Time-Triggered Protocol

TTP (Time - Triggered Protocol Class C or short: TTP / C ) is a feldbusbasierendes transmission protocol, the fault-tolerant time-triggered communication between electronic components regulates and fulfilled hard real -time requirements.

General

The special feature of this protocol is that it works with timers, ie it as control signals, the common time used. The clocks of the individual devices are continually synchronized by averaging the differences between the real arrival of the message and the message of expected arrival. This clock synchronization algorithm operates in a decentralized and fault- tolerant to any single errors. The expected arrival times are specified a priori in the MEDL (Message Descriptor List ). By MEDL each participant is aware of when he is allowed to send and when to receive.

This type of communication is called a division TDMA (Time Division Multiple Access). In a TDMA round, each participant has the opportunity for a certain time to send a message. The TDMA round is repeated as soon as the previous one is completed.

To prevent the failure of the entire communication through a defective subscriber who sends meaningless data to the channels ( the so-called " babbling idiot" ), each participant has a local bus guardian ( bus topology ), regardless of the participant to broadcast on the channels allows only during the specified in the MEDL airtime. In star topology, a central bus guardian is per channel for all participants used (cost savings).

The message from a subscriber needs to be complemented by a minimal header and the resulting frame is protected by a CRC checksum.

A further measure to be suitable for safety-critical applications, the membership service where each node maintains a list of nodes is assumed to be error-free. Should the membership list may not coincide with those of the other nodes, then takes this node to an error in himself and turns off ( passive mode ). At each transfer the membership list is updated. Should a node to send a defective frame, it is excluded from the list.

A (low ) risk of this method is that form so-called cliques, which are two or more groups, with each view themselves as free of errors and only communicate with each other. To avoid this, each node also checks whether he is a member of the largest ' clique ' and switched with a negative result in the passive mode.

TTP is used in a number of industrial production projects. Honeywell uses the protocol in the engine control system (FADEC ) for the Lockheed Martin F -16 and for the training aircraft Aermacchi M 346 and in a fly- by-wire cockpit. The Airbus A380 mega- airliner TTP is used to control the cabin pressure system produced by Nord-Micro. Hamilton Sundstrand has chosen 787 Dreamliner for the use of a TTP-based data communication platform in the electric and environmental control systems of Boeing. Alcatel uses the protocol since June 2002 as a fieldbus protocol in the station signal control system ELEKTRA 2

TTP / A

TTP / A ( Time-Triggered Protocol Class A) is another time-triggered protocol of the Time-Triggered Architecture, which is designed for real-time control of sensors, actuators and non-safety -critical control devices.

As TTP / C uses TTP / A is a TDMA scheme for collision-free transmission of messages. TTP / A is a master-slave system, which has been developed for low-cost field bus applications. TTP / A can be implemented on a standard 8-bit microcontroller with a standard UART ( Universal Asynchronous Receiver Transmitter ); This physical layer is interchangeable but in principle. The default implementation of TTP / A uses a UART coding with a start bit, 8 data bits, one parity bit and one stop bit. The communication is divided into rounds and slots in a slot a UART word is exactly sent. A round consists of a fixed sequence of slots, each slot is assigned to fix a particular message. One speaks in this context of a message or schedule Round Description List ( rodl ). Messages are thus transmitted periodically with a predictable time behavior. TTP / A supports eight different rodl so fast you can switch between different sets of data.

TTP / A guarantees a fixed response time when no transmission errors occur. If errors occur, however, this would delay the transmission of the message by a period. This is the case of sensor data is generally not a problem at these commands must be sent idempotent However, in order to guarantee a fault-tolerant performance in the time domain.

TTP / A provides an addressing scheme called Interface File System ( IFS) for up to 256 nodes, each with 64 files. A file consists of up to 256 records for each 4 bytes. The IFS provides a uniform addressing of data words in the creation of news timetables.