ControlLogix: what exactly the DC output module do?

Hi everyone,

I have a course doing lab with AB's controller and it's very fun. However, in today's lab, we were working with the "DC output" module, which makes me very confused.

My equipment is a logix5550 controller with lots of modules, the one we cared here is the so-called DC-output: 1756-OB8.

My lab assignment today is to connect a pair of output from the DC output module to a relay's coil along with a 24VDC, and another circuit is relay's contact connected to a motor (and its own power for sure, a 14VDC). The input of my controller is a built-in switch (already connected to another module (IA16) with power) and using ladder logic to connect the input and the actuator (the dc output) like:

|----] [-----( )----|
IN-0 OUT-0
The supposed result is very simple: if you close the switch, the motor will run and vice versa.


My instructor said, the DC output here is just like a swtich- if your ladder logic give it a bit of 1, it closed; otherwise it's open. But it sounds very wrong to me. Because if I use the multimeter to check the resistance of two pin of my DC output, it always shows "open loop" no matter if my switch is open or closed - that's not what a switch does. I asked him, he has no idea as well.

So my questions are:

1. What exactly did it the DC output do? The manual from AB looks too complicated to me, but It said it's a digital output? So it's only have two status? like high voltage and low voltage or what? I used multimeter to test it with R/V/A but didn't see any difference between two status, why?

2. If it's a digital output, why it's name is "DC" output?.. this confused me so much.

3. Why we need to connect an 24VDC to the output module? I mean if it really works as a switch, sure. But I hardly believe it: if so what's the difference between a DC output and a relay output then.. (we use reply output mostly before)

3. Will the two circuits I mentioned above actually work? My motor did rotate for once, but sometimes when I open the switch and closed again, it failed mysteriously and the instructor didn't know why either.

I know I may be lack of basic electric knowledge since I'm actually from mechanical engineering. Sorry about that!

I know I may be lack of basic electric knowledge

Click to expand...

You have provided your own answer.

Become proficient in basic courses on electrical and on electronic circuits. Then restart this course, hopefully with a more knowledgeable instructor.

1. What exactly did it the DC output do?

Click to expand...

according to what you've described, it SHOULD have turned on the current and made the motor run ... but ... how many amps does your lab motor pull? ... the module that you mentioned is rated for a maximum of 2 amps ... if your load is higher than that, it might have blown the module's output circuit ...

The manual from AB looks too complicated to me,

Click to expand...

well, they look that way to most people – it takes awhile to get used to this stuff ...

but It said it's a digital output? So it's only have two status?

Click to expand...

yes – OFF or ON ... or in electrical terminology, either OPEN or CLOSED ...

I used multimeter to test it with R/V/A but didn't see any difference between two status, why?

Click to expand...

the module that you're using is a SOLID-STATE type module ... that means that it uses a transistor type device instead of a real switch with actual contacts ... a test with a multimeter won't give you reliable results in a circuit like that ...

2. If it's a digital output, why it's name is "DC" output?.. this confused me so much.

Click to expand...

DC means that it's designed to work with a Direct Current circuit (as from a battery or a Power Supply) ... the other basic type would be AC – for Alternating Current (as from a household receptacle) ... BOTH of these types (DC and AC) would both be considered "Digital" since they just turn an output device ON or OFF ...

another type of module would be "analog" ... the signal through that type would be OFF or ON - or anywhere in between ...

3. Will the two circuits I mentioned above actually work?

Click to expand...

yes, if everything is done correctly it SHOULD work ...

My motor did rotate for once, but sometimes when I open the switch and closed again, it failed mysteriously and the instructor didn't know why either.

Click to expand...

was your processor in the RUN mode? ...

I know I may be lack of basic electric knowledge since I'm actually from mechanical engineering. Sorry about that!

Click to expand...

well, we've all got to start somewhere ... if your instructor really wants to know more about this, tell him to give me a call ...

Ron Beaufort said:

the module that you're using is a SOLID-STATE type module ... this uses a transistor type device instead of a real switch with actual contacts ... the multimeter test won't give you reliable results ...

did the motor run? ... if not, was your processor in the RUN mode? ... how many amps does your lab motor pull? ... the module is rated for a maximum of 2 amps ... if your load is higher than that, it might have blown the module's output circuit ...

Click to expand...

Thanks. if it works like a switch, so it has no difference (logically) with an relay module (like what we have, 1756-OW16I)?

Thanks. if it works like a switch, so it has no difference (logically) with an relay module (like what we have, 1756-OW16I)?

Click to expand...

they both are supposed to serve the same basic function – to turn an output field device (a motor – a lamp – etc.) ON or OFF ...

the OW16I module has real "switch" type contacts in it (like a relay) ...

the OB8 is a "solid-state" type which uses a transistor-type device to do the switching ... your multimeter test will give VERY confusing results on that type of module ...

My lab assignment today is to connect a pair of output from the DC output module to a relay's coil along with a 24VDC, and another circuit is relay's contact connected to a motor (and its own power for sure, a 14VDC)

Click to expand...

well, if you hooked up THAT part of the circuit correctly, then the OB8 output module shouldn't have carried the full load of the motor - only the current through the relay's coil ...

in other words, if hooked up correctly, the output module shouldn't have been damaged ... so there has to be another reason why the motor only worked once ...

good luck with your project - I'm offline for tonight ... but there are plenty of people here who can help - just be sure to give them enough DETAILS to understand what kinds of problems you're having ...

Ron Beaufort said:

well, if you hooked up THAT part of the circuit correctly, then the OB8 output module shouldn't have carried the full load of the motor - only the current through the relay's coil ...

in other words, if hooked up correctly, the output module shouldn't have been damaged ... so there has to be another reason why the motor only worked once ...

good luck with your project - I'm offline for tonight ... but there are plenty of people here who can help - just be sure to give them enough DETAILS to understand what kinds of problems you're having ...

Click to expand...

Hey, thank you very much. You answered almost all my questions, especially by noticing me that it's a solid-state device (operates by transistor). It explained the multimeter thing.

I thought DC output - judged from the name - is something supplying DC power, like the actual power (a on-table DC supply) I'm using. This confused me the most. Before asking I googled it very hard but there is little information other than manufacturer's manual when question became specific. But when I read the manual again- it does have a schematic of every module. I should read more carefully next time!

Now I have only one question: why we have all these kinds of output modules if they're all just a switch (logically)? I understand that they have different principle/mechanism, and different operating range, but why? Isn't a "real 'switch' type contacts" (relay) good enough for all the cases? Actually in the manual I can see the relay one (OW16I) has the widest range: 10...265V, 5-150V DC 16-point isolated contact module

My guess is, maybe transistor would be much faster to switch compared to relay?

I should read more carefully next time!

Click to expand...

an important lesson has been learned ...

Now I have only one question: why we have all these kinds of output modules if they're all just a switch (logically)?

My guess is, maybe transistor would be much faster to switch compared to relay?

Click to expand...

that's a good guess – and that actually IS a big part of the answer in many cases ...

another consideration is that the solid-state types have NO MOVING PARTS – so (as long as they're not mistreated "overloaded" etc.) they usually last a LOT longer than modules with actual contacts ...

and also whenever a "real" set of contacts open or close, there's usually a tiny (or maybe not so tiny) spark produced ... that sparking will eventually wear away the contact material - and start giving poor connections ... the solid-state modules don't spark - another reason that they last longer ...