Help understanding msg in rslogix 500

stu

Member
Join Date
Aug 2005
Location
England
Posts
773
hi guys
i have a question , i have two machines linked together via a msg instruction they are only using a couple of bits , what i would like to do is transfer an alarm bit to a hmi on the other machine could i use the unused bits to trigger the alarm, if so is it as easy as using the bit input to the other plc ??
on the msg instruction i understand that you enter a word address in the controller data table and the size of elements is the bits? please correct me if im wrong
and then you enter where the destination data table will be and ip address,
but how would i switch an alarm bit? i hope this makes sense
thanks stu
 
The minimum data size for a SLC500 MSG is one word, so if only a few bits of that word are used, you could use the unused bits for something else. The size in elements depends on what data you you are reading / writing. Integer data 1 element = 1 word, timer 1 element = 3 words.
 
"Size in Elements" within the MSG instruction configurator refers to the number of 16-bit consecutive words/elements.
Be careful when deploying; the MSG will overwrite anything within its configured "footprint"!
 
Last edited:
so if the plc that is sending the info could I have a switch energize say N18:2 and that will move the info to the other plc ?
-----| |----------( N18:2) is that right
 
A switch's state represents one binary bit of information; the MSG instruction will read/write a minimum of 16 bits of information.
You could MOV 1 within the intended MSG buffer integer word address when XIC Switch is True, however, this will be a waste of resources.
Address both applications at the bit level of the "messaged" integer and you will be able to transfer sixteen times more relevant data.
 
so if the plc that is sending the info could I have a switch energize say N18:2 and that will move the info to the other plc ?
-----| |----------( N18:2) is that right


N18:2 is a word reference. To use the third bit in that register as a coil reference, you would address it as N18:2/2. And yes, if N18:2 is being transferred to your other PLC, then the status of bit N18:2/2 would be reflected in the third bit of the word specified as the message target in your other PLC.
 
depending on the hmi and your communications setup, you could just set the HMI to communicate with the other SLC and pull the tag/bit directly. HMIs can communicate with multiple PLCs at once.
 
depending on the hmi and your communications setup, you could just set the HMI to communicate with the other SLC and pull the tag/bit directly. HMIs can communicate with multiple PLCs at once.

That's interesting, I'm using a panelview plus 700, how do I do that then thanks stu
 
That's interesting, I'm using a panelview plus 700, how do I do that then thanks stu


If you choose to go that route, remember that the bit transfer will not work when your HMI is off-line. While using an HMI as a virtual router is convenient, especially when transferring data between two dissimilar PLCs, it is not the most reliable method and I prefer to avoid using it unless absolutely necessary.
 
Another advantage of using the MSG instruction is that you can write data only on exception if you are transferring bits that don't change state very often and thereby reduce redundant traffic on your network.

To do this, simply move your data transfer word into a "sample and hold register" on each sweep. Then insert a rung prior to this data move that compares the value of your data transfer register with it's value on the previous sweep. Energize an internal output bit whenever the two values are not equal, then trigger the message function with that output bit.
 
Hi guys I am looking at the plc program tonight and I have found the read and write msg in the plc I want the input which say this controller N127:0 size 10 , target device N37:0 which I understand is the word from where I won't to send from if I put a switch in on N 37:0/3 will it switch N127:0/3 in the other plc becuase I did try and test it but nothing I looked at the data table and nothing moved am I doing something wrong
 
Hi guys I am looking at the plc program tonight and I have found the read and write msg in the plc I want the input which say this controller N127:0 size 10 , target device N37:0 which I understand is the word from where I won't to send from if I put a switch in on N 37:0/3 will it switch N127:0/3 in the other plc becuase I did try and test it but nothing I looked at the data table and nothing moved am I doing something wrong

You need to zoom into the MSG function block Setup Screen and determine whether you are looking at a "read" MSG command or a "write" MSG command. If the MSG function's Communication Command is "500CPU Write", then what you are attempting to do will not work. If that is the case, then you need to either add a read MSG function in the same PLC or add a write MSG function in the other PLC. A single MSG function does not provide two-way data traffic.
 
it is nearly midnight and i still cant get my head around this msg instruction, the read write instruction is already being used i am just wanting to use a couple of spare bits for an alarm plc1 has the read instruction which i would like to send the alarm to the hmi. i need to send a bit from plc2 to say alarm ,

msg instruction
this controller =n127:0
device = N36:0

N36:0/3 trigger for alarm
how where does the output alarm trigger go? i have seen the trigger in data table but im stuck /thick
learning hurts my head lol
 

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