User`s guide

A: BSC 3270 Line Control Procedures
DC 900-1340I 197
A.4 Virtual Device Procedures
The following sections describe some of the line procedures involving virtual devices.
See Section 3.4.1.11 on page 67 to create virtual devices, Section 3.4.1.12 on page 69 to
change the status of virtual devices, and Section 3.4.2.8 on page 76 to request a virtual
device status report. A review of these sections would be helpful before reading the fol-
lowing sections.
A.4.1 Virtual Printer Operation
When a device is created as a virtual printer, the ICP checks the incoming 3270 com-
mand byte for the “start print” bit set in the Write command or in the Copy command.
If the start print bit is set, the ICP responds to the data with WACK, sends the data to
the host with the
DLI_ICP_ERR_DEVICE_BUSY error code, and sets the DB bit for that device.
The host application must then send a Change Virtual 3270 Device Status command to
clear the device busy condition for that device. Figure A–3 shows a typical sequence of
events involving a virtual printer.
If an error occurs during printout (such as the printer running out of paper), the client
application must inform the ICP of the printer error so that the ICP can send the appro-
priate responses to the control station. In this case, the host sends a Change Virtual
3270 Device Status command to change the status to “device unavailable.” The ICP
considers that device unavailable until the client informs otherwise. Figure A–4 shows a
typical sequence of events involving a printer error.
A.4.2 Virtual Display Operation
A virtual device configured as a display terminal is similar to a printer device except that
the “start print” bit in the Write command or Copy command is ignored by the ICP,
and the incoming data is sent to the host without the device busy error. Figure A–5
shows how a virtual display handles a write command with the “start print” bit set.