Host Engineering Forum
General Category => Do-more CPUs and Do-more Designer Software => Topic started by: Rastus81 on February 20, 2015, 09:32:54 PM
-
Hello, I have a basic question.(I have searched for similar questions)
Whats the difference between using a start stop circuit and set/reset latches?
Is the difference in the behaviour at power failure?
Are there different scan time requirements?
Ie;
START STOP RUN
--||--------|\|-----------------( )--------
|
RUN |
--||-----
VS
START RUN
--||----------------------------(SET)---
STOP RUN
--||----------------------------(RST)---
-
Very similar. I tend to use SET/RST for "event" based triggers, and OUT coils for normal input logic "power flow" type outputs.
Also, in Do-more and DL, when you use OUT coils in a Stage (or in Do-more, in code-block), whenever the containing SG Stage becomes DISabled, all OUT coils automatically turn OFF. Outputs that are turned ON with SET do NOT automatically turn OFF when the containing SG Stage becomes DISabled.
-
Rastus81,
It looks like the push buttons in the field are using normally open contacts only. One thing to consider is what would happen if your field devices had no power supplied to them. An operator could press the stop button and because there was no power to pass on to the PLC input the output would stay energized. Just something to think about.
-
Here's one use scenario: I use SET/RST rather than normal contacts sometimes, in combination with one-shotted inputs as an easy way to treat operator inputs and maintained machine logic as multiple start and stop button presses.
I work on a type of machine where the only automatic mode isn't really a mode, it's more like a script of actions the operator could do himself from the HMI, just automated, but all the operator controls are still active. "Start the exhaust fan and run till such and such pressure and rpm is obtained, but no less than five seconds. Start the combustion air blower and run till such and such pressure and rpm is obtained, and the combustion air flow switch is made...." and so on.
Rising edge of machine logic request for an ouput is a start button push, trailing edge is a stop button push. Using edge contacts, I can no longer do a normal circuit with a latch contact due to logic rules, so SET/RST comes to the rescue.
-
Thanks guys,
With your examples, i will continue using normal coils, unless i need the extra features.
I try to steer clear of assumptions, so its good to know the details.
Regards
Rastus
-
I let to add, if you use the SET Coil and it is in a retentive range, it will still be set after a power lose, and restart if not told to reset by the program at startup. Use SET Coil with caution as it is like a light switch. Once ON, always ON until it is turned OFF.
-
Rastus81,to comment. To prevent what you are saying. I have guards in my programming to prevent that. For instance I tie one of my inputs to 24v. I then use that input in series in places to prevent a push button from working. If not voltage, then the input tied to 24v prevents any other action. I also use that 24 input to keep a machine in E-Stop condition. Just a couple examples.
-
PLCGuy, I know what you mean. But as a side topic, it would be nice if the input and output modules could monitor their own common terminals and give an ok signal which could be used in the way you mention. Maybe there are smart modules that do this?