Does the Bus Stop Here?

Communicating in the digital age


Screened unshielded twisted pair (S/UTP), also known as fully shielded (or foiled) twisted pair (FTP), is a screened UTP cable. It is a thin, flexible cable that is easy to route. UTP is small, so it does not fill up wiring ducts quickly.

There are some disadvantages as well. Twisted pair’s susceptibility to the electromagnetic interference greatly depends on the pair twisting schemes staying intact during the installation. As a result, twisted pair cables usually have stringent requirements for maximum pulling tension as well as minimum bend radius. This relative fragility of twisted pair cables makes the installation practices an important part of ensuring the cable’s performance.

Troubleshooting
Troubleshooting digital networks will often require a bit of finesse. Depending on the type of system, diagnostic devices may be built-in and can direct the technician to a potential fault on the bus. In an ARINC 429 circuit where one transmitter supplies information for up to 20 users, a physical observation of what works and what does not is often a good first step in fault isolation. Fault isolation can often be accomplished using common place electrical testing equipment.

It should be noted that digital networks operate on very low current flows and generally do not provide adequate power to illuminate test lamps. For example, ARINC 429 operates on a 10v DC range with 5 volts positive representing a digital 1 and 5 volts negative representing a digital 0. A digital volt meter is a better choice here than an analog meter as it puts a lesser load on the bus.

An oscilloscope can be an effective tool as it allows a real-time observation of bus traffic and enables the user to identify possible impedance problems or observe the presence of electrical noise.

As with the handling of any electronic equipment precautions, ensuring against electrostatic discharge is paramount. In the event a problem is detected on the network that requires further investigation to isolate the fault, all devices that are connected to the specific bus should be disconnected prior to the introduction of any test equipment that could introduce potentials outside the normal range of operation. In other words, if you plan to use a megger to locate an insulation breakdown, consider that the high electrical potentials will damage any unit still connected to the bus.

Termination resistors are sometimes installed at each end of the bus to compensate for the impedance. Although generally reliable, a failure of a terminating resistor can render the entire bus disabled. When troubleshooting, consider that terminating resistors are wired in a parallel circuit to each other; the overall bus resistance should be half the resistor value.

Bus splices are another common area for faults to occur. With the sensitivity and construction of digital networks in aircraft, proper handling techniques are essential. Pulling a twisted pair cable too tight can alter the pitch of the wire twists and increase EMI susceptibility. Installing a wire tie or cable clamp where it compresses the bus could alter the impedance, as could routing the cable with too tight a bend radius.

There have even been situations where bus cables have been routed through bungs at pressure bulkheads. The bung would squeeze the bus when the aircraft is pressurized, altering the impedance and producing a crippling effect.

Shielding is another area that can cause anomalies. Improper grounding or termination can sometimes allow a free passage for electrical noise to invade the bus or can alter the impedance by changing the capacitance value.

Many technicians are intimidated when it comes to troubleshooting data transmissions circuits. With some basic knowledge and precaution, it is not that complicated. After all, it is usually only two wires that could be either shorted or open! What could be difficult about that?

I have just come to the realization that comparing a digital data bus to an international airport terminal shuttle is probably not normal and could be considered strong evidence that a long, airplane-free vacation is probably warranted. Au revoir!

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