6100 BSC Programming Manual

Writing Applications that Use 6100 BSC
Starting the Link
Starting up the link consists of one or two tasks, depending
on whether the stations practice ID exchange.
The first of the two tasks is to connect the link. If the line
is switched, connection implies a physical and a logical action:
a telephone call is established between the two stations, and the
line state changes from DISCONNECTED to CIRCUIT ASSURANCE or
CONTROL. If the line is leased, connection is primarily a
logical action: no telephone call is necessary, but the line
state changes to CONTROL. Connection is a required task on
both sides of the link.
There are two ways for an application to connect the link.
It can make a CONTROL request and specify the CONNECT option,
or it can make its first data transfer request: WRITE, READ,
or WRITEREAD. In the second case, connection is implicit
in the request; a connection must exist before data transfer
can occur.
There may be several sequential connections in one opening of the
line. For example, a station can open the line and accept calls
from one station after another. (Of course, one link must be
disconnected before another can be established, but you don't
have to close and reopen the line.) Also, an error might break
the connection and require it to be reinstated.
If the line requires ID exchange--if it has the parameter
BSCIDEXCH--then circuit assurance is the task that directly
follows connection. The calling station sends its ID to the
called station; the called station responds, as described in
Section 1.
Figure 3-2 shows the requests used for ID exchange. The
application in the calling station makes a WRITEREAD request,
including the station ID and the ENQ in its WRITEREAD buffer.
An application in the called station makes a READ request
to receive the ID. To reply, it makes a WRITEREAD request
with its own ID and the ACK sequence in its WRITEREAD buffer.
If the called station wants to send a WACK, it includes that
sequence in its buffer. It does not use the CONTROL request.
The WRITEREAD that sends the caller's ID, or the READ request
that receives it, will establish a connection if one does not
already exist.
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