Texas Instruments 990 User Manual page 216

Throughput
A secondary station can always respond to a poll with a message
unless the poll is negative. A negative poll is sent only as a
status poll and only if the primary station cannot accept data in
response. This poll type is generally sent when the primary
station has no input buffers available to accept response data.
Unless this situation continues for a prolonged time, the
occurrence of negative polls has no effect on the application
programs attempting to send messages to the secondary stations.
B.3 TIME DELAY VALUE SELECTION
The user selects the time delay between polls during network
generation. The time delay selection must be based on the
user~s
knowledge of a secondary
station~s
function. The
secondary~s
function should be known in sufficient detail to allow the
network planner to select the appropriate time delay. During
network generation, valid responses to the MAXIMUM TIME BETWEEN
POLLS prompt are in the range of 0 to 32,000. An entry of 0
indicates that the secondary should be polled at every poll
opportunity. An entry greater than 0 indicates the desired delay
time between polls. It is prudent to choose this delay based on
the operational requirements of the secondary station. Each unit
of time entered is equal to 250 milliseconds.
B.4 POLL AND RESPONSE COMPONENTS
Each HDLC poll and and each response consists of 48 bits. For a
line data rate of 9600 bits per second (bps), the line time to
poll is approximately 48/9600 or 5 milliseconds, and the line
time to respond is the same. At 7200 bps, the line time to poll
is 48/7200 or 6.67 milliseconds, and the line time to respond is
the same. As the line data rate decreases, the line time to send
a poll and get a response increases accordingly. For example, at
300 bps, the line time to poll is 160 milliseconds and the line
time to respond is the same. The line data rate should be taken
into consideration by applications that send messages to
secondary stations and expect the addressed secondary (or task in
that secondary) to send a reply message back. Obviously, the
response time is greatly affected by the line data rate.
B-4 2270526-9701