The Controllogix 1756-ENBT Communicates over EtherNet via IP

In PLC programming, connecting to the hardware is among the most challenging steps. In Allen Bradley ControlLogix PLCs, this connection is made via a dedicated EtherNet / IP card (Note: the 1756 en2tr has its own dedicated EtherNet port and no longer needs any additional hardware). Multiple issues have been resolved since the platform was first released, as these cards have been changed and upgraded. Although many plants still use older versions of these modules, it is important to learn how to use them.

Listed in order, most recent was first (preferred) and oldest was last (obsolete):

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

Various other variants exist, but these are the most common on the field.

Our purpose in this article is to explore how to connect a ControlLogix PLC (1756-L61) to these cards and start programming. In order to follow the tutorial you will need to have access to some software and hardware. This system utilizes a ControlLogix chassis 1756-A13, along with a Programmable Logic Controller 1756-L61. Our connection will be established using RSLinx & BOOTP software.

Rotating the switch and setting the 1756-ENBT back to factory defaults

With the Rotary Switches on top, an IP address may be set and a connection may be made via the front port of the 1756-ENBT module. The function of these switches should be understood, however.

If you set the switches between 1 and 254, the IP address of the card will be The value of xxx will be determined by the switches. Using this method, an IP address can be assigned to a private network. Doing it this way has the advantage that personnel who don’t have access to or know how to use the software can easily replace the cards. So if the card fails (which is rare), you can bring it out, swap it out with a new one, and then put the switches back where they were.

In addition, the switches can be set to 888 and the card powered. The card will be reset to factory settings, which will re-enable BOOTP (if it was not already enabled). Following are the steps you need to take:

  • Power down the chassis.
  • You can remove the card now.
  • Switch the switches to “888”.
  • The card should be inserted into a chassis slot.
  • Chassis power.
  • Give the card a few minutes to boot (~2 minutes). When the factory defaults have been restored, a message will be displayed.
  • Put the chassis to sleep.
  • You can remove the card now.
  • Alternate the switch values between 1-254 and 888. Here is an example: 000.
  • Put the card back into the chassis slot.
  • Chassis power.

Using RSLinx to set a static IP address using BOOTP

The card searches for the IP address through software when BOOTP is activated. The MAC address of the card should appear in the window when the user connects an EtherNet cable to the module and launches BOOTP.

Note 1: The Ethernet port to which you are connected should have a dynamic address set up on your PC. However, a static address is not always reliable enough to see BOOTP devices.

Note 2: Only connect to one piece of hardware at a time on BOOTP. It is possible to set up more than one, but it is easy to mistakenly assign IP addresses to the wrong systems.

Writing down the MAC address of the computer you’re working on is an important step. In addition, make sure your device is set and connected correctly. It is only possible to identify the module by its MAC address written on its side, which will be provided to you via BOOTP.