PLC Address Question

[Unregistered]

Hi,
in one of my assignments i have just recieved i have been asked to answer this.

All PLC addresses are on a 8-bit (octal) bus system. Explain why this is the case?

Can anyone adivse me and where i can find some information on PLC addresses.

Cheers

 

[DanielCamacho]

Seems to me if your professor is asking this type of question he has:
1. covered this material in class or
2. he knows it is in your reading assignments...

I don't think any one here will just give you the answer...

Good luck...

 

[RussB]

Just a guess, maybe they ran out of funds to add busses to the 10 bit Address block. Besides I think that it's on the "other" side of the tracks.

 

[Steve Bailey]

It is untrue that all PLC addresses are on an octal addressing system. It is only true of PLCs that were developed in countries that don't use the metric system. PLCs that were developed in countries where the metric system predominates have decimal addressing schemes. The author of your textbook is obviously not French.

 

[TRACY H]

Ok

Cheers this won`t answer your question,but if you want to learn something about number systems? Go to Programs then accessories then calculator. When you get the caculator up click on view and choose sceintific. Here you can see the different number systems used in plc`s and pc`s. Knowing a little about number systems will help you understand addressing.

:site:

 

[mordred]

just a hint think of how many bits are in a standard PC register

 

[elevmike]

It is only true of PLCs that were developed in countries that don't use the metric system. PLCs that were developed in countries where the metric system predominates have decimal addressing schemes.

I'm not too sure about that...AD's are from Japan. But the 90-30 was brewed in the USA.

 

[Steve Bailey]

The 90-30 was a joint development effort between GE and Fanuc. Fanuc CNC model A I/O modules are 90-30 modules in a yellow plastic case. The old 90-30 handheld programmer is a Fanuc CNC handheld with a different keyboard mask.

PLCs that use hex addressing schemes were developed by alien UFO astronauts with 8 fingers on each hand.

 

[randylud]

Depends on your point of view, kinda!

Bits of a 16 bit word are 0 thru 15.
8 bit words make a byte and the bits are 0 thru 7 for the low byte and 8 thru 15 for the high byte of the 16 bits.
Now, if you choose to, you can view the two 8 bit words as octal and following the above, the low byte would be 0 thru 7, but the high byte would be 10 thru 17 in the octal system. Early PLCs like the Allen-Bradley that developed around the 1771 I/O system used octal addressing to specify the I/O points down to the bit level. It made sense to do that with the processors and the I/O rack system. That same I/O structure is still in use today with the PLC-5 family and others that came before. I personally like the SLC 500 family that uses decimal addressing just because I am used to decimal and do not have to think as much. But, it is fairly easy to get used to octal if you use it enough.

 

[RussB]

There you go Guest Student, randylud just answered your question for you. The only thing that he left out was the bit about binary and power of 2.

BTW, those 8 fingered aliens were last seen crossing the tracks between the 8 bit block and the 10 bit block. Where was your last UFO sighting?

 

[CaseyK]

Are these the same aliens that landed (unsuccessfully) in roswell, brought us velcro, and were featured in "Men In Black"?

Not to be confused by our men in powder blue, wait, wrong movie.

regards.....casey


BTW, "cheers" was not registered, we are not really sure where he was from. Shouldn't posters be registered, wait, I gave up on that arguement. Or did I?

more regards.....casey

 

[Unregistered]

Progress

In the old days octal would have referred to the base of the numbering system. The octal system having a base of 8 was used in the early 4 bit processors. They selected the four bit "nibble" from the fact that with 4 bits you can form 8 numbers, 0 - 7. Later 8 bit "byte" processors came around and they could represent numbers from 0 - 15 which made the early computer technicians choose the hexadecimal numbering system in stead of the octal system. After that 16, 32 and now 64 bit processors where developed but people continued to use the hexadecimal system.
Now from your question, I'm led to believe that in modern times a numbering system is no longer denominated after the number different numerical symbols it uses but instead after the number of bits? This really makes me feel old.