The Allen Bradley PLC has a fairly flexible memory structure. Basically, you can use the memory in the PLC to either have logic/ladder, or store data. But there is a limit to the total memory, so the bigger your program, the less data you can store; the more data, the smaller your program.
The data structure is also flexible. You tell the PLC that you want to use create a data file (from 0 to 255 (in the SLC; to 9999 in the PLC-5)), and what format the data will be in (B for Bit, N for iNteger, and so on). If you are familiar with BASIC programmming, think of it like a DIM statement (
DIM N7(100) as Integer).
By default, the PLC pre-defines certain data files. A data
file is nothing more than a one-dimentional array (for integers (e.g., N17(22)), or a two-dimentional array for more complex data structures, such as timers and counters (e.g., T4(7,PRE), C5(2,DN).
So data file zero is, by default, the output registers o0:, or, more commonly just o
. Data file one is reserved for real-world inputs; two is PLC status, three is general use bits (B3
, four is timers (T4
, five counters (C5
, six "control elements" (R6
, seven is for integers (N7), and finally data file eight is reserved for floats (F8
. Data files nine and above are for however you want to assign them. You can make file ten an integer (N10
, or a floating point file (F10
, but you can't make it both.
When PLC professionals talk about "N7 registers", we mean one of two things. We could be referring to those memory addresses that are actually in the N7 data file, or we use the term generically, and mean any data file formatted as an integer. Since I don't know if you prefer would want to put your telemetry data in N10:, or N17, or even good ol' N7:, we'll just say N7 and use the term generically.
Now to your last question.
Is it common ptarctice to map all points to the N7 register and the select points for Telemetry?
When someone is just starting with PLCs, there is alot to think about, and so they usually just use the default registers for everything. All timers use T4:, all integers use N7, and so on. This works just fine, and is a very common practice, especially on simple systems. They also put the entire program in LAD2
The more experienced programmer starts to apply some organization to his program. He puts ladder logic that pertains to a specific machine or function in a different LAD file, and also all the memory registers associated with that machine in seperate data files. This way, he can break down his tasks into smaller, more managable chunks, and can go straight to the telemetry data, because it is all in file N10:, and not have to sift through all the other information stored in N7:
Eventually, you get to the point where you NEVER use the actual N7 register for anything.
