Help choosing a PLC brand


Thread Starter

Jeff Mitchell

We design, build and install fairly small (5-300gpm) water treatment plants. We have used the GE brick style Versamax PLC and we have been a bit unhappy with the hardware, software and technical support. We are currently looking for a new PLC for our panels. We only require about 16 to 60 i/o (and sometimes a couple of analog) depending on the job. The programs are simple timing, motor controls, valves, panel lights... etc. We also need simple HMI for the plant operator to input time/date/ timing values. Remote connection via modem is also important for program edits. We are looking at Schneider Electric's Twido PLC. Will this thing work for us?? I am not a master PLC guy and need some advice with this before we invest a lot of time looking at this PLC and its' software. Is there something out there that would fit us better?? Thanks for being a nerd when I needed it.

Jeff Mitchell (wonna-be nerd)
Have you looked at the Allen Bradley Micrologix 1500? It is compact, expandable and easy to program. I work with AB PLC's exclusively so I am little biased. You can check out their entire line of PLC's on their website

Ron Beaufort

Greetings Jeff,
you said that you “design, build, and install” these systems ... question: do you also maintain them? ... or is this the customer’s responsibility? ... if this particular part of the puzzle eventually ends up in the customer’s lap, then I suggest that you consult some of them as to which brand (and model) of PLC they’d like to see installed ...

major idea: if the customer’s maintenance crew is already familiar with (just for example) Allen-Bradley ... and if they already have the Allen-Bradley software, communications cables, etc. on hand ... then your efforts to suddenly slip another brand of PLC into their sheltered little world would probably NOT go over too well ...

look at it this way ... if handled correctly, your attempts to accommodate your customer’s best interests might be seen as a valuable marketing tool ...

on the other hand, if handled incorrectly, your attempts to simply cut the bottom line might leave your customer “less-than-impressed” with the project’s final outcome ...

good luck,
Dear Mr. Jeff,

This is rajan from saudi arabia, what you decided is correct for your application. TWIDo plc is good, software handling like program developing is very easy compared to other PLCs. you may choose either TWIDo OR MICRO PLC and HMI use MAGELIS schneider product. Last 5 years i am in PLC programing side, i have worked with GE, AB and Modicon, comparing to others modicon product is easy of understanding, for beginners this is very good.

If you want any help regarding this be frank you contact me, i will help you. Because Really i am proud to work with Schneider PLCs.

Project Engineer(I&C)
Kingdom Of Saudi Arabia

Try WAGO. They offer a very flexible PLC, that is compact, modular, and power (as powerful as a mid-range AB Control Logix platform). Cost for PLC with integrated Ethernet communication is in the range of $386.00 - $495.00, software (fully compliant with IEC 61131-3 programming language) with "Free" HMI/visualization is $600.00 site license and no hardware keys. Digital I/O can be added in increments of 2, 4, or 8 Channels (buy only the I/O you need - no more 8, 16, or 32) at a cost around $8.00 per channel. Analog is also available as well as special functions such as serial modules for connection to an event printer or recorder, etc. Another point of interest is that the I/O cards use spring pressure connection technology. This is ideal for water and waste water environments due to the spring being made of stainless steel (corrosion resistant) and the fact that the dynamic clamping action is vibration proof, temperature cycling resistant, and makes for a gas-tight connection. Oh yeah, tech support is "Free". Call 1-800-346-7245 or visit us at
I have worked with Omron, GE & AB. I think that the AB Micrologix family is nice cheap and easy to program. You can buy a Micrologix 1000 for as little as $200.00 1500s are higher but offer expandability. Micrologics processors use Rslogix 500 software to program. This software is high. $1500

I think the reason that I like AB is because of the support I get from my local supplier.

Steve Myres, PE

Consider the DL06 by Automation Base I/O is 20 in, 16 out with 4 expansion slots. An expansion card is max of 4 analog or 16 discrete. So max expansion is 100 discrete or less with analog. The PLC is cheaper than the Micro 1500, the programming software is very good, and if each unit is custom (software must be debugged on running equipment) this PLC will be advantageous because unlike the A-B Micro 1500, it can do online edits. I cannot overstate how much of an advantage this is for one-off code. For a machine where the code is debugged once and the same (at least from a controls perspective) machine is built over and over, it won't matter.

Amitabh Mohan

Would recommend Micrologix 1500 due to your communication needs. Another good option is OMRON CPM1 series PLC.

Keith Dungey

Mitsubishi also manufacture a good range of small PLC's but I would also place my bet on Allen-Bradley Micrologix. We have used AB for many years on arduous duty applications and have found them to be totally reliable (built like a brick outhouse) with good backup. For the HMI, use CITECT which is easy to learn and use.

Michael Griffin

You might wish to look at the Siemens S7-200 series. I believe that one of the target markets for this product line is your type of application.
Changing the subject a little bit...

I used the S7-200 in several different projects and plants, and I consider it a superb miniPLC... But I always have excessive noise in its analog inputs. I tried the usual (connecting the unused inputs, better grounding, shielding, filtering - soft&hard) with relative small success. This kind of problem is unusual (at least in such magnitude) with other PLCs. Even the other Siemens models are very good.



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Michael Griffin

In reply to Marcos: I haven't use the analogue I/O on the S7-200 line for anything, although I have used analogue I/O on the S7-300 and especially on the S5. I believe the *accuracy* specs for the analogue on the S7-200 are not as good as for the other models, although I don't know if this has been improved lately.

There are sections in the S7-200 manual dealing with analogue accuracy and stability and ways of improving them. If you don't need high speed response, then there is programmable analogue filtering in the PLC configuration which is separately configurable for each input. I believe that these are actually software averaging and deadband functions which operate at the firmware level. Have you tried this?

The built in analogue on the S5-95U was 10 bit. This is equivalent to 0.1% of full scale which was quite acceptable resolution for most applications that I needed. The S7-200 analogue I/O cards are 12 bit, but better resolution doesn't automatically mean better accuracy.

The data format is "left justified" in the analogue word, meaning that the lower 3 or 4 bits are not actual data. This tends to magnify the apparent signal jitter by a factor of 8 or 16 although this does not reflect real changes in the signal (the lower bits are just padding).

I suppose from the above, that the following might be advisable.

1) Turn on the software filtering in the PLC configuration and set it to some reasonable value.

2) Right shift the raw analogue reading to get rid of the bit padding on the least significant bits (the amount of padding depends upon the data format).

3) You might consider dropping some of the actual data bits as well, reducing the resolution to 10 bits. It is very difficult to get 0.1% of genuine accuracy with any typical instrument system using a PLC, so you may wish to
question whether your application needs better than 0.1% resolution either.

marc sinclair


I use lots of these units, and don't find that the analogue inputs cause a problem. Some other micro PLCs only give 8 or 9 bit resolution so the lower bits on the
s7-200 may seem a little sensitive. Have you tried filtering in software? How about avoiding analogue altogether. I have used temperature to frequency conversion to avoid thermocouples, voltage to frequency converters for light level sensors, and have used pulse output to control Inverters with great success before USS. The distance sensors I used to use were 4-20mA, but are now ASI digital devices. What is it you are using the analogue for? Load cells? I would be interested to hear.

Marc Sinclair

Why do you want to ditch the VersaMax??? What software program are you using to program these, VersaPro??? What temperatures do your panels see... any below freezing or are they environmentally protected???

I have used a lot of different PLCs and they all have their +/-. Allen Bradley has been the best marketed PLC for quite some time. They are good, and verge on great, but not better. They are also extremely expensive to maintain in the SOFTWARE realm. Marketing is evidently not free.

The other replies have suggested a number of solid alternatives, Siemens, AB, Wago, etc. to your current situation.

I do think GE has a solid product for your application and pricepoint. I would probably ditch the Brick Nano/Micro PLCs and move to the VersaMax 'standard'. It would depend on your price point and investment. For low I/O count the Bricks in the 23 and 28 point count offer two communication ports and would be hard to beat in terms of Dollars. The two port are critical if you want to have a way to have a small input HMI and a dialup modem. You need two ports.

The 'standard' Versamax CPUs comes with two serial ports. One RS232 and the other RS485. One could mix and match these ports to get the cheapest combination for adding a modem connection and an HMI screen. Both Micros/Standard Versamax product do MODBUS RTU SLAVE which is important when you go to find an inexpensive HMI. They DON'T have to talk GE... they can, but they don't have to.

I would suggest Sixnet @ for industrial quality modems in both RS232 & RS485 modes. This is REALLY important if your panels see below freezing temps and greater than 90 deg F during the normal operating conditions.

GE's Quickpanels are quite high quality and relatively inexpensive. Maple Systems also does a good job of providing a high quality low price HMI => Automation Direct is another alternative for PLCs, Modems, and HMIs =>

With any and all of these products, LCD displays, modems, etc. make sure you're AOK with temperatures in terms of hot and cold that you plan to operate them within.

And, I agree, at times GE can be a huge pain to deal with. I suggest going directly to the GE SUPPLY guy in your area and giving them a what for. If they don't come to your view point, find another GE dealer. GE Supply will ship anywhere.

Carson S. Meyers

I use Omron, A/B, and Automation Direct. I agree that the Automation Direct DL-06 would be an excellent choice. They also have very good Touch Screens to use for your HMI.

Also check out the Entertron products - they are excellent!!!

marc sinclair

I've used the Twido.

It works.

I was disappointed with the build quality, and the programming software is terrible.

I'm sorry to keep bangin' on about the s7-200 - I don't work for Siemens - Honest. Check it out, the software is cheap and runs in IEC mode, the eeprom can store data logs and other files, there is a couple of modems available GSM or landline, but have a look at the internet module, mine is online most of the time at :-

Marc sinclair
Hello Rajan,

Thank you for your reply. My company was trying to work with a Water Treatment company in Saudi Arabia. Please keep in contact.

Thanks, Jeff