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Interface Control Drawings - Bridging the Gap in the Signal Chain

A document that describes the relationship between a subassembly and the whole thing becomes necessary when more than one businesses are competing to fill the socket. In many cases, a consortium forms around a form factor and settles on the input and output parameters as well as the physical aspects of the item.

The Interconnect Control Drawing Is Part of a System-Wide Focus; Safety, Reliability, Fault Tolerance on Top of the Physical Connectivity.

This is not a new concept. The PCB Designer is bound to encounter a microcosm of this in the app notes and data sheets provided by the vendors of integrated circuits. Depending on the purpose, the documentation can be pages of graphs and walls of text. Somewhere in there, you’ll find the ball map or other means of pin assignment, whether tabular or graphical.

Figure 1. Image Credit: Cisco - A 10GB/s transponder revealing the predetermined location of gold fingers. There is a connection for a laser and a detector at the other end. Whatever happened inside the case was up to the maker.

The printed circuit board can be the same way in terms of helpful silkscreen indications. Pulling back more, the product will likely have a little room for safety and regulatory marking. People who like to repair things find such information useful long after the product has been replaced by something new and (slightly?) different.

Photonics related telecom gives rise to all sorts of widgets. They fall under one umbrella or another with names like XAUI or XENPAK and so on. Those massive switches feed the line cards. The line cards, in turn, host the transponders that work as the interface between fiber optic cables and copper traces. The technology (WDM or wave division multiplexing) chopped up slices of spectrum to divide up the traffic. I’ll break that down into simple chunks.

In a way, it is similar to MIMO (multiple in, multiple out) WiFi technology that uses a number of wavelengths to propagate signals. Arbitrary colors are assigned to 18 different channels.  Recovering the clock and data was one chip. Then a multi-chip module divided 40GB/s into four streams of 10GB on the first die and then output 16 pairs of 2.5GB/s data where we could use standard materials for the PCB. (Today’s data center gear is an order of magnitude ahead in terms of data rates.)

The Black Box Isn’t Just for Passenger Jets

Smaller form factors serve the 10GB market. A multitude of form factors have sprung up over the years. The aim is usually the same two goals, improved thermal performance and lower cost. Shedding heat increases reliability while better parts cost more money. Go all in on the heatsink(s) and then work around the thermal pad trimming as little as possible as the board has to fit inside the given volume.

We added a daughter card with board-to-board connectors that had a 1 millimeter stack height. That was the most dynamic placement where the two boards shared one mm between them. The headroom on the connector side of the daughtercard was determined by the placement of the main board.

It was at this time that I got into the habit of adding the maximum height as a text on the proper layer of the part’s footprint. Grabbing that image from the main board gave me the intricate 3D space that made up the ceiling for the daughter card. It was like creating Legos except that actually making contact would be a bad thing.  Absolutely none of that mattered as long as the widget worked as advertised. As long as the interconnect requirements are met, the inner workings are irrelevant.

Figure 2. Image Credit: FS - A 40G MUX. The fins on the top of each socket point to the amount of work going on inside this multiplexer. The interest group continues to chase performance and reliability tradeoffs within the allotted space.

The term black box seems overused since they refer to telemetry that helps people understand disasters. The contents of the hardened enclosure is the data, not the hardware or even the method by which the data was acquired. The aviation industry, more than most others, will have good documentation habits around their safety equipment. If you printed out all of the docs, they would weigh more than the plane itself.

Pop culture tends to use black hole and black box in similar ways. So using the term to imply that the inner workings are a mystery is an apt way of looking at interconnect drawings. It’s the world outside that matters once everything is buttoned up. You would see some spillover from the outline and assembly drawings as a complete picture would make a good starting point. Remove unnecessary details and fill in other data as required.

Figure 3. Image Credit: ManualsLib - Disregarding the inner workings with a focus on I/O makes this representative a PCB designer would generate in support of a user manual.

Cables and Flex Configuration Beyond Pin One Designation To Include Net Names

Orientation of the connector is one thing. Once we’ve established which end is which, the details lie in the function of each pin. One thing I learned a while back is that not all PCIe lanes are created equal while they look the same. Creating the footprint of the small edge connector led to browsing the newer generations of the spec. When all of those specific lane choices have been made, it can be captured under this umbrella.

Figure 4. Image Credit: Screaming Circuits. This assembly drawing leans in the direction of an ICD for its emphasis on final dimension along with repeating data that was given elsewhere shown in parentheses. The side view puts the plan for the flex circuits in perspective removing an element of guesswork.

Aside from enumerating the copper connections, an interface control drawing can be part of a larger context where some of the following can be codified.

  • Max and minimum temperatures for storage and usage
  • Related to cooling requirements; forced air ventilation, heatsink, liquid cooling
  • Min and Max Current, over and under-voltage protection
  • ESD sensitive device alert
  • Test setup, equipment, probes, firmware
  • Other environmental concerns driven by contracts or local laws

There always has to be an “other” category as a catch-all for whatever circumstances compel this drawing into existence. Certain wings of the commercial sector will find interface control documents more appropriate than others. Anything to do with the Department of Defence will be documented in every way they can remember to list. There will be lists of lists but I believe this to be some of the more useful bureaucracy.

About the Author

John Burkhert Jr is a career PCB Designer experienced in Military, Telecom, Consumer Hardware and lately, the Automotive industry. Originally, an RF specialist -- compelled to flip the bit now and then to fill the need for high-speed digital design. John enjoys playing bass and racing bikes when he's not writing about or performing PCB layout. You can find John on LinkedIn.

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