Time-Triggered Protocol

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TTP ( Time-Triggered Protocol Class C or TTP / C for short ) is a fieldbus-based transmission protocol that regulates fault-tolerant, time-controlled communication between electronic components and meets tough real-time requirements.


The special thing about this protocol is that it works time-controlled, i. That is, it uses the common time as control signals. The clocks of the individual participants are continuously synchronized by averaging the differences between the real arrival of the message and the expected arrival of the message. This clock synchronization algorithm works decentrally and is fault-tolerant in relation to any individual faults. The expected arrival times are specified a priori in the MEDL (Message Descriptor List). Through the MEDL, each participant knows when he is allowed to send and when he has to receive.

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

In order to prevent the disruption of the entire communication by a defective participant who sends pointless data to the channels (the so-called " babbling idiot "), each participant has a local bus guardian (bus topology), which independently of the participant transmits to the channels only during the broadcast time specified in the MEDL. With a star topology, a central bus guardian is used for all participants per channel (cost savings).

A subscriber's message must be supplemented with a minimal header and this resulting frame is protected by a CRC checksum.

Another measure to be suitable for security-critical applications is the membership service, in which each node keeps a list of the nodes assumed to be error-free. If the membership list does not match those of the other nodes, then this node assumes an error in itself and switches itself off (passive mode). The membership list is updated with each transfer. If a node sends an incorrect frame, it is excluded from the list.

A (small) risk with this procedure is that so-called cliques form, that is, two or more groups that consider each other to be error-free and only communicate with one another. To avoid this, each node also checks whether it is a member of the largest 'clique' and switches to passive mode if the result is negative.

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


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

Like TTP / C, TTP / A uses a TDMA scheme for collision-free transmission of messages. TTP / A is a master-slave system that has been developed for low-cost field bus system applications. TTP / A can be implemented on a standard 8-bit microcontroller with a standard UART (Universal Asynchronous Receiver Transmitter); however, this physical layer is in principle interchangeable. In the standard implementation, TTP / A uses UART coding with a start bit, 8 data bits, a parity bit and a stop bit. Communication is divided into rounds and slots; exactly one UART word is sent in one slot. A round consists of a fixed sequence of slots, each slot being assigned to a specific message. In this context, one speaks of a news timetable or Round Description List ( RODL ). Messages are thus transmitted periodically with a predictable time behavior. TTP / A supports eight different RODLs , so that you can quickly switch between different compilations of data.

TTP / A guarantees a fixed response time if no transmission errors occur. If, however, errors occur, the transmission of the message is delayed by one period. In general, this is not a problem for sensor data, but for commands sent these must be idempotent in order to guarantee fault-tolerant behavior in the time domain.

TTP / A offers an addressing scheme called the Interface File System ( IFS ) for up to 256 nodes with 64 files each. A file consists of up to 256 records of 4 bytes each. The IFS offers standardized addressing of data words when creating message timetables.

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