Appendix A Data Transfer in Digital Aircraft Systems A.1 Serial Interfaces A large number of sensors in the airframe and engine systems require only a low volume data transfer between the sensors, the microcontroller and the central processor. Because every computer depends on the communica- tion with peripheral devices, it contains components for data transfer with external equipment. Prevalent on PCs as well as on industrial, scientific and consumer devices is the RS-232 (EIA-232) serial port as a means of control, monitoring and low volume data transfer. One port can connect to only one peripheral device. A serial port transmits and receives data one bit at a time over one wire. While it takes longer to transfer data this way, only a few wires are re- quired. Two way (full duplex) communication is available with only three separate wires, one to transmit, one to receive and a common ground wire. The RS-232 specifications include numerous additional control lines, which are used for special applications only. Generally processors internally use 8-, 16-, 32-, or 64-bit parallel data buses for faster processing. Thus data intended for transmission on a serial data line has to be converted from a parallel to a serial data stream. Like- wise data received has to be converted from a serial to a parallel data stream. These conversions can be performed by applying appropriate soft- ware, which is a method generally used on microcontrollers. A specific piece of hardware that converts data between a parallel bus and an RS-232 interface in both directions is the UART (Universal Asyn- chronous Receiver-Transmitter). The RS-232 serial port is an asynchro- nous device. For an asynchronous transmission its start is identified by a start bit and the end by one and a half or two stop bits. The data bits are sent to the receiver after a start bit. Such a data character usually consists of 7 or 8 bits. A parity bit may optionally be transmitted after the data. The transmitter and the receiver must agree on the number of data bits and the transfer rate. After converting a character to be transmitted from parallel to serial, the UART adds the start and stop bits and sends the result to the se- 200 Appendix A Data Transfer in Digital Aircraft Systems rial port. Every character received is stored, the start and stop bits are re- moved and the character is converted from serial to parallel. Then it is ready to be read by the processor. The UART usually does not directly generate or receive the external signaling levels (voltages) that are used between the devices. An interface is used to convert the logic level signals of the UART to the external sig- naling levels. All signals are measured in reference to a common ground. A positive voltage between 3 and 15 V represents a logical 0 and a nega- tive voltage between -3 and -15 V a logical 1. This switching between positive and negative is called bipolar. The zero state is not defined and is considered to be a faulty condition (this happens when the device is not operating). The dead area between +3 and -3 V is designed to absorb noise. By employing the RS-232 specification, use is made of a mature and universally available PC technology. If higher noise immunity, higher data transfer rates and more complex networks are necessary, one of two other serial interfaces is used. These are the (TIA/EIA) RS-422 and RS-485. These use two twisted wires for the data transmission. On one line a true signal is transmitted and on the other an identical signal but of opposite polarity. This produces opposing currents and magnetic fields, thus mini- mizing the emitted electromagnetic interference by cross-canceling the op- posite fields around each wire pair. Furthermore noise is coupled to both wires of the pair in the same way and thus is common to both signals. As the receiver evaluates the difference between the voltages of both wires, the effect of the noise is eliminated. This noise immunity allows for trans- missions at higher data rates and over longer distances. (TIA/EIA) RS-422 allows one transmitter to connect unidirectionally (simplex) to up to 10 receivers. Due to the lack of bidirectional capabilities allowing multipoint connections, the (TIA/EIA) RS-485 standard was cre- ated to add this capability. Its standards only describe the electrical proper- ties of the network. No transmission protocol is specified in the standard. However, several field bus standards in use specify RS-485 as electrical standard of data transmission. Among these are Profibus and Interbus-S. These widely used and proven standards can readily be adapted. RS-485 meets the requirements for a truly multi-point communication network. The standard specifies up to 32 transmitters and receivers. In a four-wire network one node is the master and all the others are slaves. The master node communicates to all slave nodes. All slave nodes communicate only with the master node. The slave nodes never listen to another slave’s re- sponse to the master. The master commands a response from only one slave at a time and thus prevents data collision. Adopting the RS-485 inter- A.2 Data Buses 201 face allows to take advantage of the widely used industrial field bus tech- nology, for which plenty of proven hardware and software is available. A.2 Data Buses A.2.1 ARINC 429 The ARINC 429 avionic data bus was specifically designed for use in civil transport aircraft and introduced in 1977. It defines how avionic devices and systems can communicate with each other. The specifications define the electrical and data characteristics and as well the protocols to be used. ARINC 429 is a unidirectional (Simplex) data bus using only one transmit- ter and at least one and not more than 20 receivers. The data bus consists of a screened twisted pair of wires with the screens usually connected to ground at both ends. Like the RS-422 and RS-485 interfaces always identical signals but of opposite polarity are transmitted along the two wires. This secures the noise immunity already described. The modulation is also bi-polar but different to the RS-422 and RS-485 interface standards. A logic state 1 is represented by a high state, which after one half of the bit length returns to zero. A logic state 0 is rep- resented by a low state, which after one half of the bit length returns to zero. This principle eliminates the need for a synchronization signal and thus the need for start and stop bits. The transmission voltage is 10±1 V between the wires. If one wire is +5 V, the other is -5 V and vice versa. Data is transmitted serially in 32-bit words. Thus the transmitter is transmitting 32-bit words or the null state. Each 32-bit word is separated by an interword gap of four bit-times. The ARINC 429 specifications define a high data rate of 100 kbps ±1% and a low data rate between 12.0 and 14.5 kbps. Each bus always operates either at high or low speed. The transmitter converts parallel binary words into 32-bit words. The receiver will convert these 32-bit serial words into parallel binary words readable by the processor to which it is connected. Bits 1 through 8 of a 32-bit word contain a system address label (SAL). This label not only de- fines a parameter but also the data type and therefore the rule how the 23 of the remaining 24 bits have to be interpreted. Three protocols are defined in ARINC 429 standards. These protocols are for the transmission of nu- meric data, discrete data and data files. The standard defines many fixed labels, which also define how the data is presented. Thus all manufacturers of different pieces of equipment al- 202 Appendix A Data Transfer in Digital Aircraft Systems ways have a common basis on which data is presented. The receivers are programmed to look for only that data which is relevant to its operation. Relevant data is identified by the label. The last bit is always the parity bit. Odd parity is specified. That means, that there must be an odd number of “1” bits in the 32-bit word. The parity bit is set according to the number of “1” bits. Since the ARINC 429 data bus transmits data in only one direction it is extremely reliable. There is a very low probability of data conflicts or data corruption. Thus it has become the mainstay of federated avionics archi- tectures. A.2.2 ARINC 629 The ARINC 629 data bus was originally developed and patented by Boe- ing and integrated into the Boeing 777 design. In 1989 it was released as an ARINC specification. Cost concerns about this avionic specific bus sys- tem and the emerging popularity of Ethernet technology have prevented further ARINC 629 implementation after the B777 development. Never- theless this standard initiated the move from the federated avionics archi- tecture – supported by the ARINC 429 bus – to the higher levels of inte- gration of the integrated modular avionics architecture. ARINC 629 is a bidirectional distributed control bus capable of support- ing up to 120 users. A 2 Mbps serial data transmission rate is specified for a twisted pair of conductors. Thus it is 20 times faster than an ARINC 429 bus. Each coupling to an ARINC 629 data bus is made via an ARINC 629 transmitter/receiver terminal.