User`s guide

94 DC 900-1317J
ADCCP NRM Programmer’s Guide
4.2.2 More Data Indicator
In normal response mode (NRM) operation, the primary station must intersperse poll-
ing operations among data transmissions and must wait for each station to respond
before proceeding to poll the next secondary station on its list. The secondary station
cannot respond until polled.
When the client sends normal data or unnumbered data by means of an NRM second-
ary station, it uses the
iProtModifier field in the data packet to indicate whether more
data follows or the data frame is the final frame in the data set. Proper use of the
iProtModifier field can improve NRM secondary data transfer efficiency by decreasing
the polling overhead required to authorize secondary data transmissions.
When the client broadcasts normal data by means of an NRM primary station, the same
more data indicator in the
iProtModifier field causes the ICP to modify its polling proce-
dures to suppress secondary station response I-frame transmissions. If the client fails to
use the more data indicator during normal data broadcasts, secondary station response
I-frame transmissions may force the ICP to initiate secondary station resets to maintain
sequence number alignment (see Section 4.4); the result may be reduced primary sta-
tion broadcast throughput efficiency and loss of secondary data transmissions.
4.2.3 Data Size and Transmit Window Size
The user’s application must select a data size and transmit window size that will ensure
efficient use of the transmission band width of the data link. For example, a direct con-
nection operating at 9600 b/s with a 256 byte data size has a negligible propagation
delay. If the primary station polls its secondary stations at every opportunity and
acknowledges received I-frames in real time, acknowledgment of data receipt occurs
within one frame transmission time (assuming that the current data frame transmission
must complete first). A window size of 2 or 3 is adequate to ensure full utilization of the
link.