Arrays in S7 Programming

OK, Folks,

Since I am between projects (my next PLC is still on order), I have not tested the code in the attachment, so I cannot be certain whether it works. However, I am laying out what I hope is a viable function for accessing data in any single-dimension array of elementary data types.

The code is written in the dark speech of STL,at which this is my first attempt. STL was chosen since it was the only S7 programming language (for S7 300 and 400) in my possessing which has capability for dealing with indirect addressing. I do not possess SCL.

The attached zip file contains an .AWL file which imports to Step 7 as an STL Source file. From there, the source may be compiled into the actual function. My default function number is 200, but this is easily adjusted in the source code before compilation if necessary.

The interface consists of one ANY-type input (Arr_In) and an INT-typeindex input (NDX) for selecting which item in the single-dimension array is to be chosen. Because this is a function, the Ret_Val output is also of the ANY-type. By using the ANY-types, the function should be able to accept an Array input and output the data type as whatever elementary data type of which the Array input consists.

On the downside, there is no fault checking for non-array inputs nor is this function intended to support arrays of a complex or user-defined data type. Also, the function was not created with multi-dimension arrays in mind.

However, if the function works as intended, the principles used in it may be adapted to complex or multi-dimension arrays.

Note on Address Registers and Compiler Warnings:
The source compiler will generate a warning about the use of address register 2 (AR2), since messing around with this register can foul the system's access to function block data. Apparently, this warning shows up whenever the LAR2 instruction is compiled. My effort to avoid side-effects relating to this usage is to save both values of AR1 and AR2 when the function is first called, and then restore these values to AR1 and AR2 just before the function exits.

Note on Usage of the ANY-type in the Editor:
I notice that when I use this function and attempt to enter symbolic references for the array input in the FBD editor, data blocks are visible, but no variable or array names are visible. I suspect that this is because none of the DB variablesare defined as the ANY-type. Indeed, further investigation reveals that a variable in a shared DB cannot be defined as an ANY type. For this reason, the symbolic array reference name must be manually entered in its entirety, i.e. the editor will not auto-complete the array name in an ANY-type function input.

Perhaps this will be of use to somebody. Since it may be some time before I can test this myself, any comments, suggestions, or criticisms are welcome.

The Captive Engineer