IBM Series 1 User Manual page 134

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Prior to presenting the following sections, it is important
to distinquish between two types of receive operations that
the teletypewriter adapter performs.
They
are:
• ! ~~~l £2ceiv~
QEeration
•
This is a receive operation in which the receive data
register is loaded with a new value. The attention
interrupt is posted upon completion of this operation.
The receive data register remains loaded until,
1)
it
is read at least once by the program ~n~ 2)
a
subsequent normal receive operation is initiated.
hn
Q~~££~n
receive
9E~ration
This is a receive operation executed with the same
timing as a normal receive operation, but the data
transmitted from the device is blocked from being
loaded into the receive data register. An overrun
receive operation is initiated if the device begins a
transmission and the adapter determines that the
receive data register (previously loaded
by
a normal
receive operation) has not been read at least once
by
the program. The character that caused the overrun is
lost.
The
character in the receive data register is
not lost and may still be read. Therefore, although
synchronization is lost# character synchronization
between the device and the adapter is not lost. An
exception interrupt is posted at the completion of an
overrun receive operation.
Interr~£1 Presentation
The teletypewriter adapter requests an interrupt (if it has
been properly prepared) under the following conditions:
1)
When a normal receive operation is completed.
2) When an overrun receive operation is completed.
3)
When a transmit operation is completed.
Condition codes
(CC)
reported at interrupt acceptance time
are as follows:
cc
valy~
2
3
4
6
7
Meaning
Exception
Device end
Attention
Attention and exception
Attention and device end
Teletypewriter Adapter 4-11