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Thread: Best Equipment for PLC

  1. #21
    Join Date
    Sep 2005
    Greenville SC
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    As Jraef said, AB is definitely the largest in the US with Siemens at #2. At the ripe old age of 69, I've used many, TI, Omron, GE, Modicon, divelbiss, Westinghouse, AD, Siemens, AB ... our customers usually told us what to use on their jobs.

    There are subtle, and not so subtle, differences. Prior to Modicon and the transitional contacts, we realized them with 2 rungs of logic. I was quite confused at those 2 rungs when I first saw them. In old statement logic ...

    store not out 1
    out 2
    store out 1

    turns output 2 on for 1 scan. A pushbutton input is often involved ...

    IMO, you can debug a machine with all but Siemens with the same general knowledge; they, more than the others, integrated "computer code" language into their ladder logic, and it was somewhat "different". Some had only unidirectional counters and timers; accomplishing what the designer needed id done differently. Some had reset of timer and counter capabilities.

    Buy one of the basic units from any; free programming software is a plus. AD's low cost HMI is a plus. Play a little. Talk with your customers' techs when you are having lunch ... they will probably be glad to discuss their systems applications ... and they KNOW theirs is the best <g>.

    Having taken and taught classes, I promise that only by doing a real job will you get detailed knowledge. But a little knowledge will give you the tools needed to learn more and accomplish jobs.

  2. #22
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    May 2017
    SCV Ca, USA
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    Quote Originally Posted by kwired View Post
    Simple inputs/outputs all one really needs to know is the mentioned ladder relay logic. One PLC to another may have some minor differences but all is based on that ladder logic.

    More complex logic functions, use of digital or analog input/outputs still probably is similar across the industry, but maybe a little harder to pick up for some people. But if you can read and even draw somewhat complex wiring diagrams, simple ladder logic used is easy to catch on to. You might get away with breaking logic rules in a hand drawing where the PLC will have certain rules that must be followed pretty strictly. Logic generally must read from left to right, one load per line is sort of common rule. Number of aux contacts per component is somewhat unlimited - compared to hard wired applications, software may still put a limit but is pretty high.
    No! Really No!
    There are a few languages that are being used in PLC programming.
    The Ladder Logic (LAD) language was invented by US engineers, the most prominent was “Dick”
    Morley. This language is more ubiquitous in US and in some English-speaking countries.

    An anecdotal account was, he invented PLC when he was drunk.
    Ladder language is considered easier to learn for electricians who are familiar with RELAY LOGIC.
    They don't have to learn Fortran or C++ coding . . . maybe a little bit of binary .

    For people who have background in electronics and computer systems, the Block Function Diagram (BPD) logic is used since it requires less space in computer display. Descriptors are often different. from LAD
    Since it requires (formal )intensive training-- it is also difficult to debug. . . but for geeks this is not a problem. You really have to know what you're doing to attempt program modification.

    The other language which is globally used is Statement Language (STL) introduced by Siemens in 1957 and popular in European countries.
    It doesn't show how lines are connected. . . it only shows the statement expressed in Boolean Theorem (binary).

    Think STL as a highly detailed specs where everything is spelled out . . . but in codes to minimize space.

    US didn't catch up until 1967 when Modicon (Gould) introduced their PLC. Morley dropped out of engineering school at MIT because he did not want to learn German.
    In 1957, there were no solid state components, so, the Germans were using germanium rectifiers and vacuum tubes.

    In the early seventies when Gould (Modicon) was just barely getting their feet wet in the PLC arena, I was working as rookie engineer for a global household product mfg company.
    In the company's drive to modernize their ageing- late fifties technology. . . they sent me (being the youngest in the gang) to train with the Modicon engineers to jump onto the bandwagon-- into this new-fangled wonders of automated production.

    Reliance and AB came later like the Japanese Fanuc. . . small units for CNC.

    At the time that we were getting involved in this technology—GM was already (pretty much ) enjoying the benefits of Dick Morley's idea.
    I did some improvements to my employers' dated equipment until I decided to bail out after 4 years.
    That didn't put a smile on my boss's face.

    And to answer OP's query where to find a good place to train, depends on what sector in the electrical industry to work for -- without having to pay for your schooling (which is quite expensive). The best place to work is in industrial environment. Commercial jobs would also help like Amazon's automated fulfilment centers. But, if you can afford it-- then go for it.

    At the time when I trained for it—it cost the company over $8000 for almost a month's training.
    Lots of automation also in modern machine shops running CNC machines.

    Money is good if you know where the money is.

    Also, the projected benefits that you expect to reap-- cannot be found in residential jobs, unless you are building fully automated homes like Bill Gates'.

  3. #23
    Join Date
    Apr 2008
    Ann Arbor, Michigan
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    190508-2125 EDT

    By 1951 and before there were point contact transistors, and copper oxide and selenium rectifiers. In the summer of 1952 I made a point contact transistor in my Brooklyn NY YMCA room. Used a 1N34 diode as the basis. Put two point contacts on the germanium and fused them with a piece of paper as separation. Sometime I might see if it still works.

    By 1954 one could buy a junction transistor for about $ 4.50 in 1954 dollars. General wages might be in the range of $ 1 per hour.

    By about 1955 you could buy a Delco power transistor. I used some in a power supply in 1956.

    I believe Chrysler had an all transistor radio by 1957, and about this time introduced the alternator with silicon rectifiers. Ford was using radios with vacuum tubes at 12 V and transistors for the power amplifier. Thus, eliminating the vibrator.

    By about 1960 the first working integrated circuit was built. See

    By 1964 there was an integrated circuit operational amplifier. See

    My recollection is that the 702 was about $35 in 1965. I believe I have an opamp with a date code around 1965.

    By the late 1960s an individual could afford to buy RTL and TTL circuits.

    By 1971 one could do a lot of things economically with integrated circuits.


  4. #24
    Join Date
    Jul 2006
    San Francisco Bay Area, CA, USA
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    I've heard the anecdote from Morely that he conceived of it all when drunk (actually it was during a hangover FROM being drunk the night before). But his timing doesn't exactly jive with known facts. GM had a project to modernize their production line controls away from relay logic to something that could be re-used when there was a model line change. In those days when you were building the 1964 to 1966 model cars, you gutted the 1961 to 1963 production line to create the 1967 to 1970 production line. When using relay panels, it involved a TON of work by both electricians and engineers, with a lot of troubleshooting etc., resulting in delays in execution. Westinghouse was selling a system called "Numalogic" that were solid state logic "modules", but you changed programming by moving wires on the front. So they were still "relay logic" just using solid state relays. A-B and Sq. D all had similar products as well, none of them were actually all that popular though.

    So in 1968 GM created a "contest" for their Hydromatic factory (automatic transmissions) to come up with a better way, based on a paper and speech one of their engineers had given at a Westinghouse convention describing these problems at GM factories. The participants were GE, who hired Bedford Associates (Morely's company) as a consultant, Westinghouse and Allen Bradley, who's team was headed up by Otto Strugger. The first to present them with a working model would win the contract for $1 million worth of the devices. PART of that program was for this system (no name for it yet) to be easily programmed by ELECTRICIANS. So from that very beginning conceptualization by GM the programming was REQUIRED to look like a relay ladder diagram. All participants had their own similar versions, but it was not referred to as Ladder Programming until some time later.

    Morely beat Strugger by only about 2 months with his Model 084 "Sequential Machine Controller, or "SMC" and won the first contract, the 084 name was because up to that point he was "0 for 84 attempts" to make it work. A-B then released their "Programmable Logic Controlle", aka the "PLC" 2 months later and got most of the contracts at GM and Ford and Chrysler after that because they already had very deeply established relationships at all of those companies. A-B vigorously defended the use of the trademarked term "PLC" for years until the trademark expired. So for most of that time Modicon, who became a separate company (when GE decided they didn't want to market this SMC) could not refer to the 084 or 184 as a "PLC", even though technically they released theirs first.
    __________________________________________________ ____________________________
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  5. #25
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    Aug 2013
    Makawao, Hawaii
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    Quote Originally Posted by Fulthrotl View Post
    AB. it's still the de facto standard in industrial control PLC's[...]
    in the United States/North America. In Europe it's Siemens. There are numerous other players out there too but by far AB and Siemens are the big dogs.

    Regardless, all PLCs use one or more of the IEC 61131-3 programming languages (Ladder Logic, Mnemonic Instruction, Sequential Function Charts, Structured Text, and Function Block Diagram). Unless you know you will be working with Brand A or Brand B equipment I would say it doesn't matter what equipment you learn on as long you learn those languages (well almost no one uses Mnemonic Instruction).

    Most installations I've seen have a preferred brand and/or series of PLC but there are usually a couple of PLCs hiding in the corner that don't match the others because they are legacy or part of a turnkey system so you better be prepared to learn other systems as well.

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