From AO of one PLC to AI of another PLC

novice · Apr 24, 2008

My project requires three CompactLogix PLCs to communicate status with each other (unit running, unit tripped, recycle valve opened, etc). I am thinking of transmitting this information in a 16-bit register, each bit representing a status information about the PLC. It will be send through an AO channel of one PLC to an AI channel of another PLC. The range is 0 to 16383, which both 1769-IF8 and 1769-OF8C do provide.

So my question: has this been done in any projects before, and what complications should I be aware of? We have always used one PLC for each job, so this is the first time the customer wants three independent PLCs in the same job.

rta53 · Apr 24, 2008

You can do this through the comm network if you network the 3 PLCs together? Are you planning to do this? Ethernet perhaps?

ghettofreeryder · Apr 24, 2008

I am gonna say this is a bad idea. If you have any interference at all, the signal will change. If this is critical, you can use a 16 pt digital card. The best solution however, would be using a form of digital communication, such as Ethernet, Controlnet, or DF1. What processor are you using?

Ron Beaufort · Apr 24, 2008

this idea won’t work ... the analog signals always “drift” by a few bits - even when the signal is supposedly “steady” ... this would translate into errors in your binary bit patterns ...

allscott · Apr 24, 2008

X3, It's not that this won't work well, it won't work at all for reasons that Ron mentioned.

TConnolly · Apr 24, 2008

You will be giving your customer never ending headaches.

For less money than the analog IO costs you can network them together and message the information between the PLCs and give your customer a professional 21sth century solution instead of a rinky-dink solution.

novice · Apr 24, 2008

ghettofreeryder said:
I am gonna say this is a bad idea. If you have any interference at all, the signal will change. If this is critical, you can use a 16 pt digital card. The best solution however, would be using a form of digital communication, such as Ethernet, Controlnet, or DF1. What processor are you using?

1769-L35E. Doesn't have ControlNet. Am trying to look into Ethernet option.

Thanks for the feedback, everyone.

TConnolly · Apr 24, 2008

Its a piece of cake to network L35Es together with ethernet and you'll be able to share large amounts of data. The L35Es can read/write directly to each other's tags. The only harware needed will be an ethernet switch. Search the forum for "MSG" to see how to use messaging.

widelto · Apr 24, 2008

novice said:
So my question: has this been done in any projects before, and what complications should I be aware of? We have always used one PLC for each job, so this is the first time the customer wants three independent PLCs in the same job.

I have a project like yours running for almost two years and I use produced/consumed tags I do not use msg, I hace 5 1769-L32E connected thru a N-tron switch and I never have a problem.

Singapore_Mats · Apr 24, 2008

as sayd above, it will give many headaches, for example if you restart one plc the other will see signal change and repond wrongly.

allscott · Apr 24, 2008

Singapore_Mats said:
as sayd above, it will give many headaches, for example if you restart one plc the other will see signal change and repond wrongly.

As I stated above, it just won't work at all, don't even try do do it.

yfcarer · Apr 25, 2008

Connect your PLCs by an ethernet network is the simplest solution for your question. It will not cost so much and it works well.

I agree with allscott's words. Using the AI->AO method for communicating such Status signals between each PLCs will not work at all at current stage.(Who knows what technology can do next??)

Craig Spinharney · Apr 25, 2008

Produced/Consumed

As widelto alluded to, I would use produced & consumed tags. Each controller would know the status of the other two without MSG instructions.

A benefit would be that the data is still sent though a controller may not be in the RUN/RemoteRUN mode.

In addition, an array or UDT could be used to facilitate future needs and the data space may already be available (and being sent).

From AO of one PLC to AI of another PLC

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