User`s guide

194 DC 900-1340I
BSC Programmer’s Guide
to control units in the order specified in the poll list given to the ICP in the Set Poll List
command (Section 3.4.1.7 on page 62). Each control unit in the poll list is polled for
input until the end of the list is reached. The BSC 3270 software then delays for the
period specified by the poll list delay option (Section 4.13 on page 95). At the end of the
delay, if any, the general polling sequence starts again.
A control unit responds to a general poll with one of the following:
• data
• device status information
• EOT (nothing to send)
If a control unit fails to respond to a general poll within the specified timeout period,
the client is notified with the
DLI_PROT_RESP_ERROR error report containing the
DLI_ICP_ERR_STATION_DOWN error code. The client can then remove the control unit from
the poll list by issuing a Set Poll List command. If the control unit is not removed from
the list, it continues to be polled during subsequent iterations through the poll list.
A.1.2 Device Selection
When the client application program writes a block of data to a link, the BSC 3270 soft-
ware issues a device selection sequence after the completion of the next general poll. The
device selection sequence is shown in Figure A–2. CUA is the control unit selection
address and DA is the device address. These addresses are derived from the CU and
device numbers specified by the client application program in the
dlWrite
pOptArgs.usProtCircuitID
field.
Figure A–1: General Poll Format
SYN SYN EOT PAD SYN SYN CUA GPA GPA ENQ PADCUA