Civil Engineering Reference
In-Depth Information
Fig. 1.13
Structure of a TTCAN “Basic Cycle”
the reference message. It is designated by a specific identifier (specified by the
system designer) which is communicated to all network participants.
The reference message itself consists—in the simple case Level I implemen-
tation—of one data byte containing the Cycle Counter (number of the actual ba-
sic cycle) and the control bit, next_is_gap. In Level II implementation, the refer-
ence message contains further information for the interpretation of NTU (refer to
Sect. 1.2.3), an additional Disc_Bit and a 16-bit information giving the actual time
of the time master. The Disc_Bit signals a disruption in the time continuity by an
external synchronization.
While in Level I implementation NTU typically is related to a bit time, in Level
II implementation NTU may be related to the physical time entity “second”.
1.3.5
Communication Sequence
Whenever the cycle time counter of the node, designated as the system clock,
reaches the reference transmission threshold (Tx_Ref_Trigger), this node transmits
its reference message, which is recognized as such by all other nodes due to the
designated identifier and which causes synchronization of their local time counters
or global time counters, in case of Level II implementation (see Sect. 1.2.6). If the
internal local or global time counter of a participant reaches its transmit threshold
(
Tx_Trigger
) signalling, the start of its designated transmission window and if the
bus is not busy, this participant transmits its message which is assigned to this time
window. In case this is an exclusive time window, a receive tag (
Rx_Trigger
) is
set in this window in all those participants which are to receive this message. This
causes error checks upon reception of this message, in order to initiate an error-
handling process in case of a problem detected.
The transmit tags consist of a time tag (column number of the system matrix),
a basic tag (designating the basic cycle in which the message is to be transmitted
for the first time) and a repetition rate (specifying the number of basic cycles after
which the message is to be retransmitted).
The receive tags contain the same information, but only the time tag is deferred
by the window width.
