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HP 27130B
Function of Read Request Length
The read request will invoke special read processing when:
1. no current receive record is available on the backplane and
2. alert 1 mode is disabled.
If
the special read processing is invoked, the firmware will perform two different types of actions
depending on the status of the frontplane. If the frontplane has a current record which can satisfy
the read request, the record will be terminated with the "text terminated by card, no more data
ll
code
and be given to the backplane to satisfy the read request.
NOTE
If
the read request has the S-bit set, any future read
requests will be satisfied by the remaining data in the
current record until the buffer is exhausted.
The preceding note has several implications. If the next read request exceeds the remalmng data
length, the read will be terminated early with the above termination code. Note that the read request
length in this case will not be satisfied. If the read request length is less than the remaining data
length, the read will be satisfied in total. If the S-bit is set, any data relnaining will be saved for fu-
ture read requests.
If
the frontplane either has no active receive read, or if the frontplane receive record is less than the
read request length, the firmware will set the down-counter to the remaining count needed to satisfy
the read. The firmware will suspend the read request until the receive record is available for the
backplane (i.e., until either the down -counter hits zero, or a terminating condition is detected).
If
a read request is terminated by the down-counter hitting zero, the termination code for the record
is "card terminated read, no more data".
Host Initiated
Text
Termination
There are many occasions when the host would like to terminate data transfer early without losing
what has already been read. For example, an application may require the read to be terminated either
by satisfying the length, or by timing out.
A control card request with subfunction 4 allows the host to terminate the frontplane receive record
with the "host initiated text termination" termination code. If no read request is active, a receive
record \vill be generated and it mayor may not contain any data. However, no record is generated if
there is no buffer space.
4-11
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