Introducing the Local Slave

Local Slave Networking Example

The Ethernet communication module supports up to three instances of the local slave functionality. The local slave functionality allows other scanners on the network to read from, and write to, the Ethernet communication module using implicit messaging. Each local slave instance can accept one exclusive owner connection and one listen only connection. Through a local slave, a remote scanner can access the communication module’s CIP Assembly object. The local slave function is especially beneficial for peer to peer data exchanges at a repetitive rate.

NOTE:

The Ethernet communication module can provide three local slave adapter instances, while simultaneously performing as a scanner. These roles are not mutually exclusive.
The local slave is exclusively an EtherNet/IP functionality.

The sample configuration, above, includes the following scanners and adapters:

A primary PLC (1) with one local slave instance enabled. The PLC performs the following functions:
- scans I/O data from remote devices (4 and 5)
- scans input data from its own local slave instance
A third party scanner (6)—which lacks adapter capability, and therefore cannot itself be scanned by the primary PLC (1)—performs the following functions:
- collects data from other sources (not part of this network)
- writes data to inputs of the primary PLC’s local slave
- scans the primary PLC’s local slave’s output data via an exclusive owner connection
A secondary scanner (7), which also scans the primary PLC’s local slave, for the very same output data scanned by the third party scanner, via a listen only connection

NOTE:

Because the third party scanner (6) and the secondary scanner (8) both receive the same data produced by the local slave, the requested packet interval (RPI) settings of the third party scanner’s exclusive owner connection are the same as those of the secondary scanner’s listen only connection.
By enabling a local slave on the primary PLC (1):
- PLC (1) allows the third party PLC (6) to write to it at a repetitive rate, even if PLC (6) is not capable of acting as an adapter.
- the secondary PLC (7) is able to scan the primary PLC (1) at a repetitive rate, rather than through application intensive explicit messaging.

The topics in this section show you how to use Control Expert software installed in the PC (2, above) to configure a local slave, and to create input and output items in support of the peer-to-peer data transfers between and among scanners.