EM3270 Manual

Product Components and Operation
Tandem EM3270 Manual—110332 1-11
Terminal Buffer Strategy
Screen Recovery
The emulator has restart and recovery capabilities that become effective in the event of
terminal or system power failure. The EM3270 emulator automatically performs
terminal initialization when a power-on status message (error 191) is received from a
terminal device. If the SHIELD option is set to OFF, the power-on error will be passed
through to Pathway, causing Pathway to perform its screen recovery.
At any time you can use the screen recovery function key to reinitialize the screen. This
function also sends a 191 error to Pathway and an appropriate signal to the host in order
to perform a screen refresh operation.
Terminal Buffer Strategy
For Tandem terminals, the EM3270 emulator reserves a page buffer for the IBM host
application at each terminal. This allows the IBM host to communicate with the terminal
as it would with a 3270 terminal without interfering with data transfer between the
terminal and any Tandem application that might be active.
This ensures that the Tandem application and the IBM host application can process
independently without destroying data in each other’s page buffers. It also provides a
means for the terminal to receive and process unsolicited messages from the IBM host
without interfering with the Tandem application session that may be in progress.
The EM3270 emulator issues a sequence of commands to establish communications
with the terminal. This sequence is identical to that used by Pathway for the same
purpose. It concludes by reading the terminal status. When the EM3270 emulator
receives this sequence from Pathway, it returns the number of page buffers along with
other status information. However, it reserves a buffer for the IBM host by returning a
value that is one less than the actual number of buffers.
You can set the number of page buffers for the device with the SET PAGES command.
The default value is eight, which reserves one page for the IBM host and seven for the
Tandem application.
Certain escape sequences sent by the Tandem application could corrupt the page buffer
reserved for the IBM host. To prevent this, the EM3270 emulator deletes the following
sequences from the data stream sent by the Tandem application:
ESC t ! Set page size to 960
ESC X ! Exit protect submode
ESC p ! Set maximum page number
The EM3270 emulator also deletes the escape sequences that would display or select the
page reserved for the IBM host.
Since IBM 3270 and Fujitsu terminals do not support multiple screen buffers as part of
an application switch, the EM3270 emulator reads the screen and saves it. The screen is
then refreshed from the other application’s screen image.