Programmable Logic Controllers

An Introduction to RSLogix5000 Tags

Tags are the method for assigning and referencing memory locations in Allen Bradley Logix5000 controllers. No longer are there any physical addresses such as N7:0 or F8:7 which use symbols to describe them. These have been replaced with tags which are a pure text based addressing scheme. This is a departure from the more conventional ways of programming PLC’s, which includes Allen Bradley’s earlier line of PLC5 and SLC 500 controllers.

Features in RSLogix5000 Release 16

RSLogix5000 Version 16 Screen Shot


Rockwell Automation unveiled some significant additions to the Logix Control Platform at the Automation Fair event in Baltimore on Oct. 25, 2006. Further additions and improvements were detailed in a presentation on RSLogix5000 Version 16 as presented below.

The Making of an Off Timer

A staple of any PLC instruction set is the timer. The most common type is the "On Timer" while its brother the "Off Timer" suffers from misunderstanding and neglect. There are quite a few PLC manufacturers (you know who you are) who simply do not offer an off timer instruction. This is a shame because they are so handy for keeping something else on (i.e. a fan or light) for a given period after something has turned off.

A Tale of Programming Two PLCs

Developing and managing several different PLCs and HMIs can be a real burden on a PC or laptop.  The problems include:

  • The different software packages can cause conflicts with each other.  Communication problems are a common example
  • Memory gets loaded up with automatically started background services even if you're not using the programs.  This kind of bloat makes everything slower.
  • Some packages don't uninstall well and leave behind a lot of files.
  • Older legacy software may only run on older operating systems.

To remedy the situation there are some options:

A Siemens S7-300 Trainer

PLC demonstration units are an essential tool for maintenance and engineering.

An Allen Bradley SLC-500 Trainer

Here's a good example of our PLC simulators used on an Allen Bradley SLC-500 trainer.  I built this unit for a customer that does automation training.  This concept is easily adaptable so if you're interested in something like this then feel free to contact me.

Modbus FAQ


1. What is MODBUS?

MODBUS is a commonly used industrial communications protocol. It allows the exchange of data between PLCs and computers. It was originally designed for Modicon (Schneider Electric) PLCs but has become widely used by many PLC manufacturers and industrial networks.

2. Why would I use MODBUS?

MODBUS is a common means of gathering data from many different sources for viewing operations, archiving and troubleshooting from a central remote location. It is widely used and a fairly simple protocol. Depending on the application a newer protocol may have more advantage.

DirectSOFT 5 Tips

Automation Direct's DirectSOFT5 programming software has a Tip of the Day feature. There's some good tips in there so I've reproduced them here for easier reading.

  • The Online Status Bar is now an integrated part of the main Status Bar on the bottom of the application.

Schneider Electric Modicon History

This text from Schneider's website was originally on at but has since been removed.  We saved it from Google's cache for our reference and your reading pleasure. 

Modicon history

Modicon created the first programmable logic controller (PLC) in the United States in 1968. The device’s success gave rise to a global industry that has expanded considerably over the past 30 years. PLCs put intelligence in machines and automated processes used in industry, infrastructure and buildings.

Cheap PLCs

Money can be pretty tight if you are on your own and wanting to learn PLCs for the first time or maybe you are just starting out in a business.  One of the most common reactions to buying a PLC for the first time is sticker shock. Yell  Just to purchase one of the most popular PLCs like Allen Bradley can run into the thousands of dollars that you don't have.  Well nothing is for free but if you're willing to cut some corners let's discuss the options for getting a PLC for education or testing that will not break the bank.