Time-Triggered Protocol
TTP (Time-Triggered Protocol) is an open and modular control system platform technology that supports the design of upgradeable, reusable and easy-to-integrate systems. As a time-triggered fieldbus, it can significantly impact the design of modern electronics and control system architectures for next-generation vehicles and industrial applications.
TTP was originally designed at the Vienna University of Technology in the early 80s. From 1998 onwards TTTech Computertechnik AG has taken over the further development of TTP, providing software and hardware solutions for this data communication protocol. Today TTP communication controller chips and IP are available from multiple sources including austriamicrosystems (SWX: AMS), ON Semiconductor (NASDAQ: ONNN) and ALTERA (NasdaqGM: ALTR).
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[edit] Definition
TTP is a dual-channel 25 Mbit/s time-triggered field bus. It can operate using one or both channels with maximum data rate of 2x 25 Mbit/s. With replicated data on both channels, redundant communication is supported.
As a fault-tolerant time-triggered protocol, TTP provides autonomous fault-tolerant message transport at known times and with minimal jitter by employing a TDMA (Time-Division Multiple Access) strategy on replicated communication channels. TTP offers fault-tolerant clock synchronization that establishes the global time base without relying on a central time server.
TTP provides a membership service to inform every correct node about the consistency of data transmission. This mechanism can be viewed as a distributed acknowledgment service that informs the application promptly if an error in the communication system has occurred. If state consistency is lost, the application is notified immediately.
Additionally, TTP includes the service of clique avoidance to detect faults outside the fault hypothesis, which cannot be tolerated at the protocol level.
[edit] Technical details
[edit] Frame, Message, Slot, TDMA Round and Cluster Cycle
Data communication in TTP is organized in TDMA rounds. A TDMA round is divided into slots. Each node in the communication system has one slot – its sending slot – and must send frames in every round. The frame size allocated to a node can vary from 2 to 240 bytes in length, each frame usually carrying several messages. The cluster cycle is a recurring sequence of TDMA rounds; in different rounds different messages can be transmitted in the frames, but in each cluster cycle the complete set of state messages is repeated. The data is protected by a 24-bit CRC (Cyclic Redundancy Check). The schedule is stored in the MEDL (Message Descriptor List) within the communication controller. The figure below gives a better understanding of those technical terms.
[edit] Clock synchronization
The clock synchronization is necessary to provide all nodes with an equivalent time concept. Each node measures the difference between the a priori known expected and the observed arrival time of a correct message to learn about the difference between the sender’s clock and the receiver’s clock. A fault-tolerant average algorithm needs this information to periodically calculate a correction term for the local clock so that the clock is kept in synchrony with all other clocks of the cluster.
[edit] Membership and Acknowledgment
A major philosophy in the design of TTP is that the protocol should transmit data consistently to all correct nodes of the distributed system and that, in case of a failure, the communication system should decide on its own which node is faulty. These properties are achieved by the membership protocol and an acknowledgment mechanism.
[edit] Commercial applications
TTP has been integrated into a number of commercial applications.
[edit] Thales Rail Signalling Solutions
The electronic interlocking system “LockTrac 6131 ELEKTRA” was designed within a cooperation of Thales Rail Signalling Solutions division and TTTech.
LockTrac 6131 ELEKTRA is an electronic interlocking system that provides the highest levels of safety and availability. The system is approved according to CENELEC standards with safety integrity level 4 (SIL4) and offers basic interlocking functions, local and remote control, automatic train operation, integrated block functionality and an integrated diagnosis system. LockTrac 6131 has two software channels with diverse software, to ensure the high safety requirements. Before getting transmitted externally, the data are checked in the safety channel. A diagnosis device saves all relevant information to allow efficient maintenance in case of failure.
[edit] FADEC
The system has been used for FADEC (Full Authority Digital Engine Control) systems.
The Modular Aerospace Control (MAC)-based FADEC for The Aermacchi M346 is scalable, adaptable and fault-tolerant. The key technology enabler in this new FADEC is the use of TTP for inter-module communication. TTP removes the complex interdependencies among modules, simplifying initial application development as well in-service changes and upgrades. It allows all modules in a system to see all data all of the time, thus ensuring seamless fault accommodation without complex channel change logic.
TTP-based Modular Aerospace Control (MAC), which is a part of the F110 full authority digital engine control (FADEC) system of General Electric, is integrated on the Lockheed Martin F-16 fighter aircraft. TTP, which is used as a backplane bus, supports high levels of engine safety, operational availability and reduced life cycle cost. A significant advantage is that all information on the bus is available to both FADEC channels simultaneously.
[edit] Environmental Systems
For the Airbus A380 TTTech developed the internal communication system for the cabin pressure control system, working together with Nord-Micro, a subsidiary of Hamilton Sundstrand Corporation.
In cooperation with Hamilton Sundstrand Corporation, TTTech developed a TTP-based data communication platform for the electric and environmental control system of the Boeing 787 Dreamliner. The TTP-designed communication platform prevents an overload in the bus system, even if several important events occur simultaneously. Additionally, TTP-based systems weigh less than conventional systems due to a lower connecter count and less wiring. Furthermore, the whole system is more flexible and has a greater modularity than conventional communication systems.
[edit] Autonomous Vehicles
The two Red Team robotic vehicles competing in the 2005 DARPA Grand Challenge were implemented with "drive-by-wire" technology, in which on-board computers controlled steering, braking and other movements. Three TTP-based TTC 200 units controlled the parking brake and throttle and transmission functions, and one TTP-By-Wire Box controlled the service brake of the H1 Hummer H1ghlander. Drive-by-wire modifications controlled the acceleration, braking and shifting of the Sandstorm.
[edit] See also
[edit] References
Kopetz, Herman; Grunsteidl, Gunter (1993-06-22 - 1993-06-24), "TTP - A time-triggered protocol for fault-tolerant real-timesystems", Fault-Tolerant Computing, 1993. FTCS-23. Digest of Papers., The Twenty-Third International Symposium on, Toulouse, France: IEEE, pp. 524–533, doi:10.1109/FTCS.1993.627355, 0-8186-3680-7
