Envoy Application Programming Manual

BISYNC Multipoint Tributary Protocol
Envoy Application Programming Manual—427159-001
5-14
Poll and Select States
Poll and Select States
Each address in the address list has an associated poll-state or select-state bit. (The poll-
state or select-state bit is in the most significant bit position of the first element of a
station address, that is, station address.<0>.) The states of the poll-state and select-state
bit are:
0 = active
1 = inactive
In the poll list, active means that the station has data to send. If a READ is outstanding
and an active station is polled, the READ completes with an error 166 (ENQ received).
Inactive means that the associated station does not have data to send. If a READ is
outstanding and an inactive station is polled, Envoy responds to the poll sequence with
an EOT sequence.
In the select list, active means that the station is able to accept data. If a READ is
outstanding and an active station is selected, the READ completes with the first message
from the supervisor station returned to the line buffer of the application process of the
tributary. Inactive means that the associated station is not able to accept (or is not
expecting) data. If a READ is outstanding and an inactive station is selected, Envoy
responds to the selection sequence with the selected sequence (RVI, WACK, or NAK).
Table 5-5
shows the two ways you can specify poll and select states for stations.
In a typical operation, the poll states for all entries in the poll list are set to inactive when
the address list is defined to Envoy. Following the call to DEFINELIST, the application
process issues a nowait READ to monitor the line. The application process then issues a
READ to the input device (for example, a terminal) for a station. When the READ of the
input device completes (meaning that a request must be made to the supervisor station to
receive the message), the application process calls the CHANGELIST procedure to set
the station’s poll state to active. In the following example, the poll state for station BB01
is set to active.
CALL CHANGELIST (linefnum,1,0); ! set entry 1 poll state
! active.
The next time station 2 (BB01) is polled, the outstanding READ completes with an error
166 (ENQ received). Envoy returns a 1 in the entry number field of the message control
word to indicate the polled station.
Table 5-5. Setting Poll and Select States
How When
By the poll-state or select-state bit in the
address list
Before the DEFINELIST procedure is
called
Call the CHANGELIST procedure and change
the poll-state or select-state bit for a specific
station
After the DEFINELIST procedure is
called