Declaring variables: VAR, VAR_INPUT and RETAIN

Everything is declared first

ST is strongly typed: there is no such thing as an undeclared variable. The section it goes in defines its role:

VAR_INPUT
  Start : BOOL;
  SetPoint : REAL := 75.0;   (* initial value *)
END_VAR

The sections

The difference that catches people

VAR keeps its value between scans; VAR_TEMP does not: it starts from zero every scan. Put a counter in VAR_TEMP and it will sit at 1 forever.

RETAIN: what survives a power cut

VAR RETAIN
  TotalBatches : DINT;
  RunHours : REAL;
END_VAR

An ordinary variable returns to its initial value on restart. A RETAIN variable is held in retentive memory and comes back as it was. Use it for totals and running hours, and not for machine state: after a power cut the machine must start from a known safe state, not from the middle of a fill.

Constants instead of magic numbers

VAR CONSTANT
  HiAlarm  : REAL := 85.0;
  OverFill : REAL := 95.0;
END_VAR

Now IF Level > HiAlarm reads itself, and the limit changes in one place.

Naming

A name starts with a letter or _, takes no spaces or symbols, and is case-insensitive (Pump and PUMP are the same variable). Plant practice: name the variable after the tag on the P&ID, such as LT101_Level.