6100 ADCCP Programming Manual

6100 ADCCP Concepts and Context
Introduction to 6100 ADCCP
069225 Tandem Computers Incorporated 1–11
Once you have assigned addresses to stations, your application does not need to refer
to the addresses because your application uses a station ID established in the
DEFINELIST request.
Control Field
The control field follows the address field. The contents and format of the control
field vary, depending on the type of frame. The control field contains the ADCCP
commands and thus provides the following functions:
Identifies the type of frame
Conveys commands, responses, and status information between communicating
stations
Requests information and acknowledges information received
Places the frame in sequence with other frames for the same station
The ADCCP protocol module builds the control field for the application, which issues
requests for protocol sequences. For example, an application in a primary station
requests a mode-setting sequence before an exchange of messages occurs between two
stations. Requests and the protocol actions that they can cause are described in
Sections 3, “ADCCP/X21 Protocol Module,” Section 4, “Writing Applications That Use
ADCCP,” and Section 5, “Writing Applications That Use ADCCP/X21.”
The control field contains the ADCCP command and has one of three different
formats, each corresponding to a category of link activity. These formats are:
Information A format used for passing application data over a link between
stations. A frame whose control field has this format is called an
I-frame.
Supervisory A format used for regulating the flow of data between stations (for
instance, to report that a station is busy or to prod a station that is slow
in responding to an earlier transmission). A frame whose control field
has this format is called an S-frame.
Unnumbered A command/response format used for conveying link commands from
a primary station to a secondary station and for conveying responses
to those commands from the secondary to the primary. A frame whose
control field has this format is called a U-frame.
The low-order bit or bits of the control field specify the frame format. The table bit
sequence of the low-order bits for each type of format is:
Bit 0 Bit 1 Frame Format
0 x I-frame
1 0 S-frame
1 1 U-frame