No one will argue with the idea that a USB protocol should probably be routed over a USB connector and standardized cable. These and other connector types have standardized pinouts, and the equipment has been qualified against well-known standards, so it seems like it’s always the lowest risk path forward.
With these connectors, you often won’t have the freedom to set whatever pinout you like without violating the standard or significant reengineering. But if you’re interfacing with another piece of equipment, you might consider modifying the pinout on the receiving board, or customizing the cable assembly to use different connectors on each end. We’ll look at some advantages of these approaches as they pertain to connectivity in multi-board systems.
Can You Customize Your Pinout?
Oftentimes, in a multiboard system, the PCBs in the system are connected to each other with a cable and connector system, and those connectors have to enforce a particular pinout. Sometimes the equipment that is being interfaced in your design was built by another company, and you have to build your device to accommodate their cable and pinout. Should you match their pinout directly, or should you customize the pinout on your board?
Below we see a simple example where the connector pinouts between two boards do not match. The receiving board has a totally custom pinout; the connector system and cable assembly must be built to accommodate this type of intentionally mismatched pinout.
The pinout on the receiving board can be made custom if the connector body allows conductor pins to be swapped in the cable assembly.
The simplest approach you’ll see to using a cable assembly in a multiboard system is to mirror the pinout on the two boards. For something like a ribbon cable, this will be your only option as the conductor pins can’t be moved around the receiving board’s connector body. In most board-to-board connectors or mezzanine connectors, you won’t have any choice; the connector bodies mate directly and this forces a mirroring of the pinout on the two boards.
In some cases, the pins on the receiving connector can instead be moved around the connector body as shown above. If the connector body is also a totally custom component, then you’ll be doing the same thing in the destination board.
Custom Pinouts Make Routing Easier
Some components, like ASICs and an MCU, may have an odd arrangement of pins that makes routing into a connector difficult. When the pinout is customized, the pins can be arranged on the PCB so that routing into the connector is much easier. This is certainly the case with BGAs and quad packages (leaded and leadless), which might have an odd arrangement of pins in a particular interface.
If you wanted to connect the boxed pins to a connector, a custom pinout would allow you to line up the connector pins to match the pin order on this MCU.
With appropriate pin placement for the connector pinout, you can prevent crossover and accommodate interfaces on orthogonal sides of the component, such as the green-colored pins in the above image.
There is one option that sits between custom pinout and standardized connector/cable assembly with fixed pinout. This would be to use a modular connector assembly, where the pinout and pin connections are fully customizable, but the connector body can be purchased off-the-shelf. Some examples of these connector systems include:
- D-sub connectors
- Industrial-grade M-series connectors
- Wire-to-board connectors
A simple conductor swap on the receiving mating connector allows the pinout to be customized, rather than just mirroring the pinout on the sending connector. Because these are modular connectors, they are available from multiple manufacturers, and you won’t lock yourself into the supply chain risk associated with having a single parts vendor.
Standard Connector, Custom Pinout or Interface
Sometimes, a particular standardized connector gives many advantages, but it might not be used for its intended standard. In other words, you can use connectors and cables like USB for something other than USB links.
While not the most common approach to connectivity in multiboard systems, you can use a standard connector and cable (USB, HDMI, Cat 5, etc.) for something other than its intended purpose, in which case you would also be customizing the pinout. This typically requires a modular connector that meets a particular standard (such as a modular RJ-45 jack for Ethernet).
This pinout coming off this RJ-45 jack can be customized for protocols other than Ethernet.
Let’s take RJ-45 connectors as an example. These are used for Ethernet cables, but networking equipment will have an RS-232 interface on the back of the enclosure that connects through an RJ-45 cable. Standards like UART are slow enough that they can run over just about any connector/cable system, including RJ-45 and standard Ethernet cables.
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