It can actually be a lot more complicated than that now. The newer PLCs can have multiple outputs in one rung and they don't have to be driven by a singular logical result. They can also have embedded numerical comparisons, one shot / edge instructions, counters and timers. The logic can get very complex.
Structures were also added some years ago to the newer controllers. Indirect addressing has been around for years. The newer controllers also have different language options. Structured Text is a popular Pascal like language. Symbolic programming is very popular now as well. The IEC 61131-3 standard describes 5 different standard languages. Dave On 3/23/2015 8:37 AM, Erik Christiansen wrote: > On 23.03.15 08:33, Mark Wendt wrote: >> So, following that logic, "off" is "on," and "on" is "off." ;-) > Taking such facetiousness at face value, I think the real situation is: > > "OFF" is "on" and "off", and "ON" is "on" and "off" > where OFF/ON is input, and on/off is output, with no and nc contacts > simply being the complement of each other. > > Even ECL logic gates have both inverted and uninverted outputs. > They also provide a logic 1 _and_ a logic 0 output for true input. > > The confusion seems to arise from the fact that relay logic, whether > ladder logic or just a rat's nest, performs its boolean operations using > many outputs (relay contacts), and just one input, if we take the > physical relay as the component. > > Once we see the logic gate as a physically dispersed collecting of input > contacts, and an output coil, then we're beginning to see the ladder > wielder's view - each rung is a logic gate with an arbitrary number of > inputs, normal (NO) or inverted (NC). The fan-out of a gate is the > number of contacts, but it is possible to run out of normal fan-out > before inverted, or vv. Fan-in is higher than anyone would normally ever > need, especially with higher voltage coils. > > And finally, on what is on, and what is off: > When an input switch or a relay is energised, then all its contacts > transition from "what they wuz" to "what they wuzn't". (So we could call > it wuzzy logic, perhaps. ;-) > > Erik > --- This email has been checked for viruses by Avast antivirus software. http://www.avast.com ------------------------------------------------------------------------------ Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
