Are these branches normal or having some difference in SCAN time?

[cjd1965]

I'm going to jump in here and vote for the fastest and least memory method of instruction placement, sorry if you disagree

We are well into the 21st century, yet some people are intent on holding on to the 1960's interpretation of ladder (programming language) as an electrical circuit.

I can't speak for other manufacturers, but in the Allen-Bradley/Rockwell world when they embarked upon development of the Logix5000 series, they ditched just about every pre-existing "rule" about instruction placement, a concept that had existed solely to make ladder programming language resemble an electrical circuit. The instruction set has evolved over the years, but there hasn't been a single new instruction that has any roots in a panel of relays. There's just 3 instructions, in the whole instruction set, that translate directly to "relay logic"... XIC XIO, and OTE. OTL and OTU don't have an equivalent, neither does ONS. Are people suggesting that maintenance techs can only understand less than 2% of the code they look at? That's an absurd notion, and doesn't give them justice.

The debate will continue, no doubt, but in my opinion we should be training our "techs" to understand that things have stepped up a gear, rather than programming our systems down to their abilities.

You would not trust your 2015 Audi A8 to be serviced by a guy who only knows 1960 VW Beetles, would you ?

Hi While I agree with what you said regards modernising code, there are hard wired relays which latch/unlatch and pulse on the market

 

[PLCMentor]

I was always taught and also teach - program for your audience. If you are a machine manufacturer then this is difficult. But as an engineer and consultant, I can program to the level of the engineers or techs at the facility.

 

[pal]

daba,
I have read your posts with a great deal of respect , but , only 3 instructions from relay logic ? !!!
As a young man I have replenished oil in relay dash pot timers which had timer done as well as timer timing functions - superceded by motorised timers . Mechanical counters , motorised cam sequencers with what are now called micro switches , pressure guages with adjustable minimum and maximum setpoints , toggle relays ( 2 coil ) to provide the Alt function found in a couple of PLC,s . If you think about you will find that a lot more than 3 instructions were based on electrical panels . Oh , don,t forget the 1 day or 7 day motorised time clocks giving real time switching .
With respect ,
Paul

 

[daba]

daba,
I have read your posts with a great deal of respect , but , only 3 instructions from relay logic ? !!!
As a young man I have replenished oil in relay dash pot timers which had timer done as well as timer timing functions - superceded by motorised timers . Mechanical counters , motorised cam sequencers with what are now called micro switches , pressure guages with adjustable minimum and maximum setpoints , toggle relays ( 2 coil ) to provide the Alt function found in a couple of PLC,s . If you think about you will find that a lot more than 3 instructions were based on electrical panels . Oh , don,t forget the 1 day or 7 day motorised time clocks giving real time switching .
With respect ,
Paul

You can quote as many (i'll say) "rare" external devices as you like, but the PLC has only digital and analog I/O, period.

The instruction set can only interface with the real world with those digital and analog I/O. What the external devices do with the PLCs excitation is controlled by the manufacturers of the external devices, not by the PLC.

The PLC simply reads inputs, and writes outputs, and the 5 instructions provided cover those adequately.

 

[rdrast]

I used to write programs to be read and understood by Homer Simpson, but stopped. This is almost 2017, I'm not going to cripple performance or my productivity to write to an audience that can't even troubleshoot a physical relay ladder diagram anymore.

If a plant electrician has access to a computer to go online with a PLC, then by now, they had better be keeping up with the current state of the art.

 

[Paullys50]

The debate will continue, no doubt, but in my opinion we should be training our "techs" to understand that things have stepped up a gear, rather than programming our systems down to their abilities.

YES!

PLC programs should be written to make it easier to troubleshoot, but there are limits.

There is a tendency to abuse descriptive tag names.

Developing good code structure/organization, code style guidelines and good naming conventions are key for troubleshooting and should be more embraced by the PLC world.

I was always taught and also teach - program for your audience.

rdrast counters this perfectly, and I can't agree more!

I used to write programs to be read and understood by Homer Simpson, but stopped. This is almost 2017, I'm not going to cripple performance or my productivity to write to an audience that can't even troubleshoot a physical relay ladder diagram anymore.

THIS THIS THIS! Programming is a business and if you cater to the KISS method your programs will be basic, and lacking of advanced features because you ran out of time and money.

EDIT: And you'll still get phone calls