User`s guide

DC 900-1340I 193
Appendix
A
BSC 3270 Line
Control Procedures
This appendix defines line control procedures for the BSC 3270 protocol.
Note
This appendix, along with Chapter 3 and Chapter 4, should be
read by programmers who are interfacing an application program
to a BSC 3270 environment. If you are programming
BSC 2780/3780, refer to Chapter 5, Chapter 6 and Appendix B.
A.1 Control Station Procedures
When a link is configured as a control station, it operates much like an IBM 3705 com-
munications controller in BSC mode. All data transfer takes place between the control
station and one of up to 32 tributary stations. The control station uses general poll, spe-
cific poll, and device selection sequences to transfer data to and from each tributary.
A.1.1 General Poll
A general poll is a request for data from all devices attached to a control unit. The for-
mat of the general poll is shown in Figure A–1. CUA is the control unit address and GPA
is the general poll indicator. The general poll indicator has a hexadecimal value of 7F
(EBCDIC) or 22 (ASCII).
The BSC 3270 software begins transmitting general polls on the line immediately after
the client starts the link by opening a session using
dlOpen (assuming the cfgLink and
enable parameters are set to “yes” in the DLI configuration file). General polls are issued