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HP 27130B
Alert 1 Mode
\\'hen this mode is enabled, and if there is no read device data request pending on the card, and if none
of the conditions discussed above were encountered to terminate the record, the finnware will notify
the host by a "data available" event that at least one character was received. The host should post a
read device data request upon receiving this notification. The card will return all characters received
from the time that the alert event status was posted, up to the execution of the read. The read buffer
must be large enough to hold the maximum number of characters; otherwise, data will be discarded
unless the S- bit is set
If any of the text termination options discussed above are enabled, they may terminate the record ear-
ly and the host will receive tha t text termination code and not the alert code for the message type.
If the alert 1 read mode is enabled and the host posts a read device data request, the card will not
suspend the read when no data are available on the I/O channel-to-card interface module
("backplane") or the MUX -to-device interface module (,'frontplane"). Instead, you will receive a
transmission log of zero length.
The alert 1 read mode is enabled by setting the alert 1 bit in the data block of the \VCC, SF 3. When
the mode is enabled, and there are data on the frontplane, and there are no data on the backplane, a
"data available" event will be generated immediately.
In summary, the alert 1 read mode has the following characteristics:
1. If
the backplane has no receive record, an event will be generated when the frontplane receives
a character which does not meet any of the terminating conditions described above.
2. If the backplane has no receive record and the front plane has an active record, the backplane
will cause the frontplane to terminate the record to satisfy the read.
3. If the backplane and the frontplane both have no active record, the read will be terminated
immediately with a transmission log of zero.
The text terminating code will be alert 1.
Type Ahead and Echoing
Type ahead mode allows the MUX to receive text before the host has posted a read for the text. The
card has enough RAM space to buffer several text lines. This will allow the device to send many lines
bef ore stopping.
If the receive buffer should become full, any new, incoming data will be lost. If echoplex is enabled,
you will notice this when the typed character is not echoed.
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