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

24-6 Industrial Communication Systems
TI (terminal 1)
TI (terminal 2)
TI (terminal 3)
M1
M2
M3
Free
space
M1
M2
M3
TG 2 TG 3
SG
TG 1
TG 2 TG 3
Basic protocol : periodic mode (when TI is sufficiently large)
TI (terminal 2)
TI (terminal 1)
TI (terminal 3)
M2
M1
M3
M4
M2
M1
SG
TG 2 TG 1 TG 3
TG 4
SG
TG 2
TG 1
Basic protocol : aperiodic mode (when TI is too small)
FIGURE 24.4
ARINC 629 basic protocol.
transmissions of each terminal. Finally, the synchronization gap (SG) is a common quiet time, longer
than any individual TG, which is used to synchronize all the terminals at the beginning of a new cycle.
These three timers are used at each terminal in the following manner:
• The TI timer starts immediately every time the terminal begins transmitting.
• The SG timer starts immediately every time the bus is sensed quiet. This timer may be reset before
it has elapsed if any activity is detected on the bus. This timer does not affect periodic mode but in
aperiodic mode, after it has elapsed we know that all the terminals have had transmit access to the
bus (as SG timer is greater than each TGi timer) and so a new cycle can begin.
• A TGi timer will start after SG has elapsed. It also starts immediately every time the bus is sensed
quiet. This timer may also be reset before it has elapsed if any bus activity is detected.
If all three timers have elapsed, the local terminal begins transmitting. As all TGi are different, only
one terminal will be allowed to transmit after any quiet time. In aperiodic mode, the terminals are
scheduled in ascending order of their TGs.
The assignment of timer values is done on a per-system basis and must address a trade-off between
bus cycle time (all terminals transmit exactly once within a cycle) and terminal transmission frequency
(all terminals must be allowed to transmit at a frequency suitable for data update rate requirements).
24.5.2 Combined Protocol
The CP has been proposed for combining periodic and aperiodic data transmission. As shown in Figure 24.5,
three levels of transmission are handled: level one for periodic transmissions, level two for shorter (and higher
priority) aperiodic transmissions, and level three for longer (and lower priority) aperiodic transmissions.
As with BP, each terminal has a unique preassigned TG. The transmit interval (TI) is only applicable during
the first periodic transmission in each cycle; for all other terminals, a concatenation event (CE) forces
all unelapsed TI timers to be canceled. This has the effect of compressing periodic transmissions at the start
of each cycle (separated only by TG delays). Two types of SGs are defined: the periodic synchronization gap
(PSG), which is used for cycle level synchronization; and the aperiodic synchronization gap (ASG), which is
needed to handle transition in a cycle between levels 1 and (and between levels 2 and 3). In order to guarantee
© 2011 by Taylor and Francis Group, LLC