Bit Access¶

Bit access selects a single bit of an integer-typed or bit-string-typed variable. The selected bit reads and writes as a BOOL.

IEC 61131-3	Section 2.4.1.2 (partial access)
Support	Supported (short form); bit form of partial-access syntax supported

Syntax¶

IronPLC accepts two equivalent forms for bit access:

Form	Example	Availability
`variable.n`	`my_byte.3`	Always supported (Edition 2 short form)
`variable.%Xn`	`my_byte.%X3`	Edition 3 partial-access syntax; see below

Both forms denote the same bit and produce the same runtime behavior — they differ only in surface syntax. Bit indices are zero-based, with 0 being the least significant bit.

Bit access composes with other variable references. The bit suffix may follow any symbolic variable, including array subscripts and structure field accesses:

my_byte.3              (* simple variable *)
my_array[i].3          (* array element *)
my_record.field.3      (* structure field *)

Valid Base Types¶

Bit access is valid on any integer or bit-string type:

Type family	Types	Valid bit indices
8-bit	`SINT`, `USINT`, `BYTE`	`0..7`
16-bit	`INT`, `UINT`, `WORD`	`0..15`
32-bit	`DINT`, `UDINT`, `DWORD`	`0..31`
64-bit	`LINT`, `ULINT`, `LWORD`	`0..63`

Accessing a bit outside the valid range raises P4025.

Example¶

Edition 3 Partial-Access Syntax¶

Note

This feature requires IEC 61131-3 Edition 3. Use --dialect iec61131-3-ed3 or the rusty dialect to enable it. See Enabling Dialects and Features for details.

IEC 61131-3:2013 adds the explicit form variable.%Xn for bit access. Semantically it is identical to the .n short form — IronPLC lowers both to the same representation. The Edition 3 form is gated behind --allow-partial-access-syntax and is enabled by default under --dialect=iec61131-3-ed3 and --dialect=rusty.

Using .%Xn without the flag raises P4033.

PROGRAM main
    VAR
        my_byte_array : ARRAY[0..1] OF BYTE := [2#00000101, 2#00000000];
        r             : BOOL;
    END_VAR

    r := my_byte_array[0].%X0;     (* TRUE *)
    my_byte_array[0].%X1 := TRUE;  (* write bit 1 *)
END_PROGRAM

The plc2plc renderer normalizes both surface forms to .n on output; the chosen bit index is preserved.

Byte / Word / Dword / Lword Partial Access¶

IEC 61131-3:2013 also defines partial access at wider granularities:

Form	Selects	Status
`.%Bn`	Byte `n` of a wider value	Not yet supported
`.%Wn`	Word `n` of a wider value	Not yet supported
`.%Dn`	Double word `n` of a wider value	Not yet supported
`.%Ln`	Long word `n` of a wider value	Not yet supported

These forms return a non-BOOL view of the underlying value and require additional codegen work. Until they land, a source that uses them raises P4033 when the flag is off and P0003 (Unmatched character sequence) when the flag is on.