Newbie Help

Hi Guys I have a couple questions for you. I would really appreciate any help explaining these questions.

The decimal value (-211) is placed into the address N7:10.
What is the state (0 or 1) of the bit N7:10/15?
What is the state (0 or 1) of the bit N7:10/2?

When using Indirect Addressing, what happens if the indirect portion of the address references a
nonexistent data table address? (Choose ALL that apply)
[Skips rung and ignores it] [Turns off outputs] [Goes into PROGRAM mode] [FAULT]

In a subroutine file (File 3), what happens to regular outputs, which are used in subroutine file 3 only, when
JSR03 instruction in the main program goes false? (Choose one)
Choose: [Stay the same] [Turn off] [Turn on]


Thanks,

Jamie

Sorry, I did not have time to read the assignment either.

First post your own answers and your reasons for the answers. We will help, somewhat, from there.

If you have a Windows calculator, or the free version of RSLogix 500 (Micro Starter Light) you can place -211 into a register even while offline and see the binary equivalent.

For the other two you can either solve them empirically if you have a unit or the emulator (which means - try it and see what happens) or else apply what you have learned about the workings of the PLC including the scan.

The decimal value (-211) is placed into the address N7:10.
What is the state (0 or 1) of the bit N7:10/15? ON
What is the state (0 or 1) of the bit N7:10/2? ON

I recreated this in Micrologix Im not to sure exactly how the N7 file works though.

When using Indirect Addressing, what happens if the indirect portion of the address references a
nonexistent data table address? (Choose ALL that apply)
[Skips rung and ignores it] [Turns off outputs] [Goes into PROGRAM mode] [FAULT]

I have no idea on this one. A brief explanation of indirect addressing would help


In a subroutine file (File 3), what happens to regular outputs, which are used in subroutine file 3 only, when
JSR03 instruction in the main program goes false? (Choose one)
Choose: [Stay the same] [Turn off] [Turn on]

I am assuming they will

In a subroutine file (File 3), what happens to regular outputs, which are used in subroutine file 3 only, when
JSR03 instruction in the main program goes false? (Choose one)
Choose: [Stay the same] [Turn off] [Turn on]

I am assuming they will turn off

In a subroutine file (File 3), what happens to regular outputs, which are used in subroutine file 3 only, when
JSR03 instruction in the main program goes false? (Choose one)
Choose: [Stay the same] [Turn off] [Turn on]

I am assuming they will turn of

That is a common misunderstanding.
When the subroutine is active it is processing logic, determining whether to turn the 'regular outputs' ON or OFF. When a subroutine is not called NO PROCESSING OF LOGIC TAKES PLACE. In other words there are no decisions concerning the state of the outputs either way. Outputs do not turn on or off on their own. It requires some logic processing. So try again.

For the other two questions, what training materials do you have? Do you have Allen Bradley manuals?

(You aren't being picked - we treat all student questions this way. We are as interested in your actual learning - as opposed to just getting questions right on a quiz - as you should be.)

Download a copy of the SLC-500 family Instruction Set Reference, 1747-RM001, from the RA Literature Library. Appendix E has a nice straightforward explanation of various addressing methods in the SLC-500 family, including indirect addressing.

You've been introduced to SLC-500 style data tables, which have a Type, a Data File Number, and a Data Table Element.

N7:10, for example is an Integer data type, Data File number 7, Word element 10.

Indirect addressing lets you substitute a value for one of those numbers.

N20:[N7:0] is an example of Indirect Addressing.

If N7:0 contains the value 0, then the above address is evaluated as N20:0.
If N7:0 contains the value 5, then the above address is evaluated as N20:5.
If N7:0 contains the value 254, then the above address is evaluated as N20:254.

But what if N7:0 contains the value 300 ? SLC-style data tables can be between 1 and 255 elements long. So the address above would evaluate as "N20:300", which doesn't exist.

One thing you'll find about PLCs is that the worst thing they can do is something unexpected. So when they are given an instruction the operating system can't deal with, they usually declare a fault and go to a safe state.

There are exceptions to this particular fault; you can do "data table spanning", so that multiple data table files can be addressed like one extra-long data table. Maybe that's extra-credit.

Thanks for the help guys really appreciate it.

I'm not sure if you understand the reason why the bits N7:10/2 & N7:10/15 are showing as being ON, or not, I apologize If you do, but a brief explanation is as follows:-

The 'N' files are SIGNED Integers, using the most significant bit (bit 15)as the sign bit to enable negative numbers to be represented, hence if bit 15 is OFF then the value of the sum of the other bits is Positive, if it's ON then the value is Negative.

To do this the processor employs 'TWO'S COMPLEMENT' on the value (Google), which basically inverts the other 15 bits and adds 1 to the sum of their value. This is why the maximum positive number that can be represented is 32767(add 1 more and you will get -32768).

This is the reason that N7:10/2 is ON when the value is -211 and OFF then +211.

Again if this was already understood I apologize.

Steve