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1756-ENBT - ControlLogix EtherNet IP Communication Allen Bradley PLC 1756-EN2T 1756-EN3T Programming

Vladimir Romanov
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One of the most challenging first steps in PLC programming is connecting to the hardware. Within the ControlLogix family of Allen Bradley PLCs, this connection is established through a dedicated EtherNet / IP card (Note: the 1756-L8x PLCs have dedicated EtherNet ports and no longer require a separate module). These cards have been changed and upgraded since the initial release of the platform and multiple issues have been resolved. However, it’s still important to know how to work with them as many plants employ older versions of these modules.

The cards are as follows; from the most recent (preferred) to the oldest (obsolete):

  • 1756-EN3T(R)
  • 1756-EN2T(R)
  • 1756-ENET
  • 1756-ENBT

There are several other variations, but those would be the most utilized cards in the field.

In this article, we will explore what it takes to connect through these cards to a ControlLogix PLC (1756-L61) and start programming. You’ll need to have access through a few pieces of software and hardware to follow the tutorial. We will be using the 1756-ENBT, a ControlLogix chassis 1756-A13 as well as a Programmable Logic Controller 1756-L61. In terms of software, we will be using RSLinx & BOOTP to establish the connection.

Rotary Switches & Resetting a 1756-ENBT Module to Factory Default

Through the use of the Rotary Switches on the top of the 1756-ENBT module, it’s possible to set an IP address and connect through the front port. However, it’s important to understand the functionality of these switches.

1756-ENBT - ControlLogix EtherNet IP Communication Allen Bradley PLC 1756-EN2T 1756-EN3T Programming

By setting the switches to a value between 1 and 254, you’re setting the IP address of the card to be where xxx is the value set by the switches. This method is the preferred way to set an IP onto the private network. The advantage of doing it this way is that the card can be easily replaced by personnel who don’t know how to use or don’t have access to the software. In other words, if the card ever fails (which is rare), you can take it out, set the switches to the same value on the new card and replace the old one.

The second option is to set the switches to 888 and to power the card. By doing so, the card will be rest to the factory setting which re-enables BOOTP (if it wasn’t already in that state). It’s important to note that you should do the following:

1756-ENBT - ControlLogix EtherNet IP Communication Allen Bradley PLC 1756-EN2T 1756-EN3T Programming
  1. Power down the chassis.
  2. Remove the card from the slot.
  3. Set the switches to “888”.
  4. Put the card into a slot on the chassis.
  5. Power on the chassis.
  6. Allow the card to boot (~2 minutes). A message will be displayed that indicates the reset to factory default.
  7. Power down the chassis.
  8. Remove the card from the slot.
  9. Set the switches to any value outside of 1-254 and 888. Example: 000.
  10. Put the card back into the slot on the chassis.
  11. Power on the chassis.
1756-ENBT - ControlLogix EtherNet IP Communication Allen Bradley PLC 1756-EN2T 1756-EN3T Programming

Setting an IP Address over BOOTP and Connecting Through RSLinx

When BOOTP is activated, the card is looking for the user to set an IP address through the software. By connecting an EtherNet cable to the module and launching BOOTP, the user should start seeing the MAC address of the card within the window.

Note 1: Set your PC to have a dynamic address on the EtherNet port to which you’re connected. In most cases, it’s still possible to see BOOTP devices with a static address, but it isn’t as reliable.

Note 2: Connect to a single piece of hardware on BOOTP at one time. It’s possible to configure multiple, but it’s easy to miss-assign an IP address on a different device.

Note 3: Write down the MAC address of the device you’re working with. Once again, it’s important to connect and set the right device. The only identifier you have is the MAC address that’s written on the side of the module and will be presented to you through BOOTP.

1756-ENBT - ControlLogix EtherNet IP Communication Allen Bradley PLC 1756-EN2T 1756-EN3T Programming BOOTP

The BOOTP window above displays two devices. One of type DHCP and the other of type BOOTP. The DHCP device, in case you’re curious, is the EtherNet adapter through which the laptop is connected. The BOOTP device is out ControlLogix EtherNet card.

By double-clicking the device of type BOOTP, the user is prompted for an IP address. Enter an IP address you wish to use for the card and press “OK”. The address you’ve selected will be used to connect through RSLinx. It’s important to note that you may change the IP through RSLinx going forward; there’s no need to go through the process above every time.

Set the IP address of your computer to match the subnet of the card. In other words, if you’ve set the IP and subnet mask of the card to be /, you’ll need to give your PC and address of with a subnet mask of If this step is skipped, you won’t find the device in RSLinx; a common problem encountered by the users.

Launch RSLinx.

Go to “Edit Drivers” and create a new “EtherNet IP” driver. Specify the local network to be the one browsed. Lastly, make sure that the same card as the one we’ve configured is selected. This is extremely important as any other EtherNet card of the PC isn’t connected to the device. Note that in our case (video), multiple adapters are shown due to Virtual Machines being installed on the laptop used. This may be confusing for many users.


At this point, the new driver should have been added to RSLinx. By opening the driver, the PC will be able to browse all the nodes on the subnet and find the card we’ve configured.


Connecting to the PLC or 1756-ENBT over USB

By connecting over USB on newer cards, the user may skip the steps above and set an IP address directly. The process if much simpler and takes a straight connection to the card and the use of the USB driver in RSLinx. Once the connection is established, the user can go into the properties of the device or the other modules within the chassis and set the required parameters.


The most important step in getting started with PLC is communication with the device. Within the ControlLogix family, the most common means to connect at the 1756-ENxx modules. They provide an EtherNet / IP access point to a ControlLogix rack and serve as the primary means of connecting to every PLC before the 1756-L8x family. The challenge lies in the user’s ability to configure and work with the 1756-ENBT cards as older versions aren’t as intuitive as some of the newer ones arnd are still dominant at most manufacturing facilities.

The cards are shipped with BOOTP enabled and allow the user to specify an IP address by connecting to them over EtherNet. However, once the card is set, the BOOTP option is disabled to keep a single IP. By resetting the card to factory settings or by accessing it through RSLinx, it’s possible to set a new address. This can be accomplished through the use of the dip switches on the card.

A rather simple alternative to the steps above would be to use a USB connection to the cards that support it.

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