Differential line input voltage limits: pin-to-ground or pair-to-pair

A frame grabber manual lists its line inputs A and B against one number: 5.5 V. The immediate question is what that number is measured across: channel A against channel B, each input against ground, or the two wires of one pair. Only one reading is correct for a given wiring, and the difference matters. The safe misreading wastes a level shifter; the unsafe one lets the first trigger pulse damage the input. Below is the rule for both wiring modes, the arithmetic that checks a camera or encoder output against the limit, and the margin line that decides between a direct connection and a divider. It is written for anyone wiring triggers, encoder signals or camera sync lines into a machine-vision frame grabber.

Two channels, one limit: what the rating measures

Inputs A and B are two separate channels, not the two halves of one signal. Each channel brings its own pins, A+ with A- and B+ with B-, and each can be wired two ways. The manual's number scopes to the wiring you choose.

• Single-ended: the signal arrives on one wire and shares the frame grabber's ground. The limit applies from each input pin to ground, once per pin, independently for A+ and for B+.
• Differential: the signal arrives as a pair on two wires. The limit applies across each pair, from A+ to A- and from B+ to B-.

It never means a rating between channel A and channel B. Those two carry different signals, so their difference is not a rated quantity; two independent drivers can sit at any voltages within their own ratings.

The connector drawing settles which row you are in: one signal pin per channel plus a shared ground row is single-ended, and two pins marked + and - per channel is differential.

What a differential input measures

A differential receiver ignores absolute voltages and acts on the difference between its two pins:

The receiver switches when the magnitude of v_diff exceeds its input threshold, which is 200 mV or less for an LVDS-class receiver, and only while both pins stay inside the receiver's input window. Interference picked up along the cable lands on both wires almost equally, so it moves v_cm and cancels out of v_diff. That is why encoder, trigger and camera sync lines ride differential pairs through a cabinet full of drives. A single-ended input measures one wire against ground, so the same interference lands straight in the signal. What sets the common-mode value in the first place, especially when camera and frame grabber do not share a ground, is treated in this reference on common-mode voltage with floating voltage sources.

Single-ended wiring: each pin against ground

Connect the signal to the channel's positive pin and complete the channel to frame grabber ground as the connector drawing shows. The limit then applies twice, once per pin. A 5 V sensor output clears it with 0.5 V to spare, while a 24 V PLC output exceeds it about 4.4 times over and needs a divider or an isolator first; the FAQ below works a divider through with the arithmetic. The channels stay independent, so whatever you drive B with never changes A's margin.

Differential wiring: each pair across its own pins

Connect the driver's output pair to the channel's input pair, polarity for polarity, and the limit now applies within each pair: A+ to A-, and B+ to B-. Real machine-vision signals sit well below it. An RS-422 driver swings at least 2 V, about 36% of this limit, and an LVDS driver about 0.35 V. A second limit hides behind the first: the receiver's input window, the absolute voltage each pin tolerates, is 0 V to +2.4 V per pin for an LVDS-class receiver. Differential wiring therefore takes two checks: the pair difference against the limit, and each pin against the window. A driver biased outside the window can damage the receiver even with a tiny pair difference, say a camera output riding at 6 V common-mode onto that 0 V to +2.4 V input.

Worked example: an LVDS camera pair into a differential input

The inputs, and where each comes from:

• 5.5 V between A+ and A- in differential mode: the input range the frame grabber manual quotes for its A and B line inputs.
• 0 V to +2.4 V per pin, with a switching threshold of 200 mV or less: the receiver requirements published in the LVDS and RS-422 compatibility application notes.
• LVDS drive levels, the pair biased near the window's midpoint and swinging a few hundred millivolts: the LVDS standard's typical operation.

The rule, in words: in differential mode the manual's number governs the pair difference, the window governs each pin, and the threshold decides whether the receiver can switch at all.

The arithmetic:

1. Bias point: the middle of the 0 V to +2.4 V window is (0 + 2.4) / 2 = 1.2 V.
2. Pair difference: an LVDS swing of about ±0.35 V peaks at 0.35 V of magnitude. That is 0.35 / 5.5, about 6% of the rating, leaving 5.15 V unused.
3. Threshold margin: 0.35 V of swing against a 0.2 V threshold leaves 0.35 - 0.2 = 0.15 V.
4. Pin levels: each pin sits between 1.2 - 0.35 / 2 = 1.03 V and 1.2 + 0.35 / 2 = 1.38 V, inside the 0 V to +2.4 V window and far inside the limit even measured to ground.

Result: the camera line connects directly, with 5.15 V of margin to the input limit, 0.15 V of margin to the threshold, and both pins inside the window. The decision rule this sets: keep the expected swing at or below 80% of the limit, here 0.8 × 5.5 V = 4.4 V. A 5 V push-pull camera sync output clears the absolute limit but crosses that design line, so look at its tolerances before wiring it direct; anything wider gets a divider or a line receiver.

Where these inputs live in a vision system

Camera Link is their classic home. The camera sends pixel and control data as differential pairs, RS-422 or LVDS depending on the Camera Link configuration, and the grabber exposes differential trigger and encoder inputs on its connector: the manual's A and B. What must match between camera and grabber is laid out in the Camera Link frame grabber compatibility guide, and wiring a quadrature encoder into those inputs is walked through in the encoder-triggered line scan capture guide.

Newer interfaces move the wiring. A CoaXPress camera takes its trigger over the camera cable itself, and a GigE or USB3 camera takes it on its own opto-isolated input, so the camera manual becomes the one to read. The MV-GX1008 is a CXP-6 frame grabber for the first path; on the second, a PoE card such as the LS-FGB-0004 carries data and power while the camera's input block takes the trigger.

One input can also serve two cameras with the right wiring, which the two cameras on one frame grabber input guide covers. When the interface is settled, start from the frame grabbers category or the accessories finder.

Checks before you connect

• Choose the wiring mode first, then read the limit in that mode's terms: pin-to-ground, or pair-to-pair.
• Take the driver's worst-case swing and bias from the camera or encoder manual, not from its supply rail.
• Run both checks: swing against the limit, each pin against the window.
• Hold the 80% line, 4.4 V of working swing here, before deciding on a direct connection.
• If a fault can drive a pin past the limit, add series resistance or a proper line receiver; sizing such protection networks for differential receivers is the subject of this input protection reference.
• Wired and still seeing nothing? Work through the frame grabber cable and wiring checks before suspecting the input.

The misreads that damage inputs

Three readings account for most dead inputs. Treating the limit as channel A against channel B compares two independent signals with no rated relationship between them. Doubling the limit because the signal uses two wires ignores that both wires carry one signal; the receiver never sees more than their difference, so the rating never doubles. Wiring a wide single-ended signal onto a pair pin and trusting the differential label for tolerance mistakes the window for the swing: the tolerance is the common-mode range, and a 24 V swing breaks it.

When signal and manual line up, with the mode chosen, the swing inside 80% of the limit and the pins inside the window, wire it direct and move on to the integration. The system configurator starts at the extras step, the systems topic page collects the surrounding guides, and the contact page reaches the applications team for a wiring you are unsure about.