Please Explain

Hey, need some clarification on the term POINTS,
I see it being referred to alot when dealing with PLC's for example: Number of I/O Points 512
example2: Data Register(D) 6144 points 0 to D6143

Thanks

points refer to the terminal points.

for instance, a Micrologix 1000 is considered a 10 point controller and has 10 inputs & 10 outputs. So, each terminal (where the wires are land) is called a point. Points are divided into inputs & outputs.

ok, I get that

but how do u explain for example a PLC with 512 I/O points, 8192 I/O device points but allowing Internal Relay (M) of 7144 points,Latch Relay (L)1048 Points, Link relay 4096 point,Data Registers of 6144 Points etc these all adding up to more points that specified in both the I/O points and I/O device Points.

Can you explain how these relate?


Is it that no device will have an excess of the 8192 points? I am confused as to the allocation...

The term 'points' as you relate is being used in a generic sense of 'position' or 'place'. It may be that some PLC has a fixed space which is then subdivided in certain defined ways.

If you can refer us to the the specific PLC and model's specification sheet we will then be able to help explain how it is using the term ('points') for its specific capabilities.

as usual, Bernie is right...it will be easier to answer questions if we know what type of controller you are using. However, the items you listed are internal points in the controller accessible in the software.

The PLC has a CPU with memory. A chunk of memory is allocated for use by the programmer - you. Lets say we have 1K of memory. This would actually be 1024 bits. This 1024 is subdivided and allocated to various functions.
We could reserve 16 bits for Input, and another 16 bits for Output. 16 points resrverved for "I", and 16 points reserved for "O". Most likely addressed as I0-I15, and O0-O15. That would be 32 points I/O.
Now we have 992 bits left. We can also reserve another 256 bits for Internal relays. You use these relays in your program for internal functions. M0-M255. Allocate another 256 bits for "L", etc."I" bits, "O" bits, "M" bits, "L" bits, etc. are all specific locations allocated in memory. None of them overlap.
You need more bits for counters, timers, and math functions.
8 bits is a Byte, 16 bits is a Word, and 32 bits is a Double Word.
A typical counter uses 3 Words. 1 for the preset number, another for the current count. The last one is divided up into Status bits - counting, done, etc.
Everything in the PLC needs an allocated (dedicated) section of memory.

A PLC computer probably has 8K or 16K of memory. 512 points of I/O is just 1 portion of that memory (half of 1K). It's easy for a CPU to turn bits on and off, in fact, that's all they do. If you want these bits to also turn on a transistor in an Output module, then extra chips are needed to go from memory to the backplane, then to the module. The same is true for Input modules. Each input point turns on 1 bit of the PLC's memory, via interface chips.
When it says you have 512 points of I/O, this is just the Module limitation. The PLC you describe is capable of communicating to 512 bits from the outside world - it's rack or backplane. This sets the limit of how many modules you can have.

In summary, you are correct. A PLC with 8K of memory will have a limitation of 8192 points total. Add up all of the points to get the total size of memory. The one you describe sounds like it has 20K or more. A higher end model.

I don't know what "I/O device points" are.

Thanks alot, keithbyll

Bernie I am using the Mitsubishi A2AS PLC

Maybe this is too simplistic, but I think of points as the dots on electrical schematics that represent connections.

since a PLC is a computer that logically represents a whole lot of physical electrical devices, points represent the same thing except in software. I/O, of course, being the exception since they are where the 'soft' points meet the 'hard' points.

Bernie's explanation was a lot better.