Stu,
By "special changes" I guess I mean any changes to the program or settings to "alert" the PLC of an HMI. BTW I am probably at the
Sub-Novice level when it comes to the HMI to PLC world. So naturally i am a little confused when you said "set up the Proface HMI so it Writes to its own Bit(B) or Integer (N) File. So that it is obvious that something is being Controlled by the HMI."
Ok, no problem, we will take it one step at a time.
HMIs Read and Write Asynchronously to/from the PLC over a Communications Link. This is independent of the PLC Program Scan, however, depending on the PLC, how it handles Servicing the HMI Communications Requests vary. The MicroLogix 1000 is a lower end PLC, so it probably only has one Processor doing everything, and will handle the Communications during what is referred to as the Housekeeping Phase.
Traditional PLCs generally have a circular Cycle that repeats continuously.
1)Read the Inputs
2)Solve PLC Program Code
3)Update the Outputs
4)Housekeeping
Do a Google Search for more information on this Cycle.
The Reading from and Writing to the PLC Data Tables happens independently of the PLC Program. No PLC Code is required to support it, other than making sure the Data you want it to Read is in the right place. If you don't do anything, the HMI will simply Read what is in the Data Table, be it right or wrong.
Getting to the Bit and Integer Files. AB PLCs allow you to create your own Data Tables. This is only limited by the Free Memory available.
So, you could create an N:10 File, and have your Proface HMI do all its Reading and Writing to those N10 File Locations. i.e. Read from N10:0 to N10:49, Write to N10:50 to N10:99. Unlike some PLC Brands, AB allows you to access the Integer Files to the Bit Level. i.e. N10:0/0
I hope that helps clear it up some. Continue Reading and Researching, and by all mean feel free to ask question here.
Stu....
