In Allen-Bradley PLCs such as the SLC 500 and PLC-5 models, files 0, 1, and 2 are exclusively reserved for discrete outputs, discrete inputs, and status bits, respectively. For example, the first bit (bit 0) of the second element in file 3 (Binary) would be addressed as B3:2/0. Elements are addressed by number following the colon after the file designator, and individual bits within each element addressed by a number following a slash mark. Within each of the Allen-Bradley PLC’s “files” are multiple “elements,” each element consisting of a set of bits (8, 16, 24, or 32) representing data. a word processing file or a spreadsheet file), usually stored on the computer’s hard disk drive. By contrast, a “file” in a personal computer is a contiguous collection of data bits with collective meaning (e.g. In the SLC 500 controller, a “file” is a block of random-access memory used to store a particular type of data. Note that Allen-Bradley’s use of the word “file” differs from personal computer parlance. The following table shows a partial memory map for an Allen-Bradley SLC 500 PLC: For these two reasons (popularity and confusion), I will focus on Allen-Bradley addressing conventions for the bulk of this section. One of the most common brands of PLC in use in the United States is Allen-Bradley (Rockwell), which happens to use a unique form of I/O addressing students tend to find confusing. Allen-Bradley (Rockwell) ControlLogix and CompactLogixĪllen-Bradley (Rockwell) Address-Based Programming.This makes it difficult to write a general tutorial on PLC addressing, and so my ultimate advice is to consult the engineering references for the PLC system you intend to program.ĭifferent generations and brands of PLCs adopt different styles of addressing, and it is helpful to explore many of these methods so that you can become more familiar with a style suited to your own programming environment. The association between I/O points and memory locations is by no means standardized between different PLC manufacturers, or even between different PLC models designed by the same manufacturer. Similarly, analog input and output channels on a PLC correspond to multi-bit words (contiguous blocks of bits) in the PLC’s memory. Virtually every microprocessor-based control system comes with a published memory map showing the organization of its limited memory: how much is available for certain functions, which addresses are linked to which I/O points, how different locations in memory are to be referenced by the programmer.ĭiscrete input and output channels on a PLC correspond to individual bits in the PLC’s memory array. This is sage advice for any programmer, especially on systems where memory is limited, and/or where I/O has a fixed association with certain locations in the system’s memory. A wise PLC programmer once told me that the first thing any aspiring programmer should learn about the PLC they intend to program is how the digital memory of that PLC is organized.
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