Many digital devices share information via busing networks. In addition certain devices such as pitot tubes and static ports are now fully electronic, providing the conversion of pressure to electrical signal within a module contained in the probe. This advancement has led to the elimination of mechanical plumbing and significantly reduced maintenance checks on air data systems.
Many are of the opinion that as many digital circuits have the ability to advise when problems occur that troubleshooting is limited to interpretation of messages or fault codes. This is an unfortunate misconception as a significant amount of the programmed fault detection circuits can not differentiate a faulty sensor from a failed wire or connector. It is true that some diagnostic software can detect and identify a failure of a digital bus, but unfortunately most cannot advise the technician exactly where the problem has occurred. Once again conventional test equipment and methods may need to be employed to recognize and then locate the cause of the malfunction.
Discrete signals are often confused with digital signals as the terms 1 and 0 are referenced. It is important to realize that these terms are not a constant but are relative to specific systems. Often circuit cards or devices controlled by a “black box” will have internal power supplies and manufacturers do not always clearly identify the output of these devices. Many components have been removed due to wrong interpretation of input and output signals. Just because a circuit has 28 volts going in does not mean it can not have inputs or outputs that are significantly different. The dilemma with thrust reverser mentioned earlier is a prime example where the thrust reverser control unit operates on 28v DC but contains an internal voltage regulator set at either 12 or 15 volts and this is the potential that is observed to determine reverser position. When switches are in the open position 12 volts is present. When specific switches close a ground is supplied to the detection circuit and the voltage drops to zero. Due to controlling resistors installed on the monitoring circuit card, the current flow in this detection circuit is limited to milliamps.
Another example involves the Honeywell Epic where in certain installations the input/output (IO) circuit cards use a 14 volt potential for the evaluation of a discrete. When the monitored device is in an open state the monitored voltage is 11 to 14 volts. When the observed component switches ON then a ground is supplied and the potential will drop into a range of 0 to 3 volts. The IO circuit card is programmed to look for either something less than 3 volts or something higher than 11 volts. In the event of a signal in the range of 3 to 11 volts the system has no way of recognizing the condition of the monitored device and may then generate a message to a central maintenance computer. In this case, a minor amount of corrosion in the ground circuit could easily restrict the milliamp current flow and cause an elevated discrete where an improper operating condition would occur.
Component Surveillance or DAU
Digital flight decks require vast amounts of information and on many current generation aircraft, almost every component that has a wire going to it can be monitored and in turn impact the operation of the system. As many avionics suites are designed with the intent to fit a wide array of aircraft, the means of providing surveillance for significant airframe components has to be conceived. An appropriate name for such a device is a data acquisition unit or DAU for short; there are other terms that describe the function of liaison between avionics and airframe such as IO circuits.
Most avionics manufacturers have created proprietary digital languages enabling communications between all of their components that require interaction. In certain cases avionics-related devices may have the need to provide data for other airframe-related computers. This is another situation where an IO circuit or DAU may play a significant role. There are numerous situations where digital interpretation has to take place and a different language is used for the output command. An example of this would be where an air data signal relating to current aircraft rate of climb is transmitted in a Honeywell language and is then sent to an IO card which translates the input and converts it to an ARINC 429 output which can then be used by the aircraft pressurization system computer to regulate the passenger cabin altitude and to a second device that will control the amount of bleed air extracted from the engines.
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