IsINF / IsNAN — Test for Infinity or Not a Number

Is Infinity (IsINF). True when the Source value is positive or negative infinity, such as the result of 1.0 / 0.0. Both signs pass power — there is no separate test for each.

Is Not a Number (IsNAN). True when the Source value is not a number, such as the result of 0.0 / 0.0.

Decimal maths can produce answers that are not measurements at all. A REAL tag can end up holding infinity, or NaN — short for "not a number". Neither is a reading you can act on, and neither announces itself.

Where a bad REAL comes from

Every one of these is the result of ordinary arithmetic on ordinary tags:

A flow calculation that divides by a speed reading is one bad scan away from the first row: the moment the line stops, the divisor is zero.

SQRT is the exception. SQRT(-9.0) gives 3.0, not a NaN, because square root works on the size of the number and ignores the minus sign. That means a negative reading gives you a plausible-looking answer instead of an obvious fault, so check the sign separately if it matters.

Why a compare won't catch it

The obvious rung — compare the tag and raise an alarm — catches one of these values and is completely blind to the other:

Infinity compares normally. It is simply bigger than every number (or smaller, if negative), so a high-limit alarm does catch it — but it tells you the limit was passed, not that the maths broke. IsINF is how you tell a genuinely huge reading apart from a divide by zero.

NaN fails every comparison, including being equal to itself. GT, LT and EQ all answer "no" forever, so a rung written to catch a bad value never fires and nothing looks wrong. IsNAN is the only test that answers yes. (NE is the mirror image: it answers true for a NaN, because "not equal" is the one comparison a NaN satisfies.)

Using them in a rung

Each instruction reads a single REAL tag and decides the rung, exactly like a contact.

Both live on the Compare tab of the ladder toolbar, next to EQ and its friends. Each rung below raises the same Sts_FlowBad alarm — one for infinity, one for NaN:

A tag can be one or the other, never both: infinity is a direction, NaN is "no answer at all". To catch either, put the two boxes on parallel branches, or write the single CMP on the next page.

Operands

source must be a tag, or one element of a REAL array — IsINF(Samples[2]) is fine. A typed-in number is not accepted, and neither is a DINT or a BOOL tag. That is not a Studio limitation: a whole number has no way to be infinite, so there would be nothing to test.

Like a contact, not like a coil

Both are input instructions, so they go to the left of the rung and something else ends it. A rung that finishes on IsINF will not compile, the same way a rung that finishes on XIC will not.

When the rung in front of them is false they are not executed at all, and the rung simply stays false.

The same tests inside CMP

CMP accepts IsINF and IsNAN as functions inside its expression, which is how you ask about two things at once:

CMP(IsINF(FlowRate) || IsNAN(FlowRate)) OTE(Sts_FlowBad)

That rung is true for infinity and for NaN, in one box. The expressions page lists everything else an expression accepts. They are the only two functions CMP takes that CPT does not — CPT has to produce a number to store, and "is this infinite?" is a yes-or-no answer.

They are not available in Structured Text.

Reading the value on screen

A REAL tag holding one of these shows as Infinity, -Infinity or NaN in Studio's tag list, so you can see at a glance which one you have.

Studio 5000 Logix Designer® spells them very differently, and a tag's Style setting changes the spelling again:

Both of the last two rows are still a NaN, and IsNAN passes power for either — read them as one condition, not as two different faults. Neither program lets you type any of these spellings into a tag, which is the same point as before: a bad REAL only ever arrives from arithmetic.