Envoy ACP/XF Application Programming Manual

EnvoyACP/XF and the Application Task
EnvoyACP/XF Application Programming Manual–132179
1-8
Establishing and Controlling a Data Link
Establishing and Controlling a Data Link
Your application establishes a link and performs line-control operations through calls to
the CONTROL procedure. These calls include numerical operation codes, several of
which vary from one protocol to another.
When you use switched-line facilities, you must set the Data Terminal Ready (DTR)
modem signal before establishing the link. You do this by issuing either a CONTROL 17
or a CONTROL 11. For both calls, you can specify that Data Set Ready (DSR) be
asserted before the call completes.
To establish a data link, the application within a primary or supervisory station must
issue a CONTROL 13 call. This call results in the transmission of one of the following
mode-setting command frames:
•
For HDLC, the primary/combined station within one station sends the Set
Asynchronous Balanced Mode (SABM) frame to the secondary/combined station
within the other. (For more information about the SABM frame, see U-Frame
Command/Response Codes on page B-8.)
•
For SDLC, a primary station sends the Set Normal Response Mode (SNRM) frame
to the secondary, or a supervisory station sends the SNRM to all its defined
tributaries by way of the EnvoyACP/XF auto-poll facility. (For more information
about the SNRM frame, see U-Frame Command/Response Codes on page B-8.)
•
For ADCCP-ABM, the primary/combined station within one station sends the
SABM or Set Asynchronous Balanced Mode Extended (SABME) frame to the
secondary/combined station within the other station. (For more information about
the SABME frame, see U-Frame Command/Response Codes on page B-8.)
•
For ADCCP-NRM, a primary station sends the Set Normal Mode (SNRM) or Set
Normal Mode Extended (SNRME) frame to the secondary station, or a supervisory
station sends the SNRM or SNRME to all its defined tributaries by way of the
EnvoyACP/XF auto-poll capability. (For more information about the SNRME frame,
see U-Frame Command/Response Codes
on page B-8.)
You can use a CONTROL 15 call to set a station to the Receive Not Ready (RNR) state.
In this state, the protocol responds with an RNR frame to any I-frame or S-frame sent by
a remote station. Your application can temporarily hold off incoming data. The
application process can use a CONTROL 16 call to return the station to the Receive
Ready (RR) state. For more information on the CONTROL procedure, see the Guardian
Procedure Calls Reference Manual. For more information about the RR and RNR states,
see S-Frame Command/Response Codes
on page B-7.
Note. If the IOP fails, EnvoyACP/XF automatically puts the link back in logical disconnect state
(LDS). Any FILE_OPEN_, SETMODE, or DEFINELIST procedures already issued are still
valid; however, the application process must re-establish the communication session using a
CONTROL 13. The application also should institute error handling on the application-to-
application level to ensure that data has reached the application layer appropriately.
Note. Only one CONTROL call can be outstanding on a station at a time. Since EnvoyACP/XF
allows multiple users to open a link, the users must synchronize the CONTROL requests.