The Time-Triggered Protocol (TTP) is an open computer network protocol for control systems. It was designed as a time-triggered fieldbus for vehicles and industrial applications. and standardized in 2011 as SAE AS6003 (TTP Communication Protocol). TTP controllers have accumulated over 500 million flight hours in commercial DAL A aviation applications, in power generation, environmental and flight controls. TTP is used in FADEC and modular aerospace controls, and flight computers. In addition, TTP devices have accumulated over 1 billion operational hours in SIL4 railway signalling applications.
History TTP was originally designed at the Vienna University of Technology in the early 1980s. In 1998 TTTech Computertechnik AG took over the development of TTP, providing software and hardware products. TTP communication controller chips and IP are available from sources including austriamicrosystems, ON Semiconductor and ALTERA.
Definition TTP is a dual-channel 4 - 25 Mbit/s time-triggered field bus. It can operate using one or both channels with a maximum data rate of 2 × 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 time-division multiple access (TDMA) 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.
Critical applications TTP is often used in mission-critical data communication applications where deterministic operation is a requirement. These operations include aircraft engine management and other aerospace applications. In these applications the TTP networks are often operated as separate networks with separate AS8202NF hardware interface devices and separate, but coordinated, configurations. The TTP protocol offers the unique feature of having all nodes on a network know, at the same time, when any other node fails to communicate or sends unreliable data. The status of each node is updated to all nodes several times each second.
Technical details Data communication in TTP is organized in TDMA rounds. A TDMA round is divided into slots. Each node has one 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.
SLOT There is one slot for each node in a TTP network. A node always transmits data (parameters) during its slot, even if the node has no data to send. However a node will only transmit the parameters that it is configured to send for the specific ROUND that the slot is in. A node may transmit parameters 1,2,3 in its SLOT during ROUND x and parameters 4,5,6 in its SLOT during ROUND y. The slot for a node is determined when the TTP network is designed using PC based utilities TTP Plan and TTP Build. The definition that causes the AS8202NF to transmit specific data or parameters for a given SLOT and ROUND is contained in the MEDL.
ROUND The TTP Round holds a slot for each node in the TTP network. The number of ROUNDS in CLUSTER CYCLE is defined using PC based utilities TTP Plan and TTP Build. This information is also contained in the MEDL. Rounds exist because a node is not required to transmit all of its parameters during its slot. To distribute bandwidth between nodes, each node transmits selected parameters in different ROUNDS.
Cluster cycle A Cluster Cycle is defined as having a number of Rounds. All nodes have transmitted all of their parameters at the end of a Cluster Cycle. The Cluster Cycle is defined as starting with the first bit of the first slot of the first round.
Balance nodes, slots and cluster cycles The number of slots is defined by the number of nodes in the TTP network. However, the number of Rounds is determined by the network designer using the TTP Plan and TTP Build utilities.
Clock synchronization Clock synchronization provides 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.
Membership and acknowledgment Time-Triggered Protocol attempts to transmit data consistently to all correct nodes of the distributed system and, in case of a failure, the communication system attempts to decide which node is faulty. These properties are achieved by the membership protocol and an acknowledgment mechanism.
Configuration requirements Each node connected to a TTP network is required to have configuration data sets resident, prior to the startup of the TTP network. The minimum number of data sets for each node is two. See hardware section and AS8202NF (below). Each node needs to know the configuration of every other node on the TTP network. For this reason, active nodes are not allowed to join an existing network without the update of configuration data sets of all nodes on the network. Typical Configuration Data Sets for each node:
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