Image Processing Reference
In-Depth Information
Positioning System (GPS) time, but has a wider horizon. The similarity to the GPS time-format
facilitates external clock synchronization using a GPS receiver.
14.5.4.2 Periodic Exchange of State Messages
TT Ethernet supports two types of communication, namely, standard event-triggered Ethernet traffic
and the time-triggered traffic that is temporally guaranteed. For the time-triggered traffic, a static or
dynamic scheduler has to define the conflict-free periods of the messages. he scheduler ensures that
no conflict between TT Ethernet messages occurs. If an event-triggered Ethernet message comes into
conflict with a time-triggered Ethernet message, then the TT Ethernet switch [SGAK] preempts
the transmission of the event-triggered message. After the completion of the transmission of the
TT Ethernet message, the switch autonomously retransmit the preempted event-triggered Ethernet
message.
Based on the time format, TT Ethernet restricts the period durations of time-triggered messages
to the positive and negative powers of two of the second. Consequently, TT Ethernet supports up to
 different period durations corresponding to the  different bits in the time format. he bit in the
binary time format that specifies the period is denoted as the period bit. The phase of a message is
specified with  bits to the right of the period bit.
14.5.4.3 Fault Isolation Mechanisms
TT Ethernet supports two configurations, namely, a non-fault-tolerant and a fault-tolerant con-
figuration. The non-fault-tolerant configuration uses a single switch, whereas the fault-tolerant
configuration requires two independent switches each of which is monitored by a guardian. The
guardian observes the behavior of the switch and disables the outputs of the switch in case an error is
detected. he error detection performed by the switch includes the monitoring of the switch delays,
thecomplianceoftheswitchtotheprecomputedmessageschedule,andasemanticanalysisofthe
time-triggered message header. he guardian has its own fault-tolerant clock synchronization algo-
rithm and receives all time-triggered messages from the switch. he time-triggered message header
in the fault-tolerant configuration is extended to contain a membership vector, the period counter,
and a signature of the scheduler.
14.5.4.4 Diagnostic Services
In the fault-tolerant configuration, TT Ethernet provides a membership vector that is calculated
according to the same algorithm as in TTP [TTTb]. In addition, the guardian of the fault-tolerant
configuration produces diagnostic messages to inform about detected errors [KAGS].
14.5.4.5 Commercial or Prototypical Components
Prototypes of TT Ethernet are available as hardware- and software-based implementations. A
hardware-based implementation is described in Ref. [SA]. A VHDL module has been developed,
which can be applied in System On a Programmable Chip systems. In addition, the VHDL module
has been applied in a Cardbus compliant PCMCIA card to provide TT Ethernet support for devices
equipped with a Cardbus interface.
In addition, a software implementation of a TT Ethernet controller has been realized [GASK].
This implementation uses a Soekris  www.soekris.com board, which incorporates a  MHz
 class processor,  Mbyte RAM, and  Ethernet interfaces (see Figure .). The TT Ethernet
controller was implemented using the Linux-operating system with the RTAI extension.
 
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