Camera opto-isolated GPIO circuit direction: which side is the input

A GPIO section in a camera technical reference draws the camera's own network on one side and a recommended external circuit on the other. The arrows read backwards at first glance. This guide is for anyone wiring triggers, strobe outputs or PLC handshakes to a machine-vision camera's I/O pins. You will learn which side of each figure you actually build, why a bare relay contact cannot drive an opto-isolated input, how to size the series resistor and the pull-up with worked numbers, and which I/O type fits your signal source.

Which side of the circuit diagram is yours?

Everything drawn inside the camera outline is the camera's own circuitry, already built; you design only the external half — one resistor per signal and nothing else. Direction never flips: an opto-isolated input always takes current in from your driver, and an output always sinks current from your pull-up.

The two halves meet at the optocoupler, which straddles the isolation barrier: the LED sits on the driving side, the phototransistor on the receiving side, and only light crosses the gap. For an input, your supply pushes current through the LED, the coupler conducts, and the camera's logic sees the edge. For an output, the camera's LED illuminates a transistor on your side, which sinks the current your pull-up feeds it. The arrow near a camera pin shows current arriving from a supply — yours or the camera's — which is why the drawing can look flipped.

Treat the figure as documentation, not a parts list: the camera half tells you what the pin demands, and you never rebuild it or need its schematic files. Pin names, pin numbers and which I/O types exist on a model live in the connector table of the same reference, and they change between camera families — read the section for your exact model, not a previous generation's.

Can a dry relay contact alone trigger the opto-isolated input?

No. An opto-isolated input is an LED that needs forward current — design for 5-10 mA at about 1.2 V — and a closed contact across the pins supplies no voltage, so no current flows. Wire the contact in series with a supply and a current-limiting resistor, or use the non-isolated input.

That is what the reference means by a driven pulse: a 5 V logic pulse through the right resistor qualifies, and so does a relay contact used as the switch inside a loop that already has a supply and a resistor. The contact can sit on the high side or the low side; the coupler only cares about the direction of current through its LED.

Here V_supply is the pulse-source voltage in volts, V_F is the coupler LED's forward voltage (about 1.2 V; data sheets print 1.1-1.5 V) and I_F is the forward current you design for, 5-10 mA. R comes out in ohms, P in watts.

What does the non-isolated output actually deliver?

Out of the box you get a 3.3 V TTL signal; the pin is a transistor with an internal pull-up to 3.3 V, and an external pull-up to your rail — up to the documented 24 V maximum — sets the high level. Design your receiver to survive the level you choose.

The sink rating sizes the pull-up. These pins take 25 mA on cameras of this class, so R(min) = 24 V / 25 mA = 960 Ω; fit 2.2 kΩ and the sink current when low is 24 V / 2200 Ω = 10.9 mA, with the internal pull-up adding less than a milliampere. Power matters when the line sits low for long stretches: P = (V_rail)² / 2200 Ω = 0.26 W, so use a 0.5 W part. A standard PLC digital input at that rail draws a few milliamperes, which this line supplies without an interposing relay.

When the line marks exposure and drives a strobe controller, wire and enable it as described in the smart camera strobe output guide.

When should you short an input to ground instead?

Use the non-isolated input when your trigger is a dry contact: it is a sinking input with an internal pull-up, so shorting it to the camera's ground fires it, drawing only a few milliamperes from the internal rail. Wire a relay contact, limit switch or PLC relay output across it.

A contact across the non-isolated input and camera ground closes a loop the camera already powers, so there is nothing to calculate. This is the relay-friendly path, and the exposed one: both sides share a ground, so a potential difference between your contact's ground and the camera's corrupts the reading.

Choose the opto-isolated input whenever the pulse source lives in another cabinet, on plant equipment or at the end of a long run, because the isolation gap keeps the two grounds apart. When pulses then vanish or double, the shared reference is the usual suspect — the camera trigger common reference guide walks that fault down.

Which I/O type fits your signal source?

Match the I/O type to your source's output stage: dry contacts go to the non-isolated input, transistor and logic drives go through the opto-isolated input behind a series resistor, and the camera's non-isolated output needs a pull-up of at least 960 Ω at that maximum rail.

Two rules settle the remaining cases. First, direction is a property of the camera's logic, not of the connector: an input takes current in, an output switches current that your supply provides. Second, pins on multi-purpose lines take their direction from software, so set the line mode before you cut metal for an adapter board. Short of pins, a trigger line can double as an output — see camera trigger lines as digital outputs.

What belongs in your bill of materials?

Three external parts per signal cover nearly every case: one series resistor per opto-isolated input, one pull-up per open-drain output, and a breakout cable that lands the camera connector on open wires. The camera side needs nothing from you, so never hunt for its internal schematic.

• One series resistor per opto-isolated input: 390 Ω 0.25 W from a 5 V source, 470 Ω 0.25 W from a 3.3 V pin, 4.7 kΩ 0.25 W from the plant rail.
• One pull-up per open-drain output: 2.2 kΩ 0.5 W for the PLC receiver, none for a 3.3 V one.
• One logic gate or small transistor when one pulse fans out to several cameras: six inputs at 5-10 mA each need 30-60 mA of drive, and the multi-camera trigger fan-out guide shows the wiring.
• A breakout cable that matches the camera's I/O connector.

Land the trigger loop on a cable that ends in open leads:

The LS-CBL-0300 runs the loop over 7 m of cable rated for drag-chain use, with an 8-core open end, so supply, resistor and contact all land on terminals of your own board. A 12-pin I/O connector takes the LS-CBL-0211 instead, open-ended again over 5 m, and frame grabber users land on the DB9F-to-open LS-CBL-0224, 7 m.

Filter by length and connector in the camera cable finder; the camera cables category holds the full range.

How much trigger delay does isolation add?

A phototransistor optocoupler stage adds microseconds, not nanoseconds: expect a few microseconds of propagation delay through the LED and transistor, against single-digit nanoseconds for a direct logic input. For line-scan or tight multi-camera work, wire the non-isolated input and spend the effort on a clean ground instead.

Coupler data sheets print rise and fall times from a few to tens of microseconds depending on the load resistor, and an external stage you add inherits that. For piece-by-piece triggering on a conveyor the delay is irrelevant; for encoder-clocked line scan or sub-10 µs multi-camera sync it is not. Budget the whole path, camera included, against your cycle time with the camera hardware trigger frame rate guide.

How do you check the wiring before the first frame?

Three checks take ten minutes: measure the loop current with a multimeter in series and expect your design value of 4.5-9.7 mA; read the line's status flag in your capture software while toggling the trigger; and scope the output line during an exposure for the pull-up level.

1. Loop current: a zero means the loop has no supply; a reading far above the design value means the series resistor is missing or mis-sized.
2. Line status: the flag in your capture software should flip with every contact closure; if it never moves, the loop or its ground return is open.
3. Camera settings: check acquisition mode and line configuration before blaming the wiring — a camera left in continuous mode ignores every correct pulse, as the GigE camera ignores external trigger guide explains.
4. Output level: trigger an exposure and scope the line; the high level should sit at your pull-up voltage or at 3.3 V, square at your trigger rate.

From here the path is short: pick the I/O type from the table, size the two resistors from the formula, and order the breakout cable that matches your connector. If a figure in your camera's reference still looks flipped, sketch the external side and send it through the contact page; we will mark it up. The rest of the trigger wiring guides sit on the systems topic page.