The ARINC® 429 Data Bus is a standard used for digital information transfer between computers. Data is transmitted in ONE direction only. The output of one computer can feed several other computers, but the receiving computers cannot return information on the same set of wires. Every major avionics computer that uses 429 for communications will have numerous transmitting and receiving circuits. Each circuit uses a 24 gauge twisted pair wire set for data transfer. Having separate wires for each circuit usually means there are literally hundreds of twisted pair wire sets running between components. (A new standard “629” is a bi-directional system that greatly reduces wire runs and weight.)

429 Data Bus information is composed of “Labels” of information. As an example: a typical digital Air Data Computer will output labels 210 - True Airspeed, 215 - Impact Pressure, 242 - Total Pressure, plus more than 30 other labels related to the aircraft flight environment. These outputs are continuously updated several times a second. They are fed to all the computers or components that require this information. Air Data information is used by the FCC, FMS, and Auto/Throttle computers. Other systems such as pressurization, ground proximity, and engine controls also require digital air data inputs. Flight information to the flight crews is displayed on the EFIS from the symbol generators. The SG’s receive 429 data from the Air Data System.
Another example of 429 digital information is the Inertial Reference System. A typical IRS will output labels 323 – Flight Path Acceleration, 324 – Pitch Angle, 360 – Potential Vertical Speed, plus others related to inertial reference. This information is output to all the components that might require it.
Analog signals are often converted to digital information to be used by different systems. Discrete signals (opens, grounds, and voltage) used for component switch positions and logic can also be converted to digital 429 data.
With all the computers and components cross-talking with 429, there is a tremendous amount of data flowing on a modern aircraft.

429 Data Bus systems have proven to be extremely reliable. When a failure does occur, troubleshooting and repair can range from a simple box replacement to a total nightmare.  Built in Test Equipment (BITE) is usually the first avenue for verifying problems. Normally, any hiccup of digital data is recorded. BITE systems are a great tool for problem isolation. Major headaches can usually be expected when the BITE system doesn’t work.
429 Data Bus readers are sometimes needed for some problems. A reader can record or simulate data on a bus. Viewing 429 information with the reader can verify missing data, but normally the receiving systems recognize missing or incorrect data and they will display these faults during a history review in BITE. Simulation is needed at times to isolate problems. I have found readers to be a unique tool for observing data traffic, but most of the time they are not needed for troubleshooting in a line maintenance environment. There are times though, when a reader can be a godsend when troubleshooting some problems.
I try to use ALL the associated components when troubleshooting data bus issues. A fault might not be recorded in one user system, but it is picked up by another. One point should be noted; a computer can have a failure in only one of its many transmit or receive circuits, only the specific connected components to that bus would be affected. System schematic manuals will usually show all interconnects for each data bus signal.

If all fails, grab a meter and have fun.

ARINC Tutorial

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Many circuits on aircraft use step down transformers for control voltages. Typical user systems are lights and 28VAC position synchros.

Troubleshooting a short that is causing a breaker trip is a fairly simple task. Isolating the user systems by disconnecting the LV connection is the first step. If the control breaker remains in, the short is "downstream" from the transformer. If the breaker is still popping, the HV line should be disconnected to isolate the transformer itself. If the breaker stays in, the transformer is shorted.

THE INFORMATION PRESENTED ON THIS SITE IS TO BE USED AS A GUIDE.

APPROVED AIRCRAFT MANUFACTURER MAINTENANCE MANUAL PROCEDURES SHOULD ALWAYS BE FOLLOWED.