Tank setpoint limiter + alarm (FBD)
Two-network FBD that clamps an operator-entered tank-level setpoint to the safe range [0, 95]% via LIMIT, and raises an alarm if the operator types an out-of-range value. Uses LT and GT comparison blocks, the LIMIT selection block, and demonstrates REAL (orange) and BOOL (black) wires carrying different data types in one diagram.
For the process-control engineer
When an operator types a tank-level setpoint into the HMI, you can't just trust the number. Maybe they meant to type 75 and hit 750. Maybe they typed -5 because they were copying from a spec sheet that used a different reference. Maybe the HMI's input field doesn't have validation and the value is whatever bit-pattern the OPC UA bridge happened to land on. The PLC always validates, and the canonical pattern is: clamp to a safe range, then raise an alarm if the requested value was out of range so a human knows to check.
Network 0 — LIMIT. SafeSetpoint = LIMIT(MN: 0.0, IN: DesiredSetpoint, MX: 95.0) is the clamp. LIMIT takes three REAL inputs and returns a REAL: the value of IN if it's in [MN, MX], otherwise MN or MX. Both bounds are inline constants — they render as yellow boxed text at their ports, no wire needed. The downstream control loop reads SafeSetpoint, never DesiredSetpoint directly. Even if the operator's input is corrupted, the tank can never be commanded outside the physically safe range.
Network 1 — out-of-range alarm. OutOfRange = OR(LT(DesiredSetpoint, 0.0), GT(DesiredSetpoint, 95.0)) is an inline expression that nests two comparison blocks inside an OR. LT (less-than) returns BOOL when its first input is less than the second; GT (greater-than) is the opposite. The OR fires when either fires — the operator's value was either too low or too high. The Alarm = MOVE(OutOfRange.OUT) then drives whatever HMI alarm channel — a red banner, a Slack notification, a SCADA event log entry.
Two data types in one diagram. Look at the wire colors in the rendered SVG: the wire from DesiredSetpoint into LIMIT.IN is REAL (orange — IEEE 754 floating-point); the wires from LT.OUT and GT.OUT into OR are BOOL (black). The LIMIT.OUT is also REAL (orange). One of the FBD engine's small but pleasant features: the renderer infers each wire's type from the source port and colors it accordingly, following the TIA Portal convention. If you accidentally wired an INT to a REAL port the colors would mismatch at the junction and you'd notice immediately.
Why not ladder? Ladder logic excels at boolean signal routing — contacts in series and parallel feeding into output coils. It has zero affordance for REAL arithmetic and comparison; you'd write the LIMIT expression as a structured-text "function block" call inside a ladder rung, which kills the visual semantic. FBD makes the math first-class. For per-scan combinational logic involving any non-BOOL signal, FBD is what the IEC 61131-3 standard expects you to use.