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I'm having trouble with Allen-Bradley MicroLogix / SLC500 PLC Addressing.
Tags: FAQ Networking Data
Allen-Bradley MicroLogix series / SLC500 series PLCs have some specific addressing format requirements for select memory locations (Bit-level and Strings, for example). Both drivers (Serial and Ethernet) follow these rules:

  1. Bit-level objects must reference the bit position to two digits! Bit 0 is 00, bit 9 is 09. For example, B3 memory, word 5, bit 0 [B3:5/0] is B3-500 (bit to 2 digits).

  2. When addressing Word-level registers, choose the file from the dropdown address. For N7:50, select N7 address type and enter 50 in the address field.

  3. If not accessing the B3/N7 address types directly via the drop-down, the File and Word must be specified to three digits. For B10 memory, word 5, bit 8 [B10:5/8], select Bfn address type and enter 01000508 in the address field. For N10 memory, word 50 [N10:50], select Nfn address type and enter 010050 in the address field.

  4. Strings are accessed by separating the sections of the address out with a period (.). String File 10, word 22 [ST10:22] is written as STfn 010.022.00 (the last 2 digits are the index or offset; usually will be .00). With String files, the length of the string needs to be set (# of elements) as well.

  5. I/O addresses are only supported with the MicroLogix PLCs. To access I/O points when using an SLC, you must map those addresses to other HMI-accessible locations (B3 memory area, for example) within the PLC program.
Content Created by David Franzwa