data, Binary Coded Decimal (BCD), and Binary Number Representation (BNR) are common ARINC 429 data formats. Data formats may also be mixed. Bits 9 and 10 are Source/Destination Identifiers (SDI) and may indicate the intended receiver or, more frequently, indicate the transmitting subsystem. Bits 1 to 8 contain a label (label words), expressed in octal (MSB 1 bit numbering), identifying the data type. The image below exemplifies many of the concepts explained in the adjacent sections. In this image the Label (260) appears in red, the Data in blue-green and the Parity bit in navy blue. Labels Label guidelines are provided as part of the ARINC 429 specification, for various equipment types. Each aircraft will contain a number of different systems, such as flight management computers, inertial reference systems, air data computers, radar altimeters, radios, and GPS sensors. For each type of equipment, a set of standard parameters is defined, which is common across all manufacturers and models. For example, any air data computer will provide the barometric altitude of the aircraft as label 203. This allows some degree of interchangeability of parts, as all air data computers behave, for the most part, in the same way. There are only a limited number of labels, though, and so label 203 may have some completely different meaning if sent by a GPS sensor, for example. Very commonly needed aircraft parameters, however, use the same label regardless of source. Also, as with any specification, each manufacturer has slight differences from the formal specification, such as by providing extra data above and beyond the specification, leaving out some data recommended by the specification, or other various changes. Protection from interference Avionics systems must meet environmental requirements, usually stated as RTCA DO-160 environmental categories. ARINC 429 employs several physical, electrical, and protocol techniques to minimize electromagnetic interference with on-board radios and other equipment, for example via other transmission cables. Its cabling is a shielded 70 Ω twisted-pair. ARINC signalling defines a 10 Vp differential between the Data A and Data B levels within the bipolar transmission (i.e. 5 V on Data A and -5 V on Data B would constitute a valid driving signal), and the specification defines acceptable voltage rise and fall times. ARINC 429's data encoding uses a complementary differential bipolar return-to-zero (BPRZ) transmission waveform, further reducing EMI emissions from the cable itself. Development tools When developing and/or troubleshooting the ARINC 429