PLC information

[cindy]

Hello, I have been taked to find out several things to do with PLC's if anyone can help I would really apreciate it, they are:

1) Describe the different forms of signals used in PLC's
2) The resolution and relationship between analogue and I/Ps, O/P's and word length.
3) network methods and standards.
4) Identify elements associated with the preparation of a PLC program

[icky812]

You can find your answers in the online tutorial. Look at the top of the page in the header.

You should also read the "New here" in red type just above your question.

bonkhead

[cindy]

Sorry the questions were a bit blunt and formal, I have read the PLC live tutorial and learn't alot. But some of the information I am trying to accuire still isn't evedent. I guess it is the way the question has been asked in the first place. The only thing I can say in response is I'm having a blonde moment.

In the tutorial there was no clear mention of the resolution and relationships between analogue I/Ps and O/Ps or word length. I have read books and donot know where to look next. Please can you even tell me where to start looking? Please!

[Rick Densing]

Analog inputs and oututs are just that- inputs and outputs. How do the discrete I and O relate?

As for resolution, when converting analog to digital form, a number of bits must be used. The more bits, the more accurate your representation of the analog signal.

To me, the definition of a 'word' is 16 bits, I wouldn't expect that to change.

If your question number 2 is a homework question, it is really poorly worded.

[icky812]

1) Describe the different forms of signals used in PLC's
2) The resolution and relationship between analogue and I/Ps, O/P's and word length.
3) network methods and standards.
4) Identify elements associated with the preparation of a PLC program

Ok, since you can't seem to find the answers, here goes... BTW, these are MY interpretations of the answers.

1) Switched DC Signals (hand switches, limit switches, relays, etc.), Switched AC Signals (same as DC), 0-10v DC Analog signals, 1-5v DC Analog signals, 0-20ma DC signals, 4-20ma DC signals, RTD temperature measurement signal, various types of Thermocouple signals, High speed counters, Network signals (ethernet, Modbus, ProfiBus, etc). There are more but this is a start. Some of these types are input only, some are I/O.

2) I/Ps and O/Ps I am assuming are individual points, ie. digital. Generally they are represented internally by individual bits. Analogue on the other hand is brought into the PLC and converted to digital. Once converted they are usually represented by a range of numbers, in Koyo plc's this range is 0-4095. Zero is zero and 4095 is the maximum of the analogue signal. You can then use the formula for a straight line to convert to engineering units (y=mc'squared').

3) Jeeze, this guy ain't asking for much! There are a bunch of different network standards in use today. Some are proprietary, some are well known. I'm just going to list the ones that I know about.

Ethernet, Modbus, Profibus, Devicenet, and K-sequence.

4) See Tom Jenkins reply to this post http://www.plctalk.net/qanda/showthread.php?s=&threadid=217.

Hope this helps...

[Rick Densing]

I think that you meant y=mx+b for scaling. Yours is a version of old Einstein's equation E=mc^2.

[Tom Jenkins]

The resolution of an A/D (Analog to Digital) convertor is:

2^Bit

where "^" means raised to the power, and Bit is the register length or number of bits used.

The max value of an A/D (Analog to Digital) convertor is:

2^Bit - 1

for 8-bit that is 2^8 - 1 = 256 - 1 = 255. This gives a resolution of ±0.4% not great but adequate for a lot of control.

for 12-bit that is 2^12 - 1 = 4095. This gives a resolution of ±0.02% which is more than adequate for most control jobs. Note that the sensors you are using are probably not nearly that accurate, so increased resolution really gains you nothing.


Some PLCs use 15-bit or 16-bit A/D conversion. I've never really seen the point, because although you can theoretically get resolution to 32,767 (±0.003%) the hardware itself isn't capable of providing accuracy commensurate with that kind of resolution. I have often obtained more accurate results with a PLC using 12-bit resolution than with other PLCs using 15 bit.

[Allen Nelson]

Originally posted by cindy
4) Identify elements associated with the preparation of a PLC program

So many possible answers - which does the prof want?


#1
Carbon, hydrogen, oxygen and nitrogen, in the form of sugar and caffeine.
Copper.
Silicon, laced with a little arsenic and gallium.


#2
Paper.
Pencil.
Brain.


#3
A signed purchase order (I never prepare a PLC program without one!)


#4
Phil's book (blatant plug)


#5
A detailed I/O list.
A detailed process description.
A thorough understanding of the PLC instruction set.
Paper.
Pencil.
Brains.

[icky812]

Thanks Rick. You are correct y=mx+b is the formula for a straight line. My brain must have took a LONG hike on that one!

banghead

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BTW, who says a PLC can't go faster than light?