User`s guide

A: BSC 3270 Line Control Procedures
DC 900-1340I 195
In order to give equal priority to input and output, at least one general poll is issued
after each device selection sequence.
A.1.3 Specific Poll
The client application program may send a specific poll on the line by issuing a Specific
Poll command (Section 3.4.1.9 on page 66) at any time while the link is active. Upon
receipt of this command, the BSC 3270 software issues a specific poll to the CU and DU
specified in the command. If the tributary has data or status to send, it appears to the
client as received data. Otherwise, BSC 3270 generates the
DLI_ICP_ERR_STATION_UP or
DLI_ICP_ERR_STATION_DOWN error report as a response.
If a reverse interrupt (RVI) is received in response to a device selection sequence, the
BSC 3270 software discards all the messages in the transmit queue and sends the client
the
DLI_PROT_RESP_LOCAL_ACK response with error code DLI_ICP_ERR_NO_CLIENT (device
busy) for each message discarded. The BSC 3270 software then automatically issues a
specific poll to that device to obtain any pending sense/status information.
A.2 Tributary Station Procedures
When a link is configured as a tributary station, its operation is similar to that of an IBM
3274 control unit with one or more “virtual” devices attached. When receiving data, the
CU specified in the station ID option (Section 4.16 on page 97) determines when the
BSC software responds on the link (unless the client is in test mode, as described below).
For transmissions, the client application program determines which device it wishes to
emulate by placing the proper CU and DU in the
dlWrite pOptArgs.usProtCircuitID field.
The CU should not differ from the configured station ID under normal circumstances.
Figure A–2: Device Selection Sequence
SYN SYN EOT PAD SYN SYN CUA DA DA ENQ PADCUA