Hardware Comparison: MicroLogix 1400 Selection Guide

As the de facto standard for industrial programmable logic controllers, Rockwell Automation’s line of Allen-Bradley PLCs covers the full spectrum of application scales, from hobbyist to multi-site systems.  In this article, we’ll examine one of the most popular and cost-effective PLC options in Allen-Bradley’s catalog: the MicroLogix 1400 family.      

MicroLogix Family Overview

The MicroLogix series sits at the micro-scale end of Allen-Bradley’s PLC offerings, just under small-scale options such as the CompactLogix series, and farther yet under large-scale solutions including the flagship ControlLogix series.  These series groupings aren’t meant to infer differences in quality or reliability – these names only refer to the scale of the applications each family is designed to serve.  Where a ControlLogix PLC may control an entire plant, a MicroLogix PLC is a much better fit to control a single machine. 

The Allen-Bradley MicroLogix family of PLCs is based on two paramount features:  compact size and cost-effectiveness.  Here’s a brief breakdown of the product families in the series: 

It’s worth pointing out that Allen-Bradley uses Bulletin Numbers to differentiate their product families, on top of the family name itself.  This can be confusing to engineers first getting into Allen-Bradley documentation, translating between family numbers and bulletin numbers. 

From here, we’ll take a closer look specifically at the MicroLogix 1400 series of PLCs, which is also identified under Bulletin 1766.  This will be helpful to understand when specifying modules or components, as each part number will be preceded by the applicable bulletin number, EG 1766-BA for a replacement battery.  Interestingly, we can see from the bulletin numbers that the MicroLogix 1400 family is the latest product developed in the group, having the highest bulletin number.  The most advanced MicroLogix series’ features are found in the 1400 for this reason. 

MicroLogix 1400 Overview

As the newest family in the MicroLogix series of programmable controllers, the MicroLogix 1400 PLC family offers a great mix of communication protocols, high-speed I/O, expanded application coverage, onboard analog inputs, and modular expandability.  Across all MicroLogix 1400 controllers, we can find the following common features:

• 20 digital inputs
• 12 digital outputs
• Expandable up to seven 1762 I/O modules, for up to 256 digital I/O points
• True online editing
• 10k word user program memory
• 10k word user data memory
• Up to 128k bytes for data logging and 64k bytes for recipe storage 
• 2 serial ports with DF1/DH485/Modbus RTU/DNP3/ASCII protocols
• Built-in Ethernet (supporting Ethernet/IP peer-to-peer messaging, Web Server, and E-mail capabilities)
• Onboard LCD screen

The MicroLogix 1400 controller is extremely powerful for its size and cost.  Advanced features such as high-speed outputs and batch recipe storage allow for more complex levels of automation, pressing the limits of what might be considered single-machine control.  We commonly see these PLCs handle multi-tank batch system, central ingredient distribution, multi-axis machine motion, and even small pilot plant system control, despite their size.  Naturally, memory, I/O, and instruction capacities will limit what applications the MicroLogix 1400 can handle, but the out-of-the-box options are robust enough to warrant strong considering before immediately jumping up to a CompactLogix PLC.   

Standard MicroLogix 1400 Models

Looking at the below chart pulled from Allen-Bradley’s product configurator, we find six different models available in the MicroLogix 1400 family.  This family of PLCs all feature onboard I/O that come with the controller, so our initial part number selections are fairly simple – we pick power supply and I/O voltages, a few I/O details, and we’re done.  

The above bulk text descriptions can sometimes present confusion in truly telling the different options apart, so another way to examine the six models is to look at the catalog number matrix itself.  The below graphic, as well as the other data tables in this article, are pulled from the Allen-Bradley document ‘MicroLogix Programmable Controllers Selection Guide’, publication #1761-SG001D-EN-P.

Now we have two tools above to see our options within the 1400 family.  In effect, our choices are limited to (4) functional areas, with (2) choices each.  We have two selections for input power type (120V AC or 24V DC); two selections for Analog I/O (included or not); two selections for supply power type (120/240V AC or 24V DC); and two selections for output type (standard relay or 24V DC fast FET switch relays).

Often when looking through catalog lists or tables for the MicroLogix 1400 series, you’ll also come across a catalog number 1766-MM1 – Memory module for MicroLogix 1400.  This is an add-on accessory, and not a different variety of the PLC configuration itself, though this number is often presented at the bottom of the PLC selection lists in Allen-Bradley’s documentation.  This module adds the benefit of removable memory to the PLC, providing a way to backup and copy your program and data files.  You can even copy programs between different PLCs using this memory module. 

Application Considerations

We can see from the above information that the MicroLogix 1400 family of PLCs is very standardized, with memory and performance specifications being rather static, and the main variation coming in the form of power and I/O options.  For this reason, our application considerations to differentiate models are primarily around I/O.

The below chart sheds additional light on the I/O configurations available in the 1400 family. 

Have a standard relay control, stop-start type application?  A 1766-L32AWA will likely be all that you need. 

Adding instrument inputs for online process or condition monitoring, such as via a pressure or temperature sensor?  Consider a 1766-L32AWAA. 

Adding even more complex process conditions, such as a high-speed flow meter input and several modulating control valve outputs?  A catalog number 1766-L32BWAA would serve the application well. 

As I/O counts increase beyond the base chassis count, you can add additional Bulletin 1762 I/O expansion modules in either DIN or panel mount styles.

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