FAQ: What is the difference between Z registers and A registers when used in Function Blocks?
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Z and A registers are both working registers that are used inside function blocks
Z registers are Global Internal Function Block registers consisting of 64 words of shared memory space within the CPU (Internal refers to internal to the function block). These registers can be represented in bit, 16-bit word, long word, and floating point, and are working registers available for use specifically within function blocks. Since they are global, several function blocks have access and can modify the stored value.
A registers are Local External Function Block registers (External refers to registers used inside the function block, but register space that is reserved outside of the function block). When a function block is defined, a block of memory is reserved by defining the starting address and the data size. This block of memory can be defined as General Global register space (M registers) or Local Drawing register space (D registers). Example: If a function block is defined at address DA00010, and to utilize 10 registers, then registers DW00010 thru DW00019 should be reserved by the programmer for the use of that specific function block. In this case, AW00000 would be mapped directly to DW00010 each scan. When the function block is being used in a drawing, to monitor an internal working register such as AW00000 in this example one would monitor DW00010.
- If D registers are used, then A registers are limited to the D register limit for local drawings - quantity of 16383 per drawing. D registers consume programming memory and expand according to need (limited to programming memory available and number of drawings).
- If M registers are used, then A registers are limited to the M register limit globally for the controller - quantity of 32767. M registers have a fixed memory space.
For more information refer to the Ladder Users Manual
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