TwinCat ST编程报错求助：TON/TOF相关语法错误排查

Hey there, let’s work through these two TwinCAT ST programming issues—they’re totally fixable once you spot the root causes!

Error 1: (TRUE AND TON_01.Q) is not a valid assignment target #

This happens because in Structured Text, the left side of an assignment (:=) must be a writable variable (like an output, memory bit, or data block variable). Expressions like TRUE AND TON_01.Q are just calculated boolean values—they’re read-only, so you can’t assign something to them.

What you might have written (wrong):

// Trying to assign to an expression—this won't work
(TRUE AND TON_01.Q) := MyOutputVar;

The fix:

If you want to set a variable based on that expression, flip it around (assign the expression’s result to your writable variable):

// Correct: Assign the expression result to a valid variable
MyOutputVar := TRUE AND TON_01.Q;

Or if you need to conditionally set a variable, use an IF block instead:

IF (TRUE AND TON_01.Q) THEN
    MyOutputVar := TRUE;
END_IF;

Error 2: Q is not an input of TOF #

TOF (Time-Delayed Off) function blocks have strict input/output pin definitions. Q is an output pin (it tells you when the timer is active), not an input. The input you use to trigger the timer is IN.

What you might have written (wrong):

// Trying to use Q as an input—this misuses the FB's pins
TOF_01(Q := MyTriggerSignal);

The fix:

Use the correct IN input pin for your trigger, and route the Q output to your variable if needed:

// Correct TOF usage: map trigger to IN, output to your variable
TOF_01(
    IN := MyTriggerSignal,
    PT := T#3S, // Set your desired delay time here
    Q := MyTOFOutputVar
);

Just double-check that you’re referencing the right pins for TON/TOF blocks—TwinCAT’s built-in FB documentation (accessible via the IDE) has clear diagrams if you need a quick refresh!

内容的提问来源于stack exchange，提问作者dieterkohlen