6100 ADCCP Programming Manual

Polling
ADCCP Link and Station Management
2–14 069225 Tandem Computers Incorporated
Station list The DEFINELIST request creates the Station List. The station list can
later be modified with the CHANGELIST request. If the local station
represents one or more secondary stations, the list contains one entry
for each secondary represented. If the local station is a primary station,
the list redefines the local station once for each remote secondary.
An entry in the station list indicates the station type, the line control
mode, and the station address. (Each time you define the primary
station, it has a different address. The address of the primary station
must match that of the remote secondary station.) The list entry also
identifies the active, RNR, NOPOLL, or ERRORSTOP condition of the
station.
Each entry in the list contains a station ID, which is a number that
ADCCP and all applications use to identify the station. Station IDs
have a range of 1 through 255 for multipoint lines. The station ID must
be 1 for point-to-point lines or 0 for a broadcast station. Any
application request that refers to a particular station uses its station ID,
rather than its address.
Scan list The SCANLIST request creates the scan list. It consists of a list of
station IDs in the order in which you want polling to occur. The Scan
List may include the same station more than once. For example, you
might want one station polled twice as often as another station. If no
application issues a SCANLIST request, the stations are polled by
increasing value of the station ID. If several applications make
SCANLIST requests, the most recent request always applies. The most
recent request even preempts a poll cycle that is in progress.
Alternate Polling In alternate polling (Option_One Polling), the primary station sends outstanding
I-frames to a secondary station as soon as the secondary station indicates that is it no
longer busy. The primary station does not wait to send I-frames until after the polling
of all other remote stations . Alternate polling is useful when the station list is long
and a timeout may occur on the remote station before all other stations are polled.
Figure 2-7 shows the sequence of frames exchanged in alternate polling. (The numbers
in the illustration indicate the station.)