Schematic question - which is proper?

These are the twiddling little details that have filled my stomache with ulcers and emptied my head of hair over the years...

I've started using remote IO, and for my schematic, I decided to represent the IO module with a modified version of it's ACAD drawing, as shown below




Now, I need to represent the coils coming off the outputs. I have always thought that coils should be represented with the positive side up on the drawing. However, the pin-out of the module is the reverse, leaving me with this -




- which I hate. However I design the lines, it's still ugly, and confusing at first glance.

Now, I'm not willing to change the pin-out of the module, since the pins are in their actual orientation. That leaves reversing the polarity of the coil -



- which is neat and clean, but I am bending the rule about coil orientation.

Sooooo... what would you do? Any suggestions/critiques? In all other respects, this is a standard IEC schematic.

Thanks!

TM

I'd throw the rule of coil orientation out and go with what you have. Maybe make the + and - a bit bigger than normal to make them stand out, but if you can avoid crossing lines you're a lot better off (IMHO).

Personally I like the second one, it is neat and easy to understand. I did not know there was a "rule" that required + to be on top, just thought you needed to show the orientation.

I see you are using Red and Black, I thought IEC was Brown and Blue for DC.

Thanks guys, that's what I was hoping to hear.

Ron, you are correct as always, IEC is Brown/Blue. However, the pigtails for these valves (I know, gotta change the symbol) were red and black from the manufacturer, so I called 'em what they were.

Thanks again!

TM

Why does almost everyone insist on drawing PLC I/O with a physical representation of the card? Why is this any more important for a PLC than for a motor starter or a relay? You don't draw all of the motor contacts and the coil and the auxiliary contacts for a motor starter together, do you? The reason is that the schematic is partially intended to assist in wiring a panel and a complete system, and partially (perhaps more!) intended to assist in field troubleshooting and maintenance.

Below is the way I draw my schematics. The PLC I/O is drawn like any other schematic component - with a schematic representation, not a physical one! Before I developed my system I have worn every single "hat" identified below. This ain't book lernin', fellers!

There are a lot of people who use drawings:

Design engineer - the physical representation is simpler, because it helps him keep track of I/O usage, commons, etc. However, he is the least important one in the chain, and ought to be smart enough to adapt anyway.

Draftsman - physical is sometimes easier, but not always (see Timothy's dilemna). Also, if you have a number of devices in parallel or external contacts in series the drawing gets very messy very quickly.

Panel builder: It doesn't much matter. They mostly are running wire point to point anyway, and it often helps identify design issues if they can understand what the circuit does.

Field electrician: The shematic representation is easier. That's why JIC did things this way! The field electrician can adapt to problems better if he knows what is going on. (But my starter doesn't have an aux contact, he says!) For the point to point wiring, again, it doesn't matter.

Commissioning engineer / technician: Here the schematic system wins hands down. You generally start addressing problems by the field device that isn't working as intended. It is worlds of help to have all the associated devices, including I/O, together so relationships can be identified. You need to know how the circuit works to troubleshoot it! Just like I do my programming to make troubleshooting easy at start-up, I do my drawings the same way. Start-up is the most expensive, time constrained, and difficult part of any project!

Service electrician/technician - everything said about commissioning goes double here.

So, Timothy, I suggest avoiding the problem altogether and draw an electrical schematic, not a PLC drawing!

Why does almost everyone insist on drawing PLC I/O with a physical representation of the card? Why is this any more important for a PLC than for a motor starter or a relay? You don't draw all of the motor contacts and the coil and the auxiliary contacts for a motor starter together, do you?

Click to expand...

Technically I do not think that is an appropriate analogy, in most cases an auxillary is used for a holding (or interlock) ckt so..it depends but is shown with the starter.



Its just my opinion but the "physical" representation of the PLC card (I/O) greatly simplifies both wiring and troubleshooting...personally I think it is an electrical representation. It is no different than showing the transformer and/or DC power supply and its connections.

How about this? It keeps the same concept.

Tim if you are trying to decide between the two, I prefer your
second drawing much more (as a electrician). It might bug you
to have the negative side on top, but as Tom said, the design
engineer is probably the least to worry about.

Actually, Ron, your diagram is a true schematic. The representation of the holding contact, for example, is independent of the physical relationship. You would and could use this same drawing for any starter, regardless of manufacturer or size. If you had multiple aux contacts, with one used to turn on a pilot light for example, the light could be shown several rungs below and grouped with other pilot lights without confusion, as long as the contact is referenced to the starter.

On the other hand, if you drew your diagram the way PLC I/O is drawn, you would need different diagrams for a NEMA style starter with the auxiliary contacts off to the side and an IEC starter where the aux contact is on top. A Cutler Hammer drawing with the coil contacts below L1 and L3 would need a different drawing from an A-B NEMA with coil terminals on top which would be still different from an AB IEC with the coil terminals beside the Line and Load terminals.

The way I like to draw PLC I/O is a schematic. When I change from ADC to Siemens I just have to change the terminal designations. My panel shop has never had trouble figuring out how to wire the I/O cards from the schematic representation, just like they could figure out how to wire anybody's starter from your schematic.

I respectfully disagree about the troubleshooting. If I have a motorized modulating valve that is malfunctioning, I want to look at the schematic with the valve as the center of attention. If the input card and the output card are on different sheets, as often happens, then while I'm looking at the output sheet it isn't clear that the input for "Closed Limit Switch" may not be coming on because the valve is in an intermediate position. It may not be clear that the valve may not be moving because a control relay is bad if I'm looking at the input sheet. Remember, the schematic isn't for the guy that knows how the system works - it's for the guy that is trying to figure it out.

Maybe I am too old and just do not know enough, there are many ways to do this I guess but based on my experience, when a plc is involved, it is very "rare" if/when any single input has a bearing on when an output activates or deactivates, that is one reason I think it is important to have a printed copy of the plc ladder. It is not unusual for that "Close Limit Switch" to be an internal bit in the plc that is based on true or false conditions from inputs or other internal bits.

That is one reason why so many electrical/maintenance types have a problem with PLC's, because they do not get it that the close this valve limit switch is ON but it is not closing.

It all depends I reckon but looking at the I/O cards in a physical representation of how it looks in reality makes it easier to me when it comes to visualizing the wiring etc.

Knowing how a system works does not mean you can memorize..well at least not me, all the I/O and associated wires. I have worked with machines that could fit in a 20ft x 10ft room but had over a 1000 I/O points.

Its just my opinion...BTW Tim do you have the option of labeling the I/O card?

rsdoran said:

it is very "rare" if/when any single input has a bearing on when an output activates or deactivates

Click to expand...

This has been my experience as well. I frequently get calls from electricians who understand my wiring diagram (not schematic) just fine (sensor A connects to input 2, motor power is controlled by output 6, etc), but don't understand how the machine works.

Of course this is also why I think good error messages and logging is essential in any PLC controlled device (well any that's the least bit complicated).

Oh well, just my 2c too.

I was about to suggest the same as brucechase ...

However, my pet gripe is this thing called -24vdc. What is it ? A true -24v when measured with respect to ground or is it the return line ?

Wait, don't answer that. I know what it is here. But when on other dwgs, then confusion comes it. I would like to see it as 'DC Return' or something like that. Unless it is a true negative voltage. But here it is not.

We (meaning ME) likes to measure voltages with respect to ground.

Hi guys! Great feedback and lively debate... I love this site...

Anywho, taking them in order -

Tom - I like your style, and I can see that the big advantage is freeing yourself from the constraints imposed by representing the wires as direct physical connections. Nevertheless, yourself and Terry Woods are the only two I've ever seen doing it this way. Which is not to say you are wrong, but perhaps your method is particularly suited to your design style?

Bruce - I'm deviating from IEC a bit here already, I'd rather not deviate any further using the JIC coil symbol. But in spirit, it's the same as #2, so thanks anyhow

Katratzi - as you said.

Ron - Yes, I do have the option to label these critters. Any suggestions?

TM

If they are cards then put in model number and point count..ie 4, 8 16, 32 point. On the drawing, that page, "maybe" provide model number info..ie 16 point relay, 4 channel analog etc. I know that some of this appears to be overkill but the more that can be provided..to me...the easier it will be for ALL to understand.

Technically DC is referenced to + (pos) and - (neg), not ground but on a 2 wire DC supply the use of the term "common" for the neg - may be easier to recognize.

Model number of each PLC card is a must! 15 years from now someone will thank you when they're trying to troubleshoot and old PLC and realize the input or output on card X or Y is bad. That someone may even be you!