PLC to PLC I/O

jwright

Member
Join Date
Feb 2003
Location
Pennsylvania
Posts
26
Hello,

I was wondering if any of you ever wire the output of a plc to an input of another plc. This would be done with stand alone plc driven machines using AB micrologix controllers and without a network.

Basically, I am sequencing plc controlled machines from a master plc. It seems to work fine. I think my main limitation here is in the loss of monitoring capability. Would this method be termed cascasing plcs or a master-slave arrangment?

Please advise as to other issues with this (pros/cons). The application is educational in nature - I am an electronics professor and do not have the budget this year to network the system.

Thanks for your help and expertise in advance.
 
Hello jwright;

In many of our projects, we do link I/O points between PLCs as a cheap and dirty way to exchange digital information. Care must be taken to examine the requirements of different modules; we have to install pull-up resistors sometimes on the circuits, for example.

I don't think there is any need to categorize and find "suitable" terms for this type of connection. "Master/slave communication" is already used in protocools of communication, "cascading" is used in PID loop definition. If you must give it a defining name, you could try something like controller or process synchronisation. I know academia (and I don't mean this with disrespect, I've taught both in college and university) thrives on pidgeon-holing definitions, but this does not help the technician in the field.
Hope this helps,
Daniel Chartier
 
Terminology

:nodi: Thanks Dainel,

Yes perhaps we do this in academia. I just want to accurately describe the process for documentation purposes - would not want to be criticized by you pros! Of course, I could always invent another term - we like to do this in academia you know o_O .

Seriously though, I had been leaning toward the term "cascading plcs" because we use it sometimes to describe a similiar arrangement with relays -- cascading relays...? just don't want to confuse folks with the terminology.

Well thanks again.

jw
 
This is an extremely common way of linking two bits of plant that need to pass signals. Generally the IO is run through interposing relays, and usually it will be specified in a "Functional Specification" as to how many are needed and what their duties will be. An example, I am dealing with installing a new strapper on one of our lines. The line is controlled by an S7-400 PLC and the strapper is fitted with a small S7-200. These communicate with a total of 5 IO each way (including one spare).

Now, to your pros and cons.
Pros:
- It is easy to setup
- More reliable and determanistic than serial
- It has no conceptial difficulties for ordinary electricians to trouble shoot
- Most manufacturers will automatically set their machines up to communicate this way (normally referring to these as "Dry Contacts")
Cons:
- Normally suitable for only small amounts of data
- Can be subject to noise if the inputs are of high impedance
- Use up sometimes limited IO
- Difficult to pass diagnostic information
- Can be difficult to upgrade, especially if you need to run new cable over some distance
- Often needs a large, heavy cable for communication between the two PLCs. Example is the first machine I worked on needed a 50 core cable to link it to the upstream line, this was later replaced by a light 2 core Modbus Plus cable.

Hope this helps,

Doug
 
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pros and cons

Thanks Doug! That's great. I appreciate your help. Do you call this plc set-up/wiring by a certain name in your work?

jw
 
jwright said:
Thanks Doug! That's great. I appreciate your help. Do you call this plc set-up/wiring by a certain name in your work?

jw

Normally we just call it "interfacing". I will be asked "Can you provide me with 4 outputs and 3 inputs for interfacing?"
Or "We will provide 6 volt free contacts on our machine and we require 4 volt free contacts from your machine." This is followed by an IO list of what these contacts will be doing.
Analog IO is sometimes used as well.
Also note, serial communications is also called interfacing, and is becomming more common.
 
interposing relay use

Doug,
Would I need to run interposing relays if I'm just using the 24VDC input PLC source voltage. This source has very limited current capability and I want to reduce the complexity of system whenever possible.
I believe the purpose of the interposing relays is to protect the outputs from excessive current of the PLC that is connected to the input of the other plc.
Thanks,
jw



 
jwright said:
Would I need to run interposing relays if I'm just using the 24VDC input PLC source voltage.
You don't HAVE TO, but isolation never hurts. If you don't like relays, you can always go solid-state.

🍻

-Eric
 
Hi,
I would suggest the use of interposing relays depending on the power-source of both PLC's. If both have the same power-source or when 2 separate sources are on the same equipotential (I hope i translate this correct) then the relays are not really needed necessary.
Gr.
Heino
 
wiring clarification

example of the plc to plc wiring that I am doing:

PLC #1: -24V (fused) to Com, Input (I1) to output (O2) of PLC#2
PLC #1: +24V to output VAC/VDC associated with O2 of PLC#2
 
You don't have to use interposing relays.
We do because we have a lot of old systems that use anything from 5V DC to 250VAC and everything in between. Manufacturers also do it, because when interfacing it is suprising the number of contractors who read "24VDC" = "110VAC", with interposing relays this is not such a problem.

If you are just doing this for the class room, then you can probably avoid using relays, or maybe just wire in one or two to show the students what they will probably encounter out in the field.
 
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jw, in my neck of the neighborhood they are called interlocks. Generally, a dry contact output is used on one of the PLCs allowing the other PLC to use a AC or DC input. As stated before, care must be taken and documentation is critical, especially with regards to safety. A panel may have its power disconnected for maintenance but interlocks may be remotely powered and still active. Yellow wire with a blue stripe for DC and red stripe for AC is a common way of denoting remotely powered wiring. Its fairly common when two pieces of equipment need to pass simple signals back and forth, such as a robot handling a product and its placing it into another machine. hope this helps.
 
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Relays make life easy in this type of situation. If using transistor outputs to inputs in another PLC, one would normally have to use open collector outputs, maybe pull up resistors etc. We do not know the capabilities of your students and whether they can understand the operation of solid state interfacing. Dry contact relays are very easy to explain and understand.

My 2 cents.
 
I too call these interlocks or sometimes permissives. Very common in packaging lines where the individual units are from different vendors. Machine A receives a permissive from machine B that it is on line and ready. Machine C passes a pemissive to machine B, and so on.
 

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