Texas Instruments 990 User Manual page 30

I ntr odu ct ion
*
Level 3. Oue to the relative sophistication of the
operating system, a TX5 secondary station can have a
maximlUll of seven messages outst andi ng: that is , the
window size of the station is seven. On the other hand,
a PM550 with no operating system has a window size of
one.
*
Level 2 (HOLe level). All the wi ndow si zes are one
because the HOLe software expects a response to every
message sent to a secondary station.
The communications system uses a p:>lling method called "data
priority polling with delay select." When the network is
generated (see Section 3), the user may specify a delay of 250
milliseconds to 8000 seconds. This delay is the maximum time
t hat may
be
allowed to el aps e between i nd i vi dual poll s to a gi ve n
secondary station during a period of low communication activity
between the primary and that secondary. When a primary task
passes a message to the network, the priority of that secondary
station rises so that the time between polls is decreased to the
minimum possible in the prevailing si tuation. If no further
activity from that secondary station is seen, its priority dro:fS
and the time between polls increases. After a specified number
of p:>lls wi th no acti vi ty, the secondary station is p:>lled in
accordance wi th the time established during network generation.
1.4.2.3 Level 3 (Packet Level).
Figure 1-7 illustrates the flow of data at the packet level. For
a transmission, the packet level generates a packet header,
attaches it to the message passed from a user task and then
forwards the combination to the next lower level (HOLe level).
For a reception, the pac ket level removes the header from the
data recei ved from the lower level and forwards the message to
the appropr i ate tas k buff er . The pac ket level al so mai ntai ns
records of the sending and receiving addresses associated with
each tas k message.
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