Duplicate Destructive Bit questions

[BeepBob]

Hello all,

Stupid newbie question...

Just making sure, a bit constantly flipping, and it is reference by 2 OTE in

different ladders, that is the main cause it is flipping usually?

I removed one OTE and the flipping stop.

 

[DBLD99]

Yes - You only should have 1 OTE in a program for each address or tag.

 

[plvlce]

Correct. It is generally poor practice to have multiple OTE of the same bit.

There are situations where it might be done without causing issues (eg in separate routines that cannot both run), but even there I would recommend against it.

 

[parky]

I have seen the use of a bit used as an out instruction more than once, this is usually used as a temporary bit i.e. it is used in the context of the logic before the output (OTE) of some logic, if used again then this is used in the next bit of logic. A typical example is where a bit of logic requires say 20 contacts, as the IDE only allows say 8, those first 8 are put onto a OTE let's call it My_Temp, this is used in the next rung as a N/O & then another 7 again onto the same <My_Temp, this is then used again with the rest of the contacts to drive something.
So something like
AND xxx AND xxx AND xxx AND xxx AND xxx OUT My_Temp
AND My_Temp AND xxx AND xxx AND xxx OUT MY_Temp
AND My_Temp ....................... OUT Y0
Some people may use it for many "Temporary" interlocks even throughout the program, unfortunately due to the rate the coms is used for on-line monitoring it can appear to be either on or off at the particular time the monitoring function reads the status. It is bad practice as it makes it difficult to find faults but seems quite common especially in the earlier PLC's where there may not be a duplicate coil detection or warning & limited memory space for variables.

 

[arpus4KM]

Like it's been said, multiple OTE will just be bad news. Last ones state will always be the one active.

If you need to use an output multiple times then set up an IO map and you can have any number of XIC turn on that single OTE. then place the OTE for the XIC you just mapped anywhere you want, this is typical for a lot of programs that have an output that is turned on when a machine can be in multiple states.

 

[BeepBob]

Sweet thank you all !

 

[BeepBob]

Just one more thing, an OTL and an OTE acting on the same bit is no no as well right?

 

[chavak]

Just one more thing, an OTL and an OTE acting on the same bit is no no as well right?

Correct,

Only exception is one OTL with multiple OTU.

But I have seen multiple OTL/OTU pairs by others. But for me, I avoid it too. Difficult to track/troubleshoot when issues arise.

_____________

 

[James Mcquade]

ONE big question that needs to be asked - are the rungs the same?

if so, i would delete one.
if not, you need to see what the difference is.
maybe the previous programmer meant to rename one output and got distracted.
i agree, one output. you can have multiple branches turning it on, but 1 output.
james

 

[parky]

Attached is a way I have seen how multiple coils have been used, especially when the logic is limited to say 8 or 10 contacts on a row, I know most IDE's nowadays can display multiple rungs using the arrow symbol but that was not always the case, yes it makes sense to use different temps if you like but I have see this so it is just a way of explaining what might be happening if the system is working ok.
In the attached the logic will work as the preceding rungs i.e. M100 will be either true or false depending on the status of those rungs, the same goes for the next set of rungs for M200.
So....
It is one of 3 things,
either the programmer knows it works
They are just being lazy
Or
They have done it to confuse or a poor attemt to make it difficult for any other person trying to decipher the code.
I came across an S5 program where timers were T256 upwards there are only 255 timers in the S5, I worked out that the timer number was loaded as an 8 bit byte not word so timer T256 was actually T255 & T257 was T1
The S5 system ignored the upper byte or only loaded the lower byte it was an obvious attemp to confuse people.