The following table shows a partial memory map for an Allen-Bradley SLC 500 PLCīy contrast, a “ﬁle” in a personal computer is a contiguous collection of data bits with collective meaning (e.g. However, enough legacy Allen-Bradley PLC systems still exist in industry to warrant coverage of these addressing conventions. Note 1 : The most modern Allen-Bradley PLCs have all but done away with ﬁxed-location I/O addressing, opting instead for tag name based I/O addressing. For these two reasons (popularity and confusion), I will focus on Allen-Bradley addressing conventions for the bulk of this section. The most common brand of PLC in use is Allen-Bradley (Rockwell), which happens to use a unique form of I/O addressing (Note 1), students tend to ﬁnd confusing. 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. 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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