wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

Protocols of the Time-Triggered Architecture: TTP, TTEthernet, TTP/A 43-7
Global time counter
32 s
61 s
8 s
4 s
2 s
1 s
500 ms
250 ms
125 ms
62.5 ms
31.2 ms
15.6 ms
7.8 ms
3.9 ms
1.95 ms
976.5 μs
488.2 μs
244.1 μs
122 μs
61 μs
30.5 μs
15.2 μs
7.6 μs
3.8 μs
1.9 μs
953.6 ns
476.8 ns
238.4 ns
119.2 ns
59.6 ns
29.8 ns
14.9 ns
2 5 2 4 2 3 2 2 2 1 2 0 2 –1 2 –2 2 –3 2 –4 2 –5 2 –6 2 –7 2 –8 2 –9 2 –10 2 –11 2 –12 2 –13 2 –14 2 –15 2 –16 2 –17 2 –18 2 –19 2 –20 2 –21 2 –22 2 –23 2 –24 2 –25 2 –26
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 Offset bits
Period 15
Period 14
Period 13
Period 12
Period 11
Period 10
Period 9
Period 8
Period 7
Period 6
Period 5
Period 4
Period 3
Period 2
Period 1
Period 0
FIGURE 43.6
Period and phase (offset).
43.5.4 Fault-Tolerant TTEthernet Configuration
In the safety-critical configuration of TTEthernet, the network provides its services in the presence
of node or communication channel failures [AKG+06]. To tolerate a fault in a communication
channel, a safety-critical TTEthernet controller, which transmits and receives redundant messages
using two distinct communication channels, is required. A message that should be redundantly sent
on two communication channels is called protected TT message. In a safety-critical configuration,
a TTEthernet switch is accompanied by a guardian controlling the input and output ports of the
switch. The guardian has knowledge about the time-triggered schedule of all protected TT messages
and dynamically disables all input ports that could possibly interfere with the transmission of a
protected TT message.
43.5.5 Clock Synchronization
The clock synchronization mechanisms in TTEthernet can be adapted to the specific requirements
of the actual applications. The initial establishment of the global time after start-up and the maintenance
of this time during operation is performed by a subset of TTEthernet controllers called
time-keeping controllers [KAGS05]. The most basic form that is supported is the central master
algorithm. In this case, a single time-keeping controller—the master—will periodically send out a
resynchronization message containing its local time. Being aware of the transmission latency of the
clock synchronization message, the individual nodes can adjust their clock according to the time
value in the synchronization message.
To tolerate a fail-stop failure of the time-master, a shadow master can be employed that takes over
when the current master stops sending synchronization messages.
In the most dependable configuration, multiple time-keeping controllers can execute a Byzantine
resilient clock synchronization algorithm that is able to tolerate an arbitrary failure of any controller.
43.6 ttP/A
The time-triggered fieldbus TTP/A is intended for the integration of smart transducers in all types of
distributed real-time control systems. Although the first targets are automotive applications, TTP/A
has been designed to meet the requirements of process control systems as well. TTP/A provides a welldefined
smart transducer interface to the sensors and actuators of a network. TTP/A is a master/slave
© 2011 by Taylor and Francis Group, LLC