6100 ADCCP Programming Manual

Application Tasks
Writing Applications that Use ADCCP
069225 Tandem Computers Incorporated 4–21
If a SABM arrives at a combined station that has not issued a SABM command,
ADCCP acknowledges the command and informs local applications. When a SABM
finishes a RECEIVETEXT request, the application knows that the mode-setting
sequence has occurred on the line.
Transferring Data Once a link is established, stations are free to exchange data on the line. To send data
to a station, an application makes a WRITEREAD call with the SENDTEXT request.
To receive data, the application makes a WRITEREAD call with the RECEIVETEXT
request.
Sending Data
A SENDTEXT request specifies the station that will receive the data by its station ID.
The request indicates whether the frame is an I-frame or is of some other type; if the
mode is NRM, it also indicates whether the poll/final bit should be set (that is,
whether the frame is the last of a related sequence of I-frames). The Level 2 Protocol
queues the frame to be transmitted, and the Level 1 Protocol builds the address and
control fields (see Section 1, “Introduction to 6100 ADCCP,” for a description of the
Level 1 and Level 2 Protocols). Several factors determine when the frame is actually
placed on the line:
Frames for a given station are transmitted in the order queued.
If the local station is a secondary station in NRM, frames are transmitted only in
response to a poll.
If the local station is a TWA primary station in NRM waiting for a response to an
earlier poll, a sequence of I-frames ending with a poll bit is held until the primary
station receives the response.
If TWA transmission is in effect, frames are transmitted only when the local station
has its turn to send.
The completion of a SENDTEXT request with good request status implies that the
remote station received and acknowledged the frame.
The number of SENDTEXT requests for each application depends on the application’s
total nowait depth and the window size for each station on the link. An application
can always tell which nowait request finishes by examining the request ID in the
response buffer.
Receiving Data
A RECEIVETEXT request is used to receive data; however, unlike a SENDTEXT
request, a RECEIVETEXT request is not specific to a station. RECEIVETEXT requests
are queued, then later completed as frames arrive from any station on the link.
A completion of a RECEIVETEXT request implies that ADCCP has received the frame
and queued an acknowledgment. The response to the request identifies the station
that sent the data.