and its objects in the MIB file. It must start with a capital letter, contain only letters and digits, and be at most 18 characters long.
Description. Optionally enter a description of the device. This will be made available to the SNMP manager as the first object for the device. It may contain any UTF-8 characters (although some SNMP managers may only display the ASCII subset correctly).
Device Type. Select the Device Type that describes this device.
Interface. Select the Modbus interface to be used to communicate with the device.
Slave ID. Enter the Modbus slave identifier. The Slave ID is what is called the "slave address" in the original Modbus specification, and is called the "unit identifier" in the Modbus/TCP specification. It should be in the range 1 to 255. The serial specification limits the range of slave identifiers to 1 to 247, for no apparent reason. The Modbus/TCP implementation guide suggests that Modbus/TCP devices that can be uniquely identified by their IP address should use slave identifier 255.