TTP/A

TTP/A is a time-triggered real-time fieldbus protocol used for the interconnection of low-cost smart transducer nodes. In contrast to TTP, which is a high-speed, fault-tolerant communication network intended for safety critical applications (SAE class C), TTP/A provides a low-speed (below 1 MB/s), low-cost protocol suitable for non-critical applications like car body electronics (SAE class A).

Time-triggered means that all relevant actions in the system, like communication or the execution of tasks are scheduled according to predefined instants in time. The schedules are repeated periodically, so that the system shows a predictable repetetive behavior. This is of special interest for sensor/actuator applications and closed control loops, where a timeliness is of utmost importance and arbitrary message delays worsen or even invalidate the result. TTP/A node The TTP/A protocol implements the communication layer and the data access according to the OMG Smart Transducer Interface standard. The interface to each transducer is provided by a so-called Interface File System (IFS), that provides a unique access to real-time data, but also to configuration and node management features. The Interface File System (IFS) makes it possible to share data between different TTP/A clusters in a transparent fashion, so that for the application no distinction between data from local and remote nodes is made. The TTP/A protocol does not require special controller hardware and can be implemented on any standard embedded microcontroller. An open-source implementation exists for the Atmel AVR architecture. Like the fault-tolerant time-triggered protocol TTP, TTP/A follows the time-triggered paradigm featuring a clock synchronization and predefined time-triggered schedules for communication and execution of tasks. Thus, TTP/A communication implements a strict TDMA scheduling. TTP/A can be used as a sub-bus for a more performant and dependable real-time system such as TTP, but can be also used to provide transducer data to a Linux system in a soft-real-time fashion. So far, TTP/A has been used only in a limited number of applications, like as an intercommunication system for mobile robots or as experimental test platform.

Principle of operation

TTP/A is a time-triggered protocol used for the communication of one active master with or among smart transducer nodes within a cluster. This cluster is controlled by the master, which establishes a common time base among the nodes. In case of a master failure, a shadow master can take over control. Every node in this cluster has a unique alias, an 8 bit (1 byte) integer, which can be assigned to the node a priori or set at any time via the configuration interface. Periodic time-triggered schedule

The TTP/A communication is organized into rounds separated by inter round gaps (IRG). IRGs are slots where the TTP/A bus is inactive for at least 13 bit times. A TTP/A round consists of one or more frames. A frame is a sequence of bytes transmitted by one node. A byte is transmitted in a slot consisting of 13 bit cells (one start-bit, eight data-bits, one parity, one stop-bits and a two bit cell wide inter byte gap (IBG)). The rounds are independent from each other. Every round starts with a fireworks byte sent by the master. The arrival of the fireworks byte is a synchronization event for every node in the cluster and identifies the round. According to the specification of the selected round, the fireworks frame is followed by data frames of specified length from the specified nodes. Each such frame is described by an entry in the round descriptor list (RODL) in the file-system of the sender and the receiver(s).

Because the slot position at which each communication action takes place is defined a priori, no further bus arbitration is necessary.

References

Citation
first = Herman
last = Kopetz
first2 = Michael
last2 = Holzmann
first3 = Wilfried
last3 = Elmenreich
title = A Universal Smart Transducer Interface: TTP/A
year = 2000
publisher = 3rd IEEE International Symposium on Object-oriented Real-time distributed Computing (ISORC 2000)
doi = 10.1109/ISORC.2000.839507

External links

* [http://www.vmars.tuwien.ac.at/ttpa TTP/A Documentation]
* [http://www.omg.org/technology/documents/formal/smarttrans.htm OMG Smart Transducer Interface]