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Using the 1761-NET-ENI for Rockwell PLC Redundancy
By Jake Nichelson
Product: IOD-4102 - Ethernet/IP Scanner Driver (Rockwell PLC Interface - Logix, PLC5, SLC505)

The purpose of this technical note is to explain how the Allen Bradley 1761-NET-ENI can be interfaced with the VIM to provide a redundant network path to Allen Bradley PLCs using DF1.


The 1761-NET-ENI is an Ethernet to RS-232 gateway and isolator. Generally speaking, this is a special-case configuration and is a form of Single Chassis Redundancy. The purpose of this configuration is to eliminate a single point of failure in only the network between DeltaV and the Allen Bradley PLC CPU. This is a special-case configuration because:

  • Serial communication is used to achieve network redundancy; and
  • A single PLC CPU is used which introduces a single point of failure between the field IO and the DeltaV Control System. This is a network-only redundancy configuration.

This configuration is recommended when faced with the following conditions:

  1. Network redundancy is required.
  2. CPU redundancy is not required.
  3. Adding and/or using an additional Ethernet module on the chassis is not an option.

This configuration is compatible with ControlLogix, CompactLogix, FlexLogix, MicroLogix, and SLC5/05 controllers. The NET-ENI can be used to communicate to the serial port of the SLC5/03 and SLC5/04 PLCs but cannot be used for redundancy with VIMs. VIM firmware IOD-4102 v3.6.3 or later is required.

Network Architecture

The Network Architecture drawing illustrates how network redundancy is achieved using the 1761-NET-ENI. In each case, the PLC is connected to the Primary PLC Network switch using an Ethernet Port. The following table explains which channels are used to establish this communication.

Allen Bradley PLC Ethernet Module Channel
ControlLogix 1756-ENBT or 1756-EN2T 0
CompactLogix On-board 1
FlexLogix 1788-ENBT 0
SLC5/05 On-board 1
MicroLogix On-board 1

Each 1761-NET-ENI will connect to the Secondary PLC Network switch via Ethernet and will connect to the respective PLC via RS-232. Each CPU has an On-board RS-232 DF1 port (Channel 0) for communication. The 1761-NET-ENI is serially connected to this port.

1761-NET-ENI Configuration

The 1761-NET-ENI must be configured before use. The NET-ENI can be configured using the ENI/ENIW Utility provided by Allen Bradley found here. This document discusses the configuration over RS-232. There are other methods for configuring the NET-ENI that are discussed in detail in Chapter 4 of the 1761-NET-ENI user manual.

The ENI Utility requires an RS-232 serial connection between the computer and the 1761-NET-ENI. The Allen-Bradley 1761-CBL-PM02 cable can be used to achieve this connection. Follow these steps to prepare the local serial port for communications with the NET-ENI:

  1. Use the serial card to connect the NET-ENI directly to the computer
  2. Open Windows Device Manager--this process is different for each version of Windows
  3. Locate the serial COM port--most likely COM1--under Ports
  4. Right-click on the COM port and select "Properties."
  5. Select the Port Settings tab and use the following settings:
    Bits per second: 19200
    Data bits: 8
    Parity: None
    Stop bits: 1
    Flow control: None
  6. Click "OK."
  7. Open the ENI/ENIW Utility
  8. Select the Utility Settings tab and Baud Rate: 19200

Next, use the ENI Utility to modify the NET-ENI IP address and baud settings and save to the NET-ENI.

  1. Select the ENI IP Addr tab and use the following settings:
    ENI Series: D232
    Baud Rate: Auto
    CompactLogix Routing: (unchecked)
    Obtain via BootIP: (unchecked)
    Obtain via DHCP: (unchecked)
    Ethernet Speed/Duplex: Auto Negotiate
  2. Specify a valid IP address, Subnet Mask, and Gateway for the network. Remember that the NET-ENI should be used as the secondary interface. The IP Address should be the same as the primary interface IP address plus 1 to the last octet. Say for example a ControlLogix 1756-ENBT/EN2T has an IP Address of, then the NET-ENI should have an IP Address of
  3. Click Save To: ENI ROM to save the changes to the ENI.
  4. After changes have been saved, connect the ENI to the PLC serial port.
  5. Power cycle the ENI by flipping the External/Cable Switch.

After following these procedures, the ENI will be ready for communication over Ethernet and the serial port will automatically configure the baud settings depending on how the settings are configured on the PLC serial port. The next step is to use RSLinx to locate the ENI and PLC on the network.

RSLogix500/5000 Controller Serial Port Configuration

The serial port of the PLC must be correctly configured before serial communications can be established to the 1761-NET-ENI. The NET-ENI has the ability to automatically detect the Baud Settings as described in the configuration procedure but the Error Detection is hardcoded to CRC. This is a serial setting that needs to match in the PLC. These are the recommended settings for PLS serial ports when using the NET-ENI for VIM to PLC communications.

RSLogix500 Channel 0 Settings

  1. Within RSLogix500, browse to the Channel Configuration: Project->Controller->Channel Configuration.
  2. Double-click Channel Configuration to edit the settings.
  3. Switch to the Chan. 0 – System tab and select the following settings:
    Driver: DF1 Full Duplex
    Baud: 19200
    Parity: NONE
    Stop bits: 1
    Source ID: (Optional)
    Control Line: No Handshaking
    Error Detection: CRC
    All other configurations: Default
  4. Click "OK" to apply the serial configuration settings.
  5. Download the configuration to the controller.

RSLogix5000 Channel 0 Settings

  1. Within RSLogix5000, right-click the Controller folder under the Controller Organizer tree and select Properties.
  2. Select the Serial Port tab in the Controller Properties window.
  3. Use the following Serial Settings:
    Mode: System
    Baud Rate: 19200
    Data Bits: 8
    Parity: None
    Stop Bits: 1
    Control Line: No Handshake
    RTS Send Delay, RTS Off Delay, DCD Wait Delay = Default
  4. Next, select the System Protocol tab.
  5. Use the following settings:
    Protocol: DF1 Point to Point
    Station Address: Optional
    Error Detection: CRC
    NAK Receive Limit, ENQ Transmit Limit, ACK Timeout, Embedded Response, Enable Duplicate Dectection = Default
  6. Click "OK" to apply the serial configuration changes.
  7. Download the configuration to the controller.

At this point, the 1761-NET-ENI and PLC should be able to communicate through a serial connection. The next step is to make the NET-ENI and PLC available in RSLinx.

RSLinx Configuration

The RSLinx setup procedure will be different depending on the PLC in question because older PLCs communicate via Ethernet while newer PLCs communicate via Ethernet/IP. See Table 2 to determine which procedure to follow for recognizing the PLC in RSLinx.

PLC Supported Communication Driver
CompactLogix Ethernet/IP
ControlLogix Ethernet/IP
FlexLogix Ethernet/IP
MicroLogix Ethernet
SLC5/03 Ethernet
SLC5/04 Ethernet
SLC5/05 Ethernet

RSLinx Setup Procedure for Ethernet Devices

This procedure will walk through adding a SLC5/05 to RSLinx using a 1761-NET-ENI.

  1. Within RSLinx, select Communcations->Configure Drivers…
  2. Select Ethernet devices under Available Driver Types and click "Add New button."
  3. Specify a name for the driver and Click "OK."
  4. Add a new station for every NET-ENI/PLC pair. The Station column is used for the Node Address of the PLC and the Host Name column is used for specifying the IP Address of the NET-ENI. The NET-ENI will be using DF1 Full Duplex to communicate to the SLC5/04. The Station Address is arbitrary since the connection between the NET-ENI and the SLC5/04 is Point-to-Point.
  5. Once the Driver has been added, it should appear under the Configured Drivers with a Running Status. If the Status is not “Running,” click "Start." Click "Close" to finish the driver configuration.

The driver should appear in the RSLinx driver tree and the PLC should appear under the driver as shown below. When using the Ethernet driver, the NET-ENI will not be viewable; the PLC will simply appear to have the IP Address of the NET-ENI. The PLC will be ready to use for RSLogix and/or VIM communication once the PLC appears under the driver in RSLinx.

Note: The NET-ENI will actually appear under the Ethernet/IP Driver tree as well, but reaching Ethernet IP devices through that driver will not be possible.

RSLinx Setup Procedure for Ethernet/IP Devices

This procedure will walk through adding a FlexLogix PLC to RSLinx using a 1761-NET-ENI.

  1. Within RSLinx, select Communcations->Configure Drivers...
  2. Select EtherNet/IP Driver under Available Driver Types and click "Add New button."
  3. Specify a name for Driver and click "OK."
  4. An EtherNet/IP Settings window will appear. Select the Browse Local Subnet radial and select the network card that shares the same subnet mask as the NET-ENI. Click "OK."
  5. Verify that the driver is running and click "Close" to finish the driver configuration.

The EtherNet/IP driver should appear in the RSLinx driver tree and the NET-ENI will appear under the driver as shown below. The PLC will be nested under the NET-ENI. At this point the PLC will be ready for using for RSLogix and VIM communication.

At this point, the 1761-NET-ENI and PLC should have valid serial communications and should be reachable through RSLogix via RSLinx. The next step is to add the NET-ENI to the VIMNet Explorer configuration.

VIMNet Explorer Configuration

The next step in the process is to add the redundant PLC to the VIMNet Explorer Configuration. This process configures the VIMs to establish communication to the primary PLC interface and the 1761-NET-ENI.

  1. Commission a redundant VIM pair following the steps described in the VIMNet Explorer help files.
  2. Next, add the redundant device under the port. Right-click the port and select "Add Device."
  3. Provide a description for the device.
  4. Click the Add button.
  5. Enter the IP Address of the primary interface and select "Single Network Card Redundancy Non Switching IP." The secondary interface IP Address is implied to be the primary IP Address + 1 to the last octet. In this example, the primary interface IP Address is so the implied secondary interface IP Address is The NET-ENI will use the secondary IP Address. The Non Switching IP selection indicates that the IP Addresses are static and do not change.
  6. Click "OK" to exit the IP definition settings; the finished device properties should look like the following:
  7. Click "OK" to finalize the device properties.
  8. Upload the configuration to the VIM pair by right-clicking the VIMs and selecting "Upload Configuration to VIM->Both cards."
  9. Click Yes at the prompt to confirm the upload.

At this point, the VIMs are configured to communicate to the PLC via the primary interface and to the secondary interface via the 1761-NET-ENI. The final step is to configure DeltaV to read and write to the PLC.

DeltaV Explorer Configuration

This process will walk through adding the redundant VIMs, the redundant PLC, and using the Special Data to communicate to the 1761-NET-ENI.

  1. After the redundant controllers have been added and commissioned, auto-sense the IO to bring in the redundant VIM cards. If the auto-sense is successful, DeltaV will indicate that “Cards were inserted into slots: 57, 59, 61, 63”.
  2. Right-click the card port and select "Properties."
  3. In the Port Properties under the Port Tab, select "Enabled" and click "OK."
  4. Right-click the Port and select "New Serial Device," provide a description, and click "OK" to confirm. This serial device will represent the redundant PLC.
  5. Right-click the Serial Device and select "New Dataset."
  6. Configure the General, DeltaV, and PLC tabs for reading or writing to the PLC. Note that Device data type will generally be 11 for the PLCs supporting NET-ENI redundancy.
  7. Follow the guidelines for configuring the Special Data tab for DF1 communications in the Ethernet/IP VIM user manual found here: See Chapter 5.0 Configuring DeltaV which contains further detail regarding DeltaV Configuration.
  8. Finally, set Special Data 4 equal to 0 or 1 and Special Data 5 equal to 0 or 8 as needed depending on the PLC. The Special Data 4 = 1 flag is required due to the node addressing for the RSLogix 5000 compatible PLC families and the Special Data 5 = 8 flag is required for NET-ENI redundancy for these PLCs. Please see the following Table for further information:
    PLC Special Data 4 Special Data 5
    CompactLogix 1 8
    ControlLogix 1 8
    FlexLogix 1 8
    MicroLogix 0 0
    SLC5/03 0 0
    SLC5/04 0 0
    SLC5/05 0 0
    In this example a SLC5/05 is being used so Special Data 4 and 5 will both be set to 0’s.

  9. Click "OK" to finish the dataset configuration.
  10. Download the configuration to the controller to finish the DeltaV configuration.

After following all of the steps up to this point, the VIMs should be communicating redundantly to the PLC using the 1761-NET-ENI.

Further Reading

Ethernet/IP VIM User Manual

1761-NET-ENI User Manual

Redundant Field Device with Single Chassis

Please contact MYNAH for any questions at: