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Old January 13th, 2022, 11:31 AM   #1
wsptay
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Plc 5

How many control blocks / messages can you have on one processor for plc 5?
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Old January 13th, 2022, 11:38 AM   #2
Operaghost
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Memory is the only limit.

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Old January 13th, 2022, 11:42 AM   #3
wsptay
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Do you know off hand how much memory does one message take?
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Old January 13th, 2022, 12:11 PM   #4
Jim3846
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I know in contrologix there was a connection limit for every processor.
Ethernet connection MSG was 1 connection.
HMI server sometimes have multiply connection per plc.
Total connections are in a manual.
I even think there is a limit in a SLC500 plc

If you look in the PLC5 instruction pdf I think it gives how much memory a MSG instruction takes plus the data table that your sending.

But PLC-5 I do not see it. I think your limited by if your msg works all the time without locking the processor up. So its a put it in and see if it works. Just remember PLC5 is not a fast processor, scan time 20-40msec usually is the fastest i have seen. Then if your doing Ethernet its better to use an enhanced PLC5. Not sure about the non enhanced PLC5.
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Old January 13th, 2022, 12:33 PM   #5
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Quote:
Originally Posted by wsptay View Post
Do you know off hand how much memory does one message take?
It varies depending on what CPU you are using and if your messages are going over Ethernet (more memory) or another network type. The MSG uses Program memory for storing the instruction and Data memory for storing the configuration of the message.

Worst case scenario is using an MG data type over Ethernet. Each MG data block consumes 56 words of data table memory. Ethernet requires two of these MG blocks for each message. So, 112 words of data table memory would be used for each message. Then you still must program the logic. It looks like the MSG instruction requires 7 words of Program memory. That is what I am observing.

So, if I had 100 messages, we would be looking at 11.2k of memory for the configuration data. If I had a PLC-5/40 controller, it has a total of 48k memory for both data and logic.

If you are not using Ethernet, cut that number in half.

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Old January 13th, 2022, 01:00 PM   #6
Operaghost
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Quote:
Originally Posted by Jim3846 View Post
I know in contrologix there was a connection limit for every processor.
Ethernet connection MSG was 1 connection.
HMI server sometimes have multiply connection per plc.
Total connections are in a manual.
"Connections" as we know them in the Logix 5000 don't tend to be the issue with the PLC-5. Usually, it is the message buffers that might be an issue. Sort of a similar concept though.

The PLC-5 has message buffers for each channel where it will queue up messages while they wait to be sent. Issues can arise when you are trying to send more data faster than the network can handle. As a result, the messages buffers can fill up.

The size of those buffers varies by connection type. The DH+ buffers I believe can hold thirty messages for each channel. Not sure exactly what Ethernet supports. I recall it being sixty-four.

Ideally if I had a bunch of messages, I would try to stage them in a sequence so they aren't all trying to send at the same time.

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Old January 13th, 2022, 03:51 PM   #7
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Quote:
Originally Posted by Operaghost View Post

Ideally if I had a bunch of messages, I would try to stage them in a sequence so they aren't all trying to send at the same time.

OG
That is typically what i see in PLC5 programs. Don't overwhelm the PLC is the rule.
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Old January 14th, 2022, 12:30 PM   #8
mylespetro
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Quote:
Originally Posted by Jim3846 View Post
That is typically what i see in PLC5 programs. Don't overwhelm the PLC is the rule.
Just used Rockwell's recommended message staging procedure on a ControlLogix handling a large amount of data literally earlier this morning, replaced an uncontrolled/rapid-fire messaging and it worked great. Processor used to get overloaded very easily and freeze requiring someone to pull the processor and reseat it, now it's communicating just as fast but with only one connection at a time.

I actually adapted the message control program from a PLC-5/RSLogix5 template sent to me by my local Rockwell representative into a Logix5000 equivalent.

Last edited by mylespetro; January 14th, 2022 at 12:34 PM.
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Old January 14th, 2022, 12:48 PM   #9
mylespetro
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For anyone interested, I've attached a zip file of Rockwell's recommended message control method, there's a KB TechNote PDF as well as a PLC5 and a Logix5000 program.
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File Type: zip Message Management.zip (721.8 KB, 5 views)
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