I recall a quote—I read this years ago so the details could be way off—to the effect of, “Good generals study tactics, and the best study logistics.” The idea here is that sometimes the less flashy and exciting aspect of an endeavor is actually more important. In the Hollywood world, trucking supplies from here to there isn’t very interesting but, in the real world, the best strategies and battle plans count for nothing if the soldiers are starving to death.
When I think back to my days working in the defense industry, I vaguely recall a similar attitude surrounding the cables and connectors for military systems—“Oh yeah, I’m sure we can find someone to design the PCBs and write the firmware, but who is going to handle the interconnects?” I also remember a particular project for which the connectors had been neglected until somewhat late in the game. I nonchalantly sought assistance from an older engineer and, after some deliberation, we realized that the issue was more complicated and burdensome than expected. Eventually, he says, “No wonder no one spec’d this out yet. . . .”
Image courtesy of TE Connectivity.
It’s certainly true that a 12-layer mixed-signal PCB with blind and buried vias is, overall, more complicated than a system of connectors and cables. But it is also true that it is possible for a project to go very wrong as a result of interconnect issues.
- Can the cables handle the signal frequencies involved?
- Is there an engineer in the building who really understands what a ground loop is? And if so, could he or she please look at this cabling diagram?
- Is the shielding adequate for the expected noise levels? Do we need twisted-pair?
- Are there mechanical or environmental concerns—abrasion, excessive bending, high temperatures, shock, vibration?
- Has anyone double-checked the pinouts?
- Do the connectors need to be keyed—i.e., must it be physically impossible to make an improper connection?
- Can the contacts in the power-supply connector handle the expected current?
- Do we have the tooling needed to assemble these things? How many different crimp tools will we need to buy?
I could probably think of a few more bullet points, but you get the idea. In some ways, these issues are more evasive—or at least more easily ignored—than the challenges and pitfalls involved in schematic design and PCB layout.
In this article, we’ll take a look at a TE Connectivity cable that is specifically designed for military and aerospace environments. It’s described as an “IEEE 1394” cable. This might seem slightly confusing at first because IEEE 1394 is associated with FireWire, which is a serial communication bus used with consumer electronics.
The FireWire logo.
We generally don’t expect to see consumer-electronics protocols or devices in, say, a jet fighter.
The issue here is that IEEE 1394 is not limited to FireWire and there is nothing inherent in the protocol that makes it inappropriate for use in military or aerospace systems. In fact, there is a specific SAE standard (it goes by the name of AS5643) that describes the “requirements for the use of IEEE-1394b . . . in military and aerospace vehicles.” This product-description document (it’s not a datasheet and I’m not sure what to call it) lists AS5643 under the “Standards and Specifications” heading and, presumably, this means that the cable is somehow in accordance with requirements set forth in the SAE document.
As you can see in the following diagram, cables these days are not as simple as we might expect.
Diagram courtesy of TE Connectivity.
One thing I appreciate about this cable is the clear and highly informative datasheet. It provides specs for impedance, crosstalk, capacitance, time delay, time delay skew between pairs, and velocity of propagation. It seems to me that these details would be helpful if you wanted to use this cable with a high-speed protocol other than IEEE 1394, assuming that you can find someone who is able to intelligently interpret this data with respect to the protocol’s signaling characteristics.
However, if you just need a rough idea of this cable’s high-frequency performance, you know that it is adequate for IEEE 1394b and this, in turn, means that it can handle serial data rates up to 3.2 Gbps.
Do you have any experience with IEEE 1394 in a harsh environment or high-reliability application? Let us know in the comments.