Multiple EtherNet/IP I/O Connections

An RMC can simultaneously have I/O connections open with up to four EtherNet/IP clients such as PLCs and HMIs. Each connection will share the same I/O data produced by the RMC, but no more than one I/O connection can be set up to send I/O data to be consumed by the RMC. Each simultaneous I/O connection must have the same RPI and input data size, and all connections must use multicast for the RMC’s produced data.

Using multiple I/O connections with an RMC requires that you understand the three types of connections supported by the RMC:

Input/Output
This connection is bi-directional: the originator (PLC or HMI) produces data consumed by the RMC and the target (RMC) produces data that is consumed by the originator. This connection type is also called an Exclusive Owner connection or the controlling connection. No more than one I/O connection can be an Input/Output connection.
Input Only
For this connection type, only the target (RMC) produces data, which is consumed by the originator (PLC or HMI). The originator will only send a heartbeat packet, sometimes at a reduced interval (less frequently than the RPI) used to allow the RMC to identify when the connection is broken.
Listen Only
This connection type is identical to an Input Only connection type, with a single exception: a Listen Only connection can only exist when one of the other I/O connection types has been established. That is, a Listen Only connection cannot be established until an Input/Output or Input Only connection has been established, and conversely when the last non-Listen-Only connection has been closed or has timed out, the Listen Only connection will automatically close as well. This connection type is the least-frequently used type. This connection is not supported by firmware 3.40.x or older.

Configuring Multiple Connections

To establish multiple I/O connections with an RMC, you simply create I/O connections from each originator (PLC or HMI) to the RMC as you would normally. However, you must obey the following rules in order for all of the connections to be successful:

All connections must be set up to use multicast (not unicast) for target-to-originator (T->O) packets.
All connections must use the same RPI value. See the Requested Packet Interval (RPI) section of the Setting up an EtherNet/IP I/O Connection topic for details on the minimum RPI supported based on the number of open connections.
All connections must have the target-to-orginator (T->O) data size set the same.
No more than one of the connections can be an Input/Output connection.
Listen Only connections will not connect until at least one Input/Output or Input Only connection has been established.
Each connection must be set up correctly for its desired connection type, as described in detail in Setting up an EtherNet/IP I/O Connection.

Using Multiple Connections

When performing communications with multiple EtherNet/IP I/O connections, the Input/Output connection (or controlling connection) behaves as normal. The Input Only and Listen Only connections can only view the data from the RMC—they can't write to the RMC. If the Sync Register is being used, then the first register in the Input Data is the Sync register and is of no use for the Input Only devices.