Information Technology Reference
In-Depth Information
The bus arbitrator then repeats the table, indefinitely. The
period
is the scan period (in ms),
the
type
defines the data type (such as INT_8 for an 8-bit integer, INT_16 for a 16-bit
integer, OSTR_32 for a 32-character string) and
time
represent the total transaction time. In
the case, the initial sequence would be:
(0 ms) (5 ms) (10 ms) (15 ms) (20 ms) (25 ms)
TEMP_1 TEMP_1 TEMP_1 TEMP_1 TEMP_1 TEMP_2
TEMP_2
TEMP_2
TEMP_2
PRES_1
PRES_1
PRE 2
PRE 2
The total time scanning must not exceed the repetition period, and the time which is not used
with a periodic scan can be used by an aperiod transfer. A macrocycle goes through the scan
sequence, and repeats. For example, in the case above the macrocycle will be:
1
st
Macrocycle 2
nd
Macrocycle
111111111111 111111111111
2 2 2 2 2 2 2 2 2 2 2 2
3 3 3 3 3 3 3 3
4 4 4 4 4 4
On a WorldFIP network there may be one or more bus arbitrators, but only one bus arbitrator
can be active at any time. When a bus arbitrator is active, the others are silent to the traffic on
the bus, and if a fault occurs on the currently active arbitrator elect a new arbitrator. This
election takes place without consultation.
Within a WorldFIP network, each station has a physical station address of between 0 and
255. The mechanism of electing the new arbitrator is a function of the stations address and
also for a time period (T3). When a dormant arbitrator detects a silence on the bus it waits for
the time period T3, and then elects itself as the bus arbitrator (if another arbitrator has not
elected itself). The time period (T3) is calculated as:
T
=
4
(
n
+
1
T
3
o
where
n
is the station address and
T
o
is basic time filler (110
ยต
s by default).
Thus the lower the address of a potential arbitrator the higher the chance it has to become the
arbitrator. After election, the bus arbitrator begins scanning. The new arbitrators must be set
up with the same elementary cycles and macrocycles as the previous arbitrator. The new bus
arbitrator can then change these by transmitting a bus arbitrator synchronisation variable,
which contains an elementary cycle number and a macrocycle number. All other dormant
bus arbitrators read these values and change their values for the macrocycles and elementary
cycles.
20.3.5 Aperiodic transfer
Variables that are not in the bus arbitrators cyclic scanning table can also be transmitted
using an aperiodic transfer. A station that can request an aperiodic transfer can be a producer
of a variable, a consumer, or both. It involves:
