S5 AWL Command: DO (Direct Operation)

DO is a system operation available only on the larger S5 CPUs (S5-135U and S5-155U, specifically CPU 928 and CPU 948). It provides direct access to peripheral I/O and memory areas, bypassing the normal process image update cycle. DO is not available on S5-90U, S5-95U, S5-100U, or S5-115U.

Syntax (S5-135U/155U only)#

DO is used as a prefix modifier for load and transfer operations:

DO  L PW 128        // Direct read of peripheral input word 128 (bypasses PAE)
DO  T PW 128        // Direct write to peripheral output word 128 (bypasses PAA)
DO  L FW 0          // Direct load of flag word 0

S7 Equivalent#

There is no direct equivalent in S7. The closest concept is peripheral access via PEW/PAW in S7-300/400 (e.g., L PEW 128 reads peripheral input directly). On S7-1500, direct peripheral access uses the same PEW/PAW notation. Programs using DO extensively require careful analysis during migration.

Why This Is Important for Migration#

DO commands indicate that the S5 program accesses I/O or memory outside the normal scan cycle. This is often performance-critical code (e.g., fast analog reads, direct hardware control). During migration, the intent behind each DO access must be understood and reimplemented using S7 mechanisms (peripheral access, interrupt OBs, or hardware interrupts).

Availability#

Part of the S5 AWL Command Reference. Maintained by PLCcheck.ai.

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