Envoy ACP/XF Application Programming Manual

Asynchronous Balanced Mode (ABM) Protocols
EnvoyACP/XF Application Programming Manual–132179
2-11
Transferring Data
within a Tandem system issues a CONTROL (fnum,18,1) call to switch the system
from disconnected to offline.
Transferring Data
Once a link has been established, your application can begin transferring data.
Transmitting I-Frames
To transmit an I-frame, the application program places the data to be sent in a buffer and
calls the WRITE procedure.
Accepting Incoming I-Frames
To accept an incoming I-frame, the application program calls the READ procedure. In
general, the application should have at least one outstanding READ request at all times.
If an I-frame arrives and a READ request is not outstanding, HDLC or ADCCP
acknowledges the frame and queues the data for the next READ request. Note that the
queued data can be lost if the processor fails before the READ request is completed.
Queuing I/O Requests
The WRITEREAD procedure initiates two requests: a WRITE (send an I-frame)
followed by a READ (accept an incoming I-frame). The READ request is not queued
until the WRITE has completed. If you issue a SETMODE (fnum,30,1) call to allow
nowait I/O operations to complete in any order, your subsequent issued requests may
complete in an unexpected order. For example, if you issue a SETMODE call, then issue
a READ request before the write portion of the WRITEREAD procedure has completed,
the READ request may complete before the read portion of the WRITEREAD
procedure. Because of this situation, do not routinely use WRITEREAD commands to
send I-frames.
Message Control Word (MCW)
For ordinary READ and WRITE operations, the two bytes (one word) of the Message
Control Word (MCW) have no value to the application program. Nevertheless, they are
included in the read count or write count. You should always set the second byte to 0,
because ABM is a point-to-point protocol and a station index is not necessary. You can
send a U-frame that includes the UI, XID, or TEST command/response code by
specifying the appropriate value in the first byte of the MCW. See the WRITE[X]
Procedure on page 4-19
Shutting Down a Data Link
When stations operate in ABM, either station can shut down the link. The local station
issues a CONTROL call (Figure 2-5) in the following form:
CALL CONTROL (filenum,18,parameter)