We have used quite a number of brands and models of PLC’s. Below is a list of some of them, and comments on what we thought of them.
Keep in mind that the main interface to the PLC is the programming software used to program and debug the PLC application software. All PLC’s look like plastic boxes with screw terminals or connectors to hook the wires to them. They are physically very similar. The programming tools form the bulk of my opinions on PLC’s, because using the programming software is the bulk of implementing a PLC.
Might as well start with the best first. I will preface my comments by saying that I was never a big fan of Allen Bradley’s line of PLC’s, especially the 1771 I/O bus. This was the flagship of the Allen Bradley line, and at one time they practically vowed that they would never turn from it. Well, they finally did, and it was the best thing they ever did. The advent of the RSLogix 5000 series of PLC’s was Allen Bradley’s final good bye to the 1771 platform.
If I were to receive a life sentence of programming PLC’s, I would prefer that it be RSLogix 5000 platform PLC’s.
RSLogix 5000 software is used to program the RSLogix (and “CompactLogix”) platforms. RSLogix 5000, in my opinion, is the best PLC programming software that I have used. That would make any RSLogix 5000 or CompactLogix PLC’s are my favorite PLC’s. There are actually a lot of other things to like about the RSLogix systems. With the RSLogix platform, AB moved to Ethernet/IP in a big way. Ethernet/IP has become widely used by manufacturers of automation components. This allows (relatively) easy connection of many different devices to PLC systems. (Siemens uses Profibus in a similar way that AB uses Ethernet/IP.)
The RSLogix series also implements motion control in a pretty usable way, which is somewhat difficult in a PLC platform. There are a lot of ways to botch motion control in the PLC world, and the RSLogix platform does the job pretty well. (For complex motion control applications, I think motion controllers are usually a better choice, but RSLogix is about as good as it gets in a PLC platform.)
The RSLogix series is the first time that Allen Bradley dropped rigid data tables with numeric addresses in favor of named variables, and user definable data structures. This system is is far superior and more flexible than the numbered data table system used in all systems prior to this platform. Ironically, this feature probably caused more backlash than any other feature. It is a radical departure, and many people were probably uncomfortable with creating named variables and not knowing where they are stored.
RSLogix 5000 also allows the creation of reusable modules of logic. I forget what they call this, but it is a powerful feature that promotes the re-use of code. Code re-use is an area that PLC’s are very weak in, and this is a big step toward creating code re-usability.
The SLC, or “Slick” line of PLC’s was Allen Bradley’s lower level of PLC’s, that originally was the little brother of the 1771 bus PLC’s (the 2, 3, 5 series PLC’s, such as the 2/17, 5/40, etc.) They were a lower cost option that AB needed to maintain market share against the many low cost options that were emerging to the flagship PLC’s that AB offered at that time.
The SLC 500 family also grew to include the “Micrologix” family, which was a minimally expandable line of “brick” style PLC’s (no rack to install I/O cards).
In general, the SLC and Microligix line of PLC’s were easy to work with, and had good programming software support (RSLogix 500 software). RSLogix 500 would be next to the top for me. It was the best thing out there before RSLogix 5000 came around.
There were quirky things about the SLC/MicroLogix world. You had to be very careful to select the proper processor, depending upon what you wanted to connect to the system. This line doesn’t have near the communication flexibility that the RSLogix line does. It was easy to get into situations where the operator interface and PLC could not be connected while the PLC was being debugged, unless you added some expensive port adapter. There were a number of ways to be burned on communications.
RS232 communications with an outside device was tricky, and sometimes required an “ASCII Basic” module. This module used some very quirky BASIC implementation. It was not a ‘delightful’ thing to work with.
Motion was possible, but not enjoyable. There was a servo module (HSRV I think), and a stepper module (HSTP). Both were rather quirky, and generally no fun to use.
For small systems with just digital I/O, the SLC/Micrologix platform was very usable. When things got complicated, the platform required a lot of product knowledge, and sometimes it felt like driving square pegs in round holes to get things done.
The PLC 2 and PLC 5 series used to be the workhorses of the AB line. The 2 is older than the 5. The 5 was sort of the predecessor to the RSLogix 5000. It was the powerhouse of the AB line.
RSLogix 5 is the latest software for the PLC 5 series. I don’t know whether it is possible to buy a PLC 5 any more, but there is no reason to do so.
In it’s day the PLC 5 was the best thing around, and so was the RSLogix 5 software.
On a side note, RSLogix 5 was primarily developed by a third party company (ICOM, I think was the name). Allen Bradley had their own software (6200 software?) which was really bad. They finally got tired of being pounded by ICOM, so they bought the company. The ICOM efforts became the guts of all of the newer AB software, which became best-of-breed, in my view. ICOM probably deserves the credit for putting AB on the map software-wise.
This is probably a strange name for this spot on the list, but Keyence actually makes small PLC’s. We were ‘forced’ to use them on some projects about 2 or 3 years ago. Once we got a copy of their newer version of software (which is not really supported in the US), I started to really like using their PLC’s. The software is quite good, especially for a Japanese PLC. (I pretty much find that across the board the Japanese PLC’s have really bad programming software. This situation may have changed over the last several years, but I haven’t used many Japanese systems lately, other than the Keyence.)
The Keyence system is on the small side (about 40 I/O for their largest ‘brick’), so they are only good for little systems. They have some expansion capacity with add-on modules, but they are not a choice for large systems.
The operator interface choices are more limited for Keyence processors, but there are very adequate screens to connect, if the system needs a screen. (Maple Systems is one manufacturer that will connect.)
Koyo (Automation Direct)
Koyo PLC’s have been marketed in the US by Automation Direct. They advertised very directly against Allen Bradley as a lower cost equivalent. They were approximately half the cost in many situations. Their PLC’s weren’t bad to work with. Their line was a little piecemeal, because they basically purchased products from many manufacturers and patched them together. Their line of operator interfaces changed pretty often, which made it likely that if you repeated a system, you might be reprogramming the operator interface because of a brand or model change. I believe that one of their small PLC’s was made by someone other than Koyo, perhaps Siemens. This meant different programming software for that series. It was all just kind of patched together somehow. It was low cost stuff, but a little quirky to work with. Definitely worth the cost savings in most cases.
Of the other Japanese offerings (Koyo and Mitsu to name two), I liked Omron. Their software was superior to Koyo or Mitsubishi when I used them. I generally don’t like any of the Japanese PLC’s, mostly because of their software. Fanuc had the same problem with their CNC attached PLC’s. The software was terrible. Frightening.
Mitsu had a comprehensive PLC line, but I never had any luck with their software. It seemed to change often, and their sales people were always bragging about how much better the new version was. It seemed to me that they were playing serious catch-up on the software side. I just didn’t like Mitsu stuff.
Fanuc PLC’s were actually GE-Fanuc stuff that originated as the series 90-xx stuff. In my opinion, GE never did a good job of this, and Fanuc didn’t make it any better. Bad software, no real effort to go the next step on the software side.
Fanuc’s PLC implementations in their CNC controls were terrible too. Nothing good to say about it.
I pretty much dodged the Siemens bullet in my career, so I don’t have much to say either way about Siemens PLC’s. From my minimal use of Siemens PLC’s, it is easy to see that rigid German engineering in the products. I worked on Siemens CNC controls many years ago, and became familiar with the (over?)engineering that is found in Siemens products.
I know a few people who have worked on both Siemens and AB RSLogix platforms, and they all prefer the RSLogix to the Siemens. I am not sure that I would have the same opinion, because I like computer programming better than PLC programming. I have a gut feeling that I might have liked the Siemens stuff if I had tried it.