Label [LBL] instruction in RSLogix 5000

[cherymax]

I understand that when the [JMP] instruction is true, it jumps/skips to the [LBL] instruction. I don't understand how the [LBL] is handeled when the [JMP] is false.


In the below scenerio... [JMP] is always false and [switch_1] always true would the light come one every time the rung is scanned or never come on?


Rung 1: [JMP]

Rung 2: [LBL]---------[switch_1]--------(lamp_1)

 

[rdrast]

It is ignored, or, if you prefer, acts a as a wire, and continue's to pass power.

 

[kamenges]

For the purposes of your question, think of a label as a place marker. It has no influence on the logic. So in your example the lamp will follow the state of the switch whether the plc arrived at the rung via the jump or just because it was the next rung in order to be scanned.

Keith

 

[danny.michael]

The LBL instruction is always true. In your example, lamp_1 would always be on when switch_1 is on, regardless of the state of the rung containing the JMP.

 

[bernie_carlton]

Another thing to remember - if your Rung 2 is the only one controlling lamp_1 and it is NOT executed because of jumps around it then the state of lamp_1 as determined on the last time the rung WAS scanned will hold. A rung which is not executed doesn't evaluate as false, it doesn't evaluate at all.

 

[tald1231]

LBL inst

What happens if you have:

[LBL]------[LAMP]

 

[bernie_carlton]

It will have the same effect as my post in 2008. If the LBL rung is JMPed to or is executed in the normal sequence the the logic would cause [LAMP] to be TRUE (I guess). But, if the rung is not executed and no other logic controls LAMP then it will retain its prior state.

 

[rdrast]

What happens if you have:

[LBL]------[LAMP]

Well, Obama launches nuclear missiles towards the moon, causing it to split into several pieces, which then rain down on the earth obliterating all life. Don't do that.

 

[Doug-P]

What happens if you have:

[LBL]------[LAMP]

This is the logical equivalent of:

-------[LAMP]