Hardware Comparison: Micro800 Selection Guide

Continuing our article series on Programmable Logic Controller selection, we’ll look next at Rockwell Automation’s line of Allen-Bradley Micro800 PLCs.  These controllers are particularly well positioned to tackle basic single-machine control, offering a versatile range of form factor, expansion, and connectivity options.  Despite their modest cost and size, the Micro800 series is packed with advanced features that you’d normally expect only of much larger platforms.   

Micro800 Series Overview

The Micro800 (and sister 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 do not translate into 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 Micro PLC is a much better fit to control a single small machine. 

The Allen-Bradley Micro800 series of PLCs is intended for low-cost, standalone machine control.  Economical in nature, these PLCs are targeted squarely at machine builders.  Below are the five models in the Micro800 family.   

Bulletin #	Family Name	Description
Bulletin 2080-LC10	Micro810	Programmable smart relay, onboard LCD, tiny 12-point form factor
Bulletin 2080-LC20	Micro820	EtherNet/IP enabled expandable PLC, up to 2 plug-in modules
Bulletin 2080-LC30	Micro830	Adds motion control with PTO, up to 5 plug-in modules
Bulletin 2080-LC50	Micro850	Adds high density I/O expansion modules, plus 5 plug-in modules
Bulletin 2080-LC70	Micro870	Multiple expansion options, high-speed counters, 2 axes motion

We’ll note here that Allen-Bradley has formulated a newer Bulletin Number standard for these and other modern products in their portfolio.  This Bulletin number has a consistent series prefix, followed by two digits to correlate to the PLC model number.  In short, all Micro800 PLCs are Bulletin 2080, with models differing by changing the LC## group.  We find that this Bulletin standard is a bit more intuitive to match up to product names than the legacy four-digit number, but it still takes some getting used to when specifying these components. 

Within each family above, you’ll find multiple part numbers representing I/O and capability variations of that controller model.  For example, in the Micro820 family, you have (3) separate catalog numbers to choose from, as shown in the below table. 

Micro800 Family Catalog Numbers
Catalog Number	Inputs	Outputs
120V AC	24V DC/ V AC	Analog 0-10V (shared with 24V DC)	Relay	24V DC SRC	Analog 0-10V
2080-LC20-20QWB(R)	–	12	4	7	–	1
2080-LC20-20QBB(R)	–	12	4	–	7	1
2080-LC20-20AWB(R)	8	4	4	7	–	1

This article is intended to compare the families of PLCs, and will not get into the specific configurations for each available catalog number within each family.  The reader is encouraged to select the family that best matches their application needs, and then further drill down into that family’s standard configurations to arrive at the right PLC catalog number for the project.  

Micro800 Series General Selection  

Let’s next dive into the hardware side of the Micro800 series PLCs, taking a look first at general selection criteria, and then looking further into connectivity attributes that set models apart from others. 

First Details to Know:

• There are 5 models of Micro800 PLCs
• Models vary in base I/O quantity – 10, 12, 16, 20, 24, and 48 point options (1 point = 1 address)
• Power supply options include 120/240V AC, 24V DC, or 12V DC
• Base output options include 24V DC Sinking or Sourcing, or Relay outputs
• Base input options include 110/220V AC, 24V AC/DC, or 12V DC inputs

A word about IO on the Micro800:  the literature will describe “points” as base selections in the catalog number.  This term refers to embedded digital I/O points physically located on the PLC enclosure.  When you specify a particular PLC model, you’ll decide further how you want the onboard points to be allocated, as there is some flexibility in how these points can be split between discreet and analog, input and output types.  In addition to points, you’ll see reference to Plug-In Modules.  These are the next option for expanding I/O, and refer to small clip-in cards that attach to the PLC chassis’ face, adding more I/O points to the base quantity.  Finally, you’ll see separate information regarding Expansion Modules, which are full-depth cards that attach to the side of the PLC, adding even more I/O points to the total.  To summarize, your Micro800 will come with a certain quantity of fixed I/O Points, expandable with Plug-In Modules on the face, and further expandable with Expansion Modules on the side. 

The Micro810 has fixed I/O Points and cannot be further expanded. 

The Micro820 and Micro830 can be expanded using Plug-In Modules only. 

The Micro850 and Micro870 can be expanded using both Plug-In Modules and Expansion Modules. 

Another way to think about these add-on options:  Plug-In Modules expand the PLC without increasing its footprint by snapping onto the face, whereas Expansion Modules do increase the PLC’s footprint by attaching on the side. 

Plug-In Modules and Accessories

From the above section, we learned that Plug-In Modules snap onto the face of the Micro800 PLC and expand I/O points.  This naturally still leaves more questions.  What types of I/O can we use?  What about communication and interface points?  Any other features available?  Let’s see! 

Plug-In and Accessory Modules are categorized into functional groups as follows:

• Digital Input, Output, Relay, and Combination
• Analog Input and Output
• Thermocouple and RTD
• Trimpot Analog
• Memory Backup and High Accuracy RTC
• RS232 / 485 Serial Port 
• DeviceNet
• High Speed Counter
• Micro810 LCD
• Micro810 USB
• Remote LCD (for Micro820 only)
• External Power Supply
• Embedded Serial Port Cables

This impressive list of expansion options covers a wide range of capabilities, opening up substantial connectivity potential for single-machine applications.  In each of the functional groups listed above, you’ll find a variety of selections to fit your exact needs, more than we can cover here.  Check Allen-Bradley’s selection guide documents carefully when looking for the options you need on your project – many plug-in modules only work with specific Micro800 models based on physical, functional, and processing compatibility requirements. 

Expansion Modules and Accessories 

Still need more I/O points for your project than you can achieve with Plug-In Modules?  That’s where Expansion Modules come in.  Micro800 Expansion Modules further increase your connectivity choices in high-density discreet and analog I/O, including thermal sensor RTD and Thermocouple channel types.  It’s worth noting that Expansion Modules are primarily for I/O additions, whereas Plug-In Modules are where you’ll find all of the more advanced communication and accessory options. 

Among the most common options in Micro800 Expansion Modules, you’ll find 32-point 12/24V DC Sink/Source Input, 8-point 120/240V AC Triac Output, 8-channel Voltage/Current Analog Input, 4-channel Voltage/Current Analog Output, and 4-channel RTD and TC Specialty Input types.  You’ll also find a power supply module for powering up to four expansion modules. 

Overall Series Breakdown

The Micro800 PLC series, with its five product families therein, presents more selection options than we can cover in this short article.  Below we’ll present a feature comparison table to consolidate most of the details discussed above, allowing for a more visual review of the differences between families. 

Micro800 Series Feature Breakdown
Attribute	Micro810	Micro820	Micro830	Micro850	Micro870
12-point	20-point	10-point	16-point	24-point	48-point	24-point	48-point	24-point
Communication ports, embedded	USB 2.0 (with USB adapter)	10/100 Base T Ethernet port (RJ-45) RS232/RS485 non-isolated combo serial	USB 2.0 (non-isolated) RS232/RS485 non-isolated combo serial	USB 2.0 (non-isolated) RS232/RS485 non-isolated combo serial 10/100 Base T Ethernet port (RJ-45)
Embedded digital I/O points (1)	12	19	10	16	24	48	24	48	24
Base analog I/O channels	Four 24V DC digital inputs are shared as 0…10V analog inputs (DC input models only)	One 0…10V analog output Four 24V DC digital inputs can be configured as 0…10V analog inputs (DC input models only) and via plug-in modules	Via plug-in modules	Via plug-in modules and expansion I/O
Number of plug-in modules	0	2	2	2	3	5	3	5	3
Maximum digital I/O (2)	12	35	26	32	48	88	132	192	304
Expansion I/O supported	—	—	—	All Expansion Modules (see Catalog)
Types of accessories or plug-ins supported	LCD display with backup memory module   USB Adaptor	Micro800 Remote LCD   All plug-in modules (see Catalog)	All plug-in modules (see Catalog)
Power supply	Embedded 120/240V AC and 12/24V DC options	Base unit has embedded 24V DC power supply, optional external 120/240V AC power supply available
Basic instruction speed	2.5 μs per basic instruction	0.30 μs per basic instruction
Minimum scan/cycle time (3)	<0.25 ms	<4 ms	<0.25 ms

(1) See Number and Types of Inputs/Outputs for Micro800 Catalogs on page 8.

(2) For Micro820 and Micro830 controllers, the number of maximum digital I/O assumes 8-point digital I/O plug-ins (for example, 2080-IQ4OB4) are used on all available plug-in slots. For Micro850 and Micro870 controllers, the maximum number of digital I/O supported includes the base, plug-ins, and expansion I/O.

(3) Including reading and writing I/O, program execution, and communications overhead.

(4) 2080-MEMBAK-RTC is not supported on Micro820 and Micro870 controllers. 2080-MEMBAK-RTC2 is not supported on Micro820 controllers.