ECC Forum

RiverCityBrewer wrote:My question is, now, how do I have my din assert the output (as needed) and also still allow process control to assert the output? I've tried a few iterations but usually end up with the output "sticking" closed.

If you want to tie the DIN to controlling the output in ladder logic, then you cannot also control/override in a process.

If this is the way you want to do it, I suggest wiring the output control using a 3 position On-Off-On switch. This will allow you to not only manually control the output, but also lets you force it to process control or force the output off.

The control is all low voltage on the 5V side and easy to implement.
I did a diagram in this post: http://forum.embeddedcc.com/viewtopic.php?f=17&t=333

JonW wrote:

RiverCityBrewer wrote:My question is, now, how do I have my din assert the output (as needed) and also still allow process control to assert the output? I've tried a few iterations but usually end up with the output "sticking" closed.

If you want to tie the DIN to controlling the output in ladder logic, then you cannot also control/override in a process.

If this is the way you want to do it, I suggest wiring the output control using an 3 position On-Off-On switch. This will allow you to not only manually control the output, but also lets you force it to process control or force the output off.

The control is all low voltage on the 5V side and easy to implement.
I did a diagram in this post: http://forum.embeddedcc.com/viewtopic.php?f=17&t=333

Thanks Jon,

My brain was in software mode instead of simply looking at the wiring... easy fix!

Thanks,

Joe

Ladder is not the last and final logic applied before the hardware is actually updated.

If you have a ladder that says Din On -> Pump On, then the reverse will also be true, i.e. Din Off -> Pump off. So you need to add a qualifier in front like Pump Off -> Din On -> Pump On. Make sense?

-BD

You sure that will work BrunDog? I think if all conditions are not met, then the pump would stay off (regardless of process control).

I suck with ladder, but there is certainly a way to do it! Maybe with a logical OR?

-BD

It depends on exactly what you're trying to do, but this might work. What this does is allow the switch (DIN1) to control the output as long as the process is not asserting the output. This means that the switch will not work to turn the output off if the process is asserting it on. It does allow you to use the switch to manually turn the output on/off if the process isn't running. In general, I'd say Jon's solution is better but I enjoy trying to solve these problems with Ladder Logic.