Some help for a student

[Guest PCT Student]

Hi, I was wondering if someone could answer a question for me. I'm a student at a tech school and on our last PLC test one of the questions was, "How many words does a coil take if the plc uses 8-bit words?" Now I know if they are 16-bit words it uses 1 word, but would 8-bit uses 2 words? If not could you explain why not? Thanks for any help.

[TechJunki 0]

First, I assume by "coil" you mean a solenoid valve or similar device. Then if I understand your question correctly the answer is 1 word, actually less then 1. From my knowledge I have never had to use, nor have ever seen more than a single bit used to turn a single "coil" on. Therefore whether you are using a 16Bit word or an 8Bit word the "coil" would, either way only take a single bit of either word to energize the ouput to the"coil". I hope this answers your question correctly. JJ

[Guest]

I'd love to know what your instructor considers the "correct" answer to this one ... please come back and keep us posted ... good luck, Ron

[Jay Anthony 118]

And of what value is the answer????

[KinK 0]

LOL

[Guest HalBishop]

I bet his instructor knows all about the retroencabluator.

[Guest PCT Student]

Thanks for all the replies, the answer was one word. Although I still don't see how you can take a 16-bit address and use an 8-bit word for it. The problem was the instructor is one of those 'I'm always right' guys who never admits when he is wrong, so I wanted to make sure he was right about the answer. Thanks again for the help.

[IO_Rack 181]

I don't think you understand what TechJunki explained to you. One 8 bit word: 7 6 5 4 3 2 1 0 One 16 bit word: 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 In the 8 bit word, you have 8 availible outputs. In the 16 bit word you 16 available outputs. You require only 1 output from either to energize a coil. If you manipulate the word to use only 1 output then you affect all other outputs within the word. I hope that is not confusing. Edited 7 Apr 2005 by IO_Rack

[Guest PCT Student]

Actually what the question was about was the number of words it would take to program different things. Like it takes one word of memory to program a contact or coil, or two to five words to program a timer or counter, using 16-bit words. My confusion was how could you program a 16-bit address using only one 8-bit word. I hope this clears up my question, I don't have my book with me, so I can't go into more detail. Thanks for all the responses.

[TechJunki 0]

I give up...... anyone else??

[IO_Rack 181]

Number of 8 bit words to energize coil = 1/8

[ianbuckley 0]

I think he (the professor) is talking about words of program memory used for an output coil instruction.

[Guest PCT Student]

Yes

[Gerry 13]

It would be one very small PLC with extremely limited instruction set to fit the output energise instruction into one byte. The instruction has to include the op code (e.g. OTE) and the physical memory address of the bit to operate on.

[RMA 0]

I think the real answer is that this is another of those poorly phrased questions, of which we, unfortunately, see far too many! The answer may indeed be one word (line) of program command, but as Gerry mentioned, that is likely to translate to a lot more words of memory which get used.