A Time-Triggered Fieldbus Protocol: TTP/A

TTP/A communication structure

In a time-triggered protocol, the schedules for all messages are predefined and peridocal. So each node in an ensemble knows a priori when to send or when to expect to receive some message. For an overall communication cluster. Time-triggered systems are predictable, it is relatively easy to verify a communication schedule, and the diagnosis of a defect component is facilitated. However, TT systems are inflexible, therefore they are mainly used in dependable systems such as automotive (Flexray protocol) and avionic applications (Safebus, TTP).
With TTP/A, there is, however, also a time-triggered fieldbus protocol. A fieldbus connects devices like sensors, controllers, and actuators, and, therefore must support a higher flexibility than traditional TT applications. On the other hand, the periodic behavior of message schedules is beneficial for sensor/actuator applications and closed control loops, where timeliness is required and arbitrary message delays worsen or even invalidate the result.
TTP/A manages these contradictionary requirements for timeliness, predictability and flexibility by reserving a part of the schedule for monitoring and configuration. Although this means, that the effective communication rate for real-time traffic is reduced, it, unlike in other protocols, guarantees this rate, regardless of other traffic.

TTP/A node size vs. debit card

The hardware requirements for TTP/A are low – even for embedded systems standards. The protocol runs on any microcontroller providing a UART interface, a few kilobytes of flash memory and around 100 bytes (sic!) of RAM. A generic implementation is available as open source software.

Links on TTP/A:

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About Wilfried Elmenreich

Understanding the communication networks of the future.
This entry was posted in Protocols, Real-Time Networks and tagged , , , , . Bookmark the permalink.

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