RSLogix5000: Using ONS in branches.

RSLogix5000 ver 12.02

I understand how a ONS works on a rung, basically only allowing the rung to be true for one scan, until it goes false again.

If I use a ONS on a branch, how does it change the way it acts?

Does it only allow that branch to be true once, until it goes false?



I attached an example of my code, it's written in 500 which doesn't allow ONS on branches, but 5000 does. I just used this to illustrate since my laptop does not contain 5000 software.

Basically, I am using a toggle bit for 2 pushbuttons, that latch and unlatch an operation. I also have outside conditions (outside of the 2 pushbuttons) that latch and unlatch the same operation. I wanted the toggle bit to follow the operation latch so that when the outside condition was no longer true, the pushbutton only has to be hit once to unlatch, instead of twice (once to "latch" the already latched operation, and again to unlatch).

I know it works, (maybe some hidden bugs, but I haven't found them yet) but I don't quite grasp how the ONS works inside the branch.

It doesn't matter what platform, a standard ONS instruction simply acts as a 'TRUE' contact for one and only one scan after it's input conditions go true.

In the case of your latches with the 'Outside Condition', this makes sure that the toggle bit can trigger the latch to one state or another, but have no other action even if it remains in one state.

To acertain degree, the platform does matter. As Tharon stated, you can't do what he did on an SLC processor.

The PLC and development software abstract alot of stuff from the end user when you use things like oneshot instructions. What it comes down to is the plc needs to keep track of when the logic preceeding the oneshot is true and needs to file this information away. For whatever reason, the SLC line couldn't do this for multiple occurences on a single rung. The PLC5 and Logix family can.

Don't think of this so much on the rung/branch level as much as in terms of before and after the ONS. The ONS will affect anythng after itself in line with the right power rail. This is more meaningful in the Logix family where you can string output instructions in series. But the effect of the ONS is felt after itself until the branch hits the rail.

Keith

kamenges said:

To a certain degree, the platform does matter.

Keith

Click to expand...

Also, on the Softlogix5 product, the ONS is not to be used in branches. They function OK in series rungs but not in branches.

Don't think of this so much on the rung/branch level as much as in terms of before and after the ONS. The ONS will affect anythng after itself in line with the right power rail. This is more meaningful in the Logix family where you can string output instructions in series. But the effect of the ONS is felt after itself until the branch hits the rail.

Click to expand...

That makes sense.

Thanks to everyone for their help.