Understanding LL frequent forms

I am updating a machine first commissioned in 1998 with a FANUC 90/30 plc
From hours of study, it appears that the original logic documentation is incomplete and in many cases incorrect for this particular process tool. I am beginning to suspect that both the paper copy, and the logic stored in EEPROM is actually a generic package, with a few modifications to suite particulars as needed. I certainly can not fault a manufacturer for such expediency. If there is nothing connected to an input or an output, No harm, no foul. Correct me if I'm wrong.

As a complete tyro to ladder logic programs I can't help but notice some of the logic patterns occur frequently. The diagram below is an extract of one I am struggling with.


|342M 344M 139M 163M 344M
| ------] [-----]/[-----+--]/[-----]/[---------------------------------------------( )-- |
342M 344M |
|------]/[-----] [-----+

In particular, the internal reference %M0344 is used as an open contact, a closed contact, and a coil all in the same rung. Please shed some light on nature of this arrangement of elements. (doubling confusing for me, is that none of these references are connected to anything other than other internal references for several "layers" of logic both forward to an output, and backwards to an input.

I feel that if I can get my head around this "building block" It will allow me to see much more in these pages and pages of ladder diagrams.

I thought I saw a reference explanation of such an arrangement "somewhere on the net". but I did not study the content at the time, and now I can't seem to find the search terms that lead back.

Will someone(s) care to share a simple description of the above contact structure?

All comments welcome

Thanks in advance

Cal

|      342M    344M       139M    163M                                            344M

|------] [-----]/[-----+--]/[-----]/[---------------------------------------------( )

|      342M    344M    |

|------]/[-----] [-----+

Looks to me like a typical flip-flop circuit. 342M is likely a one-shot ('Positive Transition Coil' in GE speak). Each time 342M fires, output 344M changes state. If 139M or 163M are on, 344M will return to the OFF state.



-Eric

|      342M    344M       139M    163M                                            344M

|------] [-----]/[-----+--]/[-----]/[---------------------------------------------( )

|      342M    344M    |

|------]/[-----] [-----+

Just in case you need a description as to how it is exactly toggling, here's a crude one:

Firstly you should know that 342M has to be a One-shot output for this logic to work and that in turn means it remains high for only one logic scan when triggered by a positive edge.

Let's forget 139M and 163M NCs for the moment so that the ladder looks like this:

|      342M    344M                                                        344M

|------] [-----]/[-----+----------------------------------------------------( )

|      342M    344M    |

|------]/[-----] [-----+

Now for the running commentary :

342M goes high
Since 344M is low and since 344M NC is used this makes logic of branch 1 before 344M coil true
This in turn cause 344M to become High
Thus the 344M NO becomes high and 344M NC becomes low
But by this time 342M has become low as it is a one-shot
So by the time 344M becomes high the first branch logic becomes false and the second branch logic becomes true
Thus, 344M gets "sealed-in" or "latched" via Branch 2

Now next time 342M becomes high you can see that the 342M NC will break the latch and turn 342M off.

Hope the explanation does not confuse you further.

I just did a search of the logic to see if it were a one shot rising (^) in FANUC speak.

Nope, 'just a simple instruction as shown

I may have missed something, There are (^) in the diagrams elsewhere.

Thanks for the replies!

CalG said:

I just did a search of the logic to see if it were a one shot rising (^) in FANUC speak.

Nope, 'just a simple instruction as shown

I may have missed something, There are (^) in the diagrams elsewhere.

Thanks for the replies!

Click to expand...

I am not familiar with GE Fanuc, but if there is a one shot falling instruction in them and 342M is used as a one shot falling, then also the logic will work since one shot falling also stays high for only one scan cycle, the only difference being that it gets triggered on a negative transition/edge.

Hi
There is a rising or falling edge 1 shot instruction for 90/30 but it is only for a coil.

It is a coil with an up or down arrow inside it. Obviously you can use contacts of the 1 shot coil where you need then.

There is no 1shot contact without using a coil as in other PLC platfroms

CalG said:

I just did a search of the logic to see if it were a one shot rising (^) in FANUC speak.

Nope, 'just a simple instruction as shown

I may have missed something, There are (^) in the diagrams elsewhere.

Thanks for the replies!

Click to expand...

I have seen that flip-flop circuit used with HMIs to allow a single pushbutton bit from the HMI to serve as both a Start and Stop pushbutton.

Although the bit operates for 1 scan in the PLC program, as if it were a one-shot, this is accomplished by using the "set bit" function in the HMI and then unconditionally clearing the entire register array of %M bits that is mapped for HMI pushbutton inputs at the end of the Main program block.

Using this method, the single pushbutton on the HMI screen functions as if it were a toggle pushbutton. The advantage of using this method versus actually toggling the bit with the HMI pushbutton is that the toggle pushbutton bit won't become "out of sync" with the motor's (or solenoid's, etc.) run state if conditions in the PLC program (interlocks or alarms) drops out the run request for the motor.

And now, as Paul Harvey would have said, Page 2...

The original programmer made it more complicated than necessary. As others have noted, the construction of the rung suggests a flip/flop, but that construction requires that the triggering event (in this case 342M) be a one-shot. But Cal reports that the coil instruction for 342M is a standard coil. What gives????

The answer is that 342M is a one-shot, but doesn't use the one-shot coil included in the instruction set. Here are the two rungs immediately preceding the one that Cal posted.

AUTO_EN  M0343 RY3APR1  M0432  M0448             M0342

--] [-----]/[----]/[-----]/[----]/[---------------( )-

 

AUTO_EN RY3APR1  M0432  M0448             M0343

--] [-----]/[-----]/[----]/[---------------( )-

That is the classic work-around ladder to create a one-shot when the PLC doesn't include one in its function set.

Ha!

Nice catch, Steve. I remember making "roll your own" 1-shots now that you mention it, but it's been long, long ago.

Steve Bailey said:

That is the classic work-around ladder to create a one-shot when the PLC doesn't include one in its function set.

Click to expand...

But Cal said that there is One shot rising in the instruction set and that they have also been used in the program.

CalG said:

I just did a search of the logic to see if it were a one shot rising (^) in FANUC speak.

Nope, 'just a simple instruction as shown

I may have missed something, There are (^) in the diagrams elsewhere.

Thanks for the replies!

Click to expand...

Anirban Hazra said:

But Cal said that there is One shot rising in the instruction set and that they have also been used in the program.

Click to expand...

If it is an OEM machine assembly, then the manufacturers may have simply copied an older program from a previously used PLC that didn't offer a one shot instruction.

Although the homemade one shot is no longer required with the newer PLC, it still works. Maybe you've never heard this old truism: "If it ain't broke -- don't fix it."

I would expect a program installed in 1998 likely has had more than one programmer messing with it over the years which would account for the perceived inconsistencies in MO's.

But Cal said that there is One shot rising in the instruction set and that they have also been used in the program.

Click to expand...

Apparently one-shots (^) were used elsewhere, but not for this set of rungs.

Old programs with old ways get copied into new programs without changing or updating for the now-available functions. Probably part of the program was copied and left the way it was with the self-built one-shot, even though a one-shot instruction was then available on this PLC model.

Hmmm, at least 3 great minds are thinking alike.

If it is an OEM machine assembly, then the manufacturers may have simply copied an older program from a previously used PLC that didn't offer a one shot instruction.

Click to expand...

I don't know whether the machine came from an OEM or was built in-house, but I do know the facility where it's located. I know they used the old GE Series One, later the Series One plus, still later the 90-30. They were still using the 90-30 when I quit my job as Automation Specialist for their GE distributor to go into business on my own.

The original Series One didn't have a one-shot instruction and the roll-you-own solution was written up in an applications manual that GE published. When the Series One Plus was introduced, it included the one-shot in its function set. They called it the SOR (set-out-reset)instruction.

Jeesh! You guys know your stuff!

Lancie1 said:

Apparently one-shots (^) were used elsewhere, but not for this set of rungs.

Old programs with old ways get copied into new programs without changing or updating for the now-available functions. Probably part of the program was copied and left the way it was with the self-built one-shot, even though a one-shot instruction was then available on this PLC model.

Hmmm, at least 3 great minds are thinking alike.

Click to expand...

It must be nice!

These particular references are indeed tied to the HMI which is a simple push button affair. (and in the factory repair cycle at present:-(

And Yes, The program is old, looks like many portions are "generic", and it surely has been modified over the years.

Though I have not run across any falling one shots in the blocks, there are many rising one shots (^). and All in the "NEW" process specific blocks.
I would do a text search if I could figure how to get the ASCII down arrow (CTL Y) to leave it's mark. (Now that we have "icons" it seems we don't need regular ASCII symbols...Oh Well)

It's starting to piece together.

Would anyone care to offer the "streamlined" version using the one shot instruction? As mentioned, This is a pattern seen frequently in the logic, and if it were simplifed, could make a more compact ladder sequence.

I am looking at removing some rungs that were associated with features not found on this machine .

Thanks to all

Cal