In this article, we will discuss different modules of Allen Bradley PLC ControlLogix hardware along with their properties and functionality. Allen Bradley PLC ControlLogix 1756 is one of the most famous PLCs used in plants.
Allen Bradley PLC ControlLogix 1756 Hardware
The hardware of Allen Bradley PLC ControlLogix 1756 consists of Chassis, Power Supply module, Controller or CPU, IO modules, Redundancy module, ControlNet module, and Ethernet module.
Let us see all these modules in detail.
1. Chassis
The Allen Bradley PLC ControlLogix system is a module-type system. So it requires Chassis for installation. The chassis used for ControlLogix is only a horizontal back panel mounting type as shown below. The leftmost-sided slot is dedicated to the power supply module.
Other modules can be placed in any slot. The size of the slot for the power supply module is also quite big because the power supply module is itself big in size. The chassis has a backplane communication as well as a power distribution facility. The backplane of the chassis provides a high-speed communication path between the modules installed and also provides power.
Different chassis along with their model numbers available are;
| Model Number | Slots |
| 1756-A4 | 4 Slot Chassis for ControlLogix |
| 1756-A7 | 7 Slot Chassis for ControlLogix |
| 1756-A10 | 10 Slot Chassis for ControlLogix |
| 1756-A13 | 13 Slot Chassis for ControlLogix |
| 1756-A17 | 17 slot Chassis for ControlLogix |
2. Allen Bradley PLC ControlLogix Hardware –Power Supply Module
As the name suggests the power supply module is used to power the ControlLogix chassis which has different modules installed. The power supply module has a dedicated location in the chassis as discussed in the chassis section.
The different models of power supply modules available are;
| Model Number | Ratings |
| 1756-PA72 | 85-265 VAC Power Supply Module (10 Amp @ 5V) |
| 1756-PA75 | 85-265 VAC Power Supply Module (13 Amp @ 5V) |
| 1756-PB72 | 19-32 VDC Power Supply Module (10 Amp @ 5V) |
| 1756-PB75 | 19-32 VDC Power Supply Module (13 Amp @ 5V) |
Depending upon the requirement of the modules installed in the chassis, an appropriate type of power supply module is chosen.
3. Allen Bradley PLC ControlLogix Hardware – Controller or CPU
The controller or CPU is the brain of the PLC system. 1756-L71 and 1756-L72 are the most commonly used controller for Allen Bradley PLC ControlLogix 1756 series. The 1756-L71 controller has a user memory of 2 MB and IO memory of 0.98 MB. While the 1756-L72 controller has a user memory of 4 MB and IO memory of 0.98 MB. The controller has a dedicated USB port and a battery for program backup also. The controller also has a key slot. The key is used to change the modes of the PLC. 1 USB port is provided for programming, configuration, online edits, and firmware updates.
Other versions of the controller are 1756-L61, 1756-L62, and many more.
4. IO modules
Allen Bradley PLC ControlLogix 1756 series has two types of Input-Output modules. It can be either a Removable Terminal Block (RTB) or an Interface module (IFM). RTB and IFM are just doing the job of a terminal block to connect the field Input Outputs to the Input-Output Cards. Different model numbers of Input-Output cards are 1756-IF8, 1756-IF16, 1756-OF8, 1756-OF16, 1756-IB8, 1756-IB16, 1756-OB8, and 1756-OB16. Now, these all words and numbers seem confusing. It is easy to identify the Analog Input, Analog Output, Digital Input, and Digital Output cards of these model numbers by knowing the text written before B16.
1756-*B16 in place of * you can find I and O. The letter I is used for the Input card and the letter O is used for the Output card.
1756-I*16 in place of * you can find F and B. The letter F is used for the analog card and the letter B is used for the digital card.
1756-IB** in place of * here, you can find numbers. These numbers indicate the number of channels used in that card.
So, a card having model number 1756-IF16 is a 16-channel analog input card. While 1756-OB8 is a card with 8 digital outputs.
5. Redundancy Module
Allen Bradley PLC ControlLogix 1756 series is used in a redundant configuration. So for establishing the redundancy, a redundancy module is needed. 1756-RM is the redundancy module used to establish the controller redundancy. The redundancy module establishes the redundancy for two controllers installed in identical racks having the same components and configuration. The redundancy modules also provide system redundancy monitoring as well module health monitoring through LED on it which are PRI, COM, and OK.
6. ControlNet Module
To connect the PLC system to the network as well as Remote Input Output modules, a ControlNet module is used. 1756-CNB is a non-redundant ControlNet module. 1756-CNBR is a redundant ControlNet module. ControlNet modules have a special BNC connector for connecting the PLC to the network as well as Remote Input Output modules. ControlNet module also has a USB port provided for temporary local programming purposes.
7. Ethernet Module
Allen Bradley PLC ControlLogix 1756 series uses an Ethernet module for communication. 1756-EN2TR is the model number used for communication. It is an EtherNet/IP (Ethernet Industrial Protocol) with a dual-port interface communication module. The Ethernet module supports real-time I/O messaging, general messaging, and information exchange for applications like Real-Time Control, Time Synchronization, and motion control diagnostics information. The 1756-EN2TR Module has 10 – 100 Mbps EtherNet/IP communication rates and uses dual copper.