Internal system cabling 101

The PC/104 embedded system design issue of efficient internal cabling and will provide helpful information to aid engineers in eliminating some of the common problems that internal connectors and cabling can cause.

This Fundamentals 101 column will address the PC/104 embedded system design issue of efficient internal cabling and will provide helpful information to aid engineers in eliminating some of the common problems that internal connectors and cabling can cause.

In the beginning
In the beginning stages of PC/104 embedded system design, engineers should seriously consider the internal cabling requirements for interconnecting individual PC/104 circuit boards to the external enclosure connectors and other internal devices. A system-level wiring diagram is always a good starting point. More important, careful review and analysis of physical cable sizes, types of cables, and the sensitivity of the signals within the cables and their relative placement to other internal cables is key to design success.

“Getting down to the wire”
To start, an engineer would commonly create a three-dimensional CAD drawing of the embedded system enclosure showing the placement of the circuit boards, the power supply, and any off-board components. This three-dimensional CAD drawing typically identifies and measures everything except the internal cabling required “within” the enclosure. It is not uncommon for a serious design issue to surprisingly appear when assembling the first complete system prototypes. For example, the internal cabling may take up more space than that expected or, worse, there may not be enough room for some of the internal cables. This problem can quickly develop into a serious design issue if the internal cabling restricts airflow for internal system cooling or the cable is just a little bit too short. In addition, forcing the cable connection can create mechanical stresses on the circuit board and cable.

The initial enclosure drawings should include internal cabling and routing with an accurate representation of cable size, flexibility, and mechanical interference associated with all the other system components.

Cables
Typically, the connectors on individual PC/104 circuit boards dictate the type of cable that should be utilized. This could range from shielded cables to ribbon cables to twisted pair or individual wires.. The design engineer still must determine if the type of cable will maintain the signal integrity requirements for each type of transmission signal. In addition, the engineer must review the required cabling type and physical routing for the noise environment that may be present within the enclosure itself, as well as the environmental conditions the embedded system will be exposed to in the end application. A lot of time can be wasted debugging a recurring system failure only to realize that a sensitive, low-voltage signal was inadvertently routed next to a noisy power supply line.

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Figure 1

Board-level connectors
The environmental system requirements and system repair requirements will determine the type of connectors needed when selecting PC/104 circuit boards and mating cables. Embedded systems that might be exposed to an environment consisting of shock and/or vibration may require a type of locking connector. Embedded systems that might have multiple people servicing them or any type of future upgrade capabilities may require restricted use of multiple identical connectors and that all connectors be keyed.

Design engineers must seriously consider Murphy’s Law for embedded systems that utilize connectors. If a connector is capable of backwards installation or if a user can accidentally swap two cables because each utilizes the same connector, at some point in the future one of these mishaps may occur. If you have a special connector that requires a particular substitution on a vendor’s circuit board, as long as it does not require a change to the bare board footprint and mechanically there is sufficient clearance, many PC/104 vendors will work with you to install a custom connector that meets your embedded system design requirements.

Putting it all together
It is always important to design an embedded system that utilizes the best choice of circuit boards and the proper enclosure to meet the end application’s design requirements. But it is equally important to take into account the end system’s internal cables and connectors to satisfy the space requirements, signal integrity requirements, and environmental requirements.