Seal-Ins and Physical Outputs

I have been talking with a new hire about programming practices good/bad. We received a program from one of our customers from an internal controls engineer. As I was looking through his program I found that he was using in his output logic the physical output address used to seal itself in.

In the past I had learned that any seal-ins would be done in manual mode, auto mode, or home sequencing. When we were discussing this internally our new hire gave some valid points as to how this was beneficial, but if I had never seen this done before then I have to ask what are to cons to this. He stated that, for the most part, an output like an air cylinder is either extended or retracted.

Being in the industry for a little over 3 years i know this not to always be the case. As well, all the old timers that i learned from never did this.

If anyone can shed light on this practice please feel free to enlighten me if this is a good or bad practice.

http://www.plctalk.net/qanda/archive/index.php/t-51948.html

http://www.plctalk.net/qanda/showthread.php?t=69347

Here are a couple of previous threads on the issue that may help you out.

Regards,

I did not find anything actually relating to using a physical output address to seal-in itself. i am aware of seal-ins and how they work. My question relates to using plc logic and not a real world physical relay wiring. the above image shows using Q2.0 output coming on then using that same address used as seal in to itself. to me this over complicates the output logic in more complex logic.

It is a 'relay logic' deriving method and it has its pros and cons.

PLC ladder logic was created and developed similarly to the old hard wiring functional methodology of implementing automatic electro-mechanical controls. In 'old school' automation, any control 'loads' (relays or power contactors), after being electro-mechanically 'engaged' were 'maintained' in the ON state by one of their NO (Normally Open) auxiliary contacts in series with all the required ON state conditions.

The method is still widely used (myself included) for ensuring of a 'self-resetting' logic in case a OTE ON state required condition is not true anymore without additional 'unlatching' implemented logic.

Also since OTEs are the most important (and dangerous!) PLC instructions, modern platforms such as AB/RA Logix will automatically reset (turn OFF) any functional OTEs upon a Power Off/Power On cycle thus creating of a "Safe State' for a restarting system.

I prefer seal-ins to latch bits as well, although I usually don't directly reference hardware outputs in my control ladder. I create an array of bits that mirrors my hardware outputs, use those as the OTEs in my control program, and then schedule a subroutine AFTER the control routine that uses the created array bits to fire the actual output bits.

I did not find anything actually relating to using a physical output address to seal-in itself.

Click to expand...

When I have a signal available to confirm that the device controlled by the output is doing what it is supposed to, I'll use that to seal in the logic. For example, an auxiliary contact from a motor starter. You don't always have a feedback signal, so sometimes you don't have an alternative to using the output to seal itself.

LoganB that is what i was getting at. i use my auto logic and any conditions to turn on an internal memory address output that is then used to control the physical output. Never a set or reset or the physical output to keep the physical output on.

somehow it does not seem like good practice. Other than making the outputs routines complicated i cant come up with any scenarios where this is not a good practice to use such type of logic.

Attached is how i usually create auto logic and then address the physical output control.

auto and outputs. the extra stuff is the customer standard and not in our usual programming methods. things like faults, resets, or light grids are not usually found in the outputs.