IMPlEMEntIng Board-to-Board IntErconnEct In SPacE-conStraInEd InduStrIal aPPlIcatIonS
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here are many design considerations when creating an industrial automation control. In addition to the functional technical specification, there are the
mechanical constraints of enclosure size. As factory floor space is at a premium, the space available for control cabinets is limited, so the pressure is on for manufacturers to squeeze more and more functionality into a smaller space. A popular method of increasing circuit density is vertical and horizontal board stacking, segmenting an application with a ‘mother and daughter’ board arrangement. Not only does this improve space utilisation, but it also offers the opportunity for daughter
board in-service upgrades and a more convenient approach for maintenance. However, this mechanical approach
presents several challenges, especially for high-speed data signals between host processors and sub-systems. This article highlights the issues associated with maintaining signal integrity, maximising space efficiency, and accommodating manufacturing tolerances.
THE INTERCONNECT CHALLENGES OF SPACE-CONSTRAINED SYSTEMS As processors become increasingly sophisticated and highly integrated system-on- chip (SoC) ICs enable machine learning algorithms at the edge, industrial equipment designers seek to incorporate their powerful features into new products. For the engineering team, fully utilising the new SoC devices typically requires additional supporting components, which increases the board footprint. Many industrial applications are already space-constrained, with defined factory floor space limits on how many control cabinets each item of machinery is allowed. Squeezing more electronics-based systems
into the available space of a control cabinet while providing ease of serviceability for maintenance purposes led to the adoption of the DIN rail mounting format. DIN rail enclosures are available in various sizes and offer quick, convenient, and standardised mounting for a wide range of control systems. However, despite DIN rail’s many advantages, individual enclosures may still stipulate minimum separation distances to allow for adequate heat dissipation and airflow. Placing more electronic
Figure 1: The Phoenix Contact FP 0.8 board-to-board
connectors offer a versatile arrangement with a variety of stacking heights. (Source: Phoenix Contact)
components within a given DIN rail enclosure has led engineers to be more creative with PCB layout, often adopting a motherboard or backplane PCB that accommodates sub-system ‘daughter’ boards. This
54 connEctIng InduStrY 20th annIvErSarY EdItIon
By Mark Patrick, Mouser Electronics
approach increases the density of electronic systems within a given space, subject to potential thermal management requirements, allowing increased system functionality.
THE TECHNICAL CONSIDERATIONS OF BOARD-TO-BOARD CONNECTIVITY Splitting a complex control system over separate PCBs is an ideal solution to increasing the overall system density. However, engineers must account for several technical considerations when adopting this approach. Many feature-rich industrial control systems today rely on compute- intensive SoCs and processors such as GPUs and neural processing units (NPUs). These high-throughput devices achieve stunning processing performance and typically interface with high-speed memory, optical transceivers, and vision sensors. Working with these high-performance
processors demands careful PCB design. The layout of power supply rails, high-speed data lines, analogue sensor inputs, and data converter ICs all require specialist attention. The separation of ultra-low-voltage analogue tracks from electrically noisy data lines and the placement of decoupling capacitors close to loads are all familiar topics for PCB design engineers. However, incorporating separate PCBs and
the associated connectors into the layout adds further board complexity. The selection of suitable board-to-board connectors warrants a detailed investigation; the main topics include: Connector EMC/EMI capabilities:
High-speed data lines can create considerable high-frequency noise and create significant electromagnetic interference (EMI) radiating from PCB tracks and connector terminations. This EMI may interfere with signals on adjacent connector pins, resulting in erratic system behaviour. Using shielded connector assemblies and dividing up the internal connector layout to incorporate ground pins helps minimise the impact of EMI. Likewise, sensitive analogue
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