Sinking, sourcing, PNP, NPN: how to read an Allen-Bradley input card without guessing

I have watched this argument run on the same forums for close to twenty years, and it opens the same way every time. Somebody lands a proximity switch on a perfectly good input card, the sensor LED lights when the target goes by, the input never moves, and the thread underneath fills with people confidently contradicting each other about what sinking means. There is a PLCTalk thread from February 2025 actually titled "Oh no, not another sourcing sinking PNP NPN post," which tells you how worn out everyone is with it.

Here is the part that took me too long to notice. Most of those people are not wrong. They are using two different naming conventions and nobody says which one out loud. Below is how Rockwell words it in its own manuals, why that wording catches people off guard, and the one measurement that skips the argument entirely.

Rockwell names the card, not the sensor

Open any 1769 Compact I/O installation instruction and the same footnote sits under the wiring diagram. Sourcing and sinking describe current flow between the I/O module and the field device. A sourcing I/O circuit supplies current to a sinking field device. A sinking I/O circuit is driven by a field device that sources current. Field devices tied to DC common are the sinking ones; field devices tied to +V are sourcing.

Follow that through and you land somewhere most people do not expect. A PNP sensor sources current. So the card that accepts a PNP sensor is, in Rockwell language, a sinking input.

The MicroLogix 1100 user manual says it even flatter. A sinking input energizes when high level voltage reaches the input terminal, and you wire the supply VDC negative to that group's COM. A sourcing input energizes on low level voltage, and the supply VDC positive goes to COM.

Read that twice. On a MicroLogix 1100, the input Rockwell calls sinking is the active high one.

Two identical terminals, different COM connection: this is what determines whether the input is sinking or sourcing.

The other convention, and why both survive

A lot of vendors, trainers and technical writers do the opposite. They name the input after the sensor it takes, so an input that accepts a PNP sensor gets called a sourcing input. That is not sloppiness. It is internally consistent too, just anchored to the field device instead of the module circuit.

The trouble is what happens when the two meet. Two spec sheets can both print the phrase "sourcing input" and mean opposite wiring. Neither one is going to tell you which convention it picked.

I stopped arguing about the vocabulary years ago. Ask what the common terminal is tied to and the whole thing evaporates in about fifteen seconds.

The check that settles it

COM tied to DC negative means the group takes PNP sensors, and the input goes active once its terminal sees +24V. Rockwell's word for that is sinking.

Flip it around and COM goes to DC positive: now the group wants NPN sensors, and the input only goes active when its terminal gets pulled down toward 0V. That one Rockwell calls sourcing.

Neither is a property of the card sitting in the box. On a MicroLogix 1100 any of the embedded DC input groups can be set up either way depending on how DC COM gets wired. The 1769 Compact input cards work the same: each isolated group runs in sink or source.

A 1769-IQ16 splits its inputs into two isolated groups, 0 through 7 and 8 through 15. Step up to a 1769-IQ32T and that becomes four groups of eight. Each group can go its own way, which means one card can legitimately carry PNP sensors on one group and NPN on another. It also means whoever wired the panel before you may have done exactly that without writing it down anywhere.

The thresholds that actually decide whether an input turns on

Almost every article on this subject says 0V is off and 24V is on. The cards do not work that way, and the band in the middle is where the intermittent faults live.

On a 1769-IQ16 and a 1769-IQ16F, off state runs up to 5V DC and on state starts at 10V DC minimum. Anything between those two numbers is not guaranteed to be read as either. Operating range is 10 to 30V DC at 30 degrees C, tightening to 10 to 26.4V DC at 60 degrees C. Minimum on-state current is 2.0 mA into a nominal 3 kilohm.

Now put a 1769-IQ32T next to it. Same family, same "24V DC sink/source" printed on the front. Off state maximum is 11V DC. On state minimum is 19V DC. Operating range is 20.4 to 26.4V DC at 60 degrees C, and minimum on-state current is 3.0 mA into 5.6 kilohm.

The plain 1769-IQ32, without the T, is not the same electrical spec as the IQ32T either, going by Rockwell's published thresholds. One letter in the catalog number, and you cannot assume the two behave the same on the bench.

A sensor carrying a couple volts of drop, or a long run with some voltage loss on it, can feed a 1769-IQ16 all day and then sit under 19V on an IQ32T and never turn it on cleanly. Swap one of those cards for the other during a repair and the sensors that were fine yesterday go intermittent, with nothing wrong in the wiring.

Same controller, different thresholds

It gets finer than card to card. On a MicroLogix 1100, the 1763-L16BWA and 1763-L16BBB split their embedded DC inputs into two groups with different electrical specs. Inputs 0 through 3 are the high-speed ones and need 14 to 24V DC to be on, at a minimum of 2.5 mA at 14V. Inputs 4 and up are the standard ones: on from 10 to 24V DC, minimum 2.0 mA at 10V. Off state on both is 0 to 5V DC.

So, a sensor with real voltage drop on it can work fine on input 6 and go flaky on input 1, same controller, same panel, different result. Nothing about the wiring changed. The electrical spec did.

 

Isolated groups on one card: COM 1 and COM 2 can be connected differently.

What the manuals actually publish

Everything in this table comes from the Rockwell installation instructions and user manual for each part, not from a distributor listing.

Sources: publications 1769-IN007B, 1769-IN064A, 1769-IN072A and 1763-UM001D. The MicroLogix 1100 manual also carries a warning worth repeating: the 24V DC sensor supply on that controller is for input devices only and must not be used to power output circuits.

What it looks like when it is backwards

Say you pull a failed 1769-IQ16 and the replacement goes in with COM 1 landed on +24V, because someone reads the old drawing wrong and nobody double-checks it against the actual sensors. The panel runs PNP proximity switches. Power up, walk a target past the sensor, and its output LED lights on cue. The input on the card never moves.

Nothing is damaged. The sensor output and the card common are both sitting near +24V, so there is no potential across the input circuit and no current through the opto. The card is doing precisely what it was wired to do.

Then people go looking for it with a meter and find 24V at the input terminal, measured to ground, and conclude the card is bad. Measure between the input terminal and that group's own COM. Measured to ground you will get a completely believable number on a card that is never going to turn on.

One more that catches people: leakage. Two-wire sensors pass a small current even when they are off. The IQ16 publishes 1.5 mA maximum off-state current and the IQ32T 1.7 mA. Put enough leaky two-wire proxes on a group and an input can sit on with nothing in front of it.

Six steps in the panel

1.    Find which COM terminal serves the input you care about. On a 1769-IQ32T that is one of four; on an IQ16 one of two.

2.    Meter that COM against DC negative and against DC positive. Whichever reads near zero tells you how the group is tied.

3.    COM on DC negative means the group wants PNP sensors. COM on DC positive means NPN.

4.    Check the sensor body or its datasheet for PNP or NPN before the wire lands, not after.

5.    Measure the input terminal against that group's COM, never against ground.

6.    Compare what you read to the card's published on-state minimum, not to 24V. On an IQ32T that number is 19V.

You need to measure between the input terminal and COM of the same group, not to ground.

Questions that come up every time

What is the difference between a PNP and an NPN sensor?

A PNP sensor switches the positive side, pushing current out toward whatever it feeds. NPN works the other way: it switches the negative side and gives the circuit a path down to 0V. PNP is the common choice across North American and European equipment; NPN shows up far more on machinery built in Asia, which is why imported machines so often arrive with the opposite convention from the rest of the plant.

Is a sinking input active high or active low?

In Rockwell documentation, active high. The MicroLogix 1100 manual states plainly that a sinking input energizes when high level voltage is applied to the input terminal. If you are reading a document from another vendor, do not assume it uses the word the same way. Go look at COM.

Can I mix PNP and NPN sensors on the same card?

Not inside a single isolated group, because the group is committed by how its COM is wired. Across groups, yes. A 1769-IQ32T has four isolated groups and each one can be wired independently, so one card can serve both types. The same holds for the embedded input groups on a MicroLogix 1100. Just document it, because the next person will assume the whole card is wired one way.

Before you order a replacement

If a card is genuinely dead and you are matching a part number out of an existing panel, copy the suffix exactly. IQ32 and IQ32T are not interchangeable on thresholds, and neither are the high-speed and standard input groups on a MicroLogix 1100 once your sensors have any voltage drop on them.

We keep 1769-IQ16, 1769-IQ16F, 1769-IQ32, 1769-IQ32T and the ControlLogix and POINT I/O input modules on the shelf as surplus, with a two-year warranty on everything. If you are unsure which variant came out of your panel, send the catalog number off the side of the old card and we will match it.