Connector Pinout Design for PCBs

A connector pinout is a small piece of design work with a disproportionate effect on how well a board performs. The same set of signals can be assigned to the same connector in several ways, and the choices determine how much crosstalk the cable carries, whether the returns are where they need to be and how easy the assembly is to wire correctly. Pin mapping is also a documentation problem, because the connector is where two drawings have to agree.

Pin Mapping Between Boards

The connector is the point at which two designs meet, and the pinout has to be defined once and used by both. Where each side defines its own assignment, the result is a cable that must cross signals to compensate, with the crossing occurring in the worst possible place electrically and the best possible place for a wiring error.

The pin mapping should be documented in a single table that is referenced by both drawings, and the table should show the signal, the direction, the pin number and the mate. The convention for numbering rows and positions must be stated, because a mirrored connector that is not clearly described will be wired backwards at least once during development.

Keying and polarity belong in the same document. A connector that can be mated in two orientations will eventually be mated in the wrong one, and the resulting damage is often more expensive than the keying. Where keying is not available, a clear marking on the board and on the cable is the minimum protection.

Ground Assignment

Ground pins do more than complete the circuit. They provide the return path for every signal, and they also set the impedance of the signal pins by their proximity. A connector with a single ground pin at one end forces all the return current through that one path, which produces a shared inductance between signals and couples them together.

The practical arrangement is to distribute ground pins so that each signal has a nearby return. A ratio of one ground to every two or three signals is common at moderate speeds, and a ratio approaching one to one is used for high speed or high pin count interfaces. The assignment should be made from the signal list rather than from the mechanical drawing alone.

Where a connector carries several different interfaces, the grounds can be grouped by interface rather than distributed uniformly. This works when the groups are separated physically, and it allows a noisy interface to have its own returns rather than sharing with a sensitive one. The grouping should be visible in the pinout document.

Crosstalk in the Connector

A connector has more crosstalk than the board traces leading to it, because the pins run parallel to each other at a close spacing over the length of the connector body and through the mating interface. The coupling occurs between adjacent pins and between pins that share the same return path, and it grows with the number of signals switching simultaneously.

The first defence is the assignment itself. Aggressive signals such as clocks and switching outputs should not be placed next to sensitive inputs, and the highest speed signals should be separated by grounds. Where the pinout is constrained by a standard, the ordering may be fixed, and the board layout then has to compensate by routing and by termination.

The second defence is the routing into the connector. Traces that approach the connector in parallel over a long distance couple before they even reach the pins, and the coupling adds to what the connector itself produces. Fanning the traces out early and keeping the parallel section short are both effective, as is the 3W rule applied to the approach.

Signal Return Path

Every signal that leaves the board must have a return path back to it, and the path should be defined rather than incidental. In a ribbon cable or a shielded cable, the return is the adjacent ground conductor or the shield. In a connector without enough grounds, the return may take a path through another signal, which couples the two together.

Where a cable is shielded, the shield should be connected to the board ground in a way that provides a low impedance path at the frequencies of interest. A single wire from the shield to a distant ground point provides no useful return at high frequency, and the correct approach is a connection directly to the board ground plane at the point where the connector is mounted.

The board edge is a common place for the return path to be broken. If the connector sits over a split in the ground plane, the return current has to divert around the split, which raises the impedance of the path and increases the emissions. The connector should be positioned over continuous ground and the mounting and shield connections should be bonded to it.

Power Pins and Current Rating

Power pins have to be sized for the current and for the temperature rise that is acceptable, and a connector’s rating is usually given per pin at a stated temperature rise with all pins loaded. Derating for a group of adjacent loaded pins is often necessary, because the pins heat each other.

Distributing power across several pins reduces the current per pin and also reduces the inductance of the supply path, which matters for a switching load. Where the current is high, the connector choice becomes a thermal decision as much as an electrical one, and the pin assignment should reflect the current map of the board.

The mating contacts contribute resistance and inductance as well, and a connector that has been used many times may have a higher contact resistance than a new one. Where a connector is cycled regularly, its rating should account for that rather than assuming the specification at first mate.

Documentation and Verification

The pinout document is the deliverable, and it should be unambiguous, complete and controlled. It should show the connector from the mating face as well as from the board side, with the pin numbering convention stated, and it should list every pin including the unused ones, because an unlisted pin is a pin whose status is undefined.

Verification is done with a continuity test on the first assembly, and it should be performed before power is applied rather than after. A simple check of each signal against its assigned pin and against ground catches the wiring errors that otherwise destroy a board on first power-up.

The final check is between the two drawings. Comparing the board pinout against the cable pinout, function by function, is a five minute task that catches the mirrored connector, the transposed pair and the pin that was moved during a revision. It should be done every time either drawing changes.

Practical Rules

Define the pinout once and reference it from both drawings, distribute grounds so that every signal has a nearby return, and separate aggressive signals from sensitive ones. Derate the power pins for the group and bond the shield directly to the board ground.

Verify the assembly with a continuity check before applying power, and re-check the pinout whenever either drawing is revised. The pinout belongs with the build documentation so that the quality records show the assembly was checked against the correct revision rather than against a remembered one.

FAQ

How many ground pins does a connector need? Enough that every signal has a nearby return. Ratios from one ground per two or three signals up to one per signal are typical, depending on speed.

Why do connectors have more crosstalk than traces? Because the pins run parallel at a close spacing through the connector and the mating interface, and they often share a return path.

What should be checked before first power-up? Continuity of every signal to its assigned pin, and of ground to the shield and mounting. It is much cheaper than replacing the board.