Texas Instruments 990 User Manual page 213

Throughput
Appendix B
Throughput
B.l POLLING METHODS
The two processes involved in the control of a multipoint polled
line are polling and addressing. Polling is used by the master,
or primary, station to solicit input messages from the secondary
stations. Addressing is used to send messages to secondary
stations. In the polling process, the primary station invites
the secondary stations one by one to send any messages that are
waiting to be transmitted to the primary station. A secondary
with a message sends it as a reply to the poll. If a secondary
has no message to send, the reply is a negative poll indicating
that the secondary has no data to send. The primary station then
polls the next station. This process is referred to as roll call
polling.
The primary invites each secondary to senq by working through a
polling list until all secondary stations in the list have been
polled. The primary then goes back to the top of the polling
list and begins the roll call again. This polling list is a
fixed-format data structure that is generally stored on a disk
file and loaded into memory prior to the start of communications
activity. It should be modifiable so that the list of stations
can be changed as the network grows (or decreases) in size.
There are many variations of roll call polling, but none of them
eliminate the roll calling aspect of the process. In general,
any scheme that causes each secondary station to be polled as the
primary works its way through a polling list can be termed roll
call polling. Some variations of the roll call process are as
follows:
*
*
Prioritized roll call Each secondary station is
assigned a priority value so that higher priority
stations are polled more frequently than lower priority
stations.
Time-delayed roll call
assigned a time value that
between polls. The poller
polling list but issues
time value has expired.
Each secondary station is
defines the time to wait
still works its way through a
polls only to stations whose
2270526-9701 B-1