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ADDITIONAL NOTES
(Cont'd)
MODES OF
OPERATION
RECEIVE MODE
6. Code translation must be accomplished by programming when using
codes other than EBCDIC.
7. A DDI does not have to be sent immediately following the ACK. The
DDI can be held up to 18 seconds from the time the DD2 and BP
characters were sent by the transmitting communication control.
When a DDI is not received in this time period, a time-out error is
detected by the control and indicated in the transmitting program.
A time-out error also is detected and indicated to the receiving
program. These time-outs cause the interrupt indicator, standard
device byte (22
=
1), and the sense byte (2 2
=
1) to be set in each
processor and the communication controls return to the Idle Mode.
• The 70/653 Communication Control has three operating modes: Idle
Mode, Receive Mode, and Transmit Mode. In addition, an Automatic Call
Function is available. The neutral condition of this control is the Idle Mode.
In this mode the control is operational and is responsive to signals from
the processor and the communication line. These signals cause the com-
munication control to enter either the Receive or the Transmit Mode.
Once either mode has been established, operation continues in that mode
until the end of a transmission, or until an error condition has been detected.
• In the Receive Mode, the communication control transfers data and
control character, received from the communication line, into the main
memory of the processor.
When the Unattended Answer feature of the data set is used, the
receipt of a Ring Indicator (RI) signal starts a 19-second timer in the
control and it assumes that the communication line connection has been
established. The RI signal is generated by the data set upon receipt of
dialing pulses from the communication line. When nothing is received
from the line within 19 seconds, the control disconnects the line and returns
to the Idle Mode. No error is indicated upon detection of this condition.
The first seven characters received are the Idle Line (I-L) characters.
These characters are used for synchronization and are not transferred
into main memory. Recognition of the I-L characters causes the control
to set the interrupt indicator, standard device byte (22
=
1), and the sense
byte (2
1
=
1). The 19-second timer, set when the Ring Indicator was
received, is reset. At this point, a Read command is issued.
The next characters received from the line are DD2 and Block Parity.
These characters are not transferred into main memory. The Block Parity
is compared to the accumulated Block Parity of the receiving communica-
tions control. If these Block Parity characters are equal and no previous
errors have been detected, the control sets the interrupt indicator, standard
device byte (2
2
=
1), sense byte (2
4
=
1), and the 19-second timer. The
timer is set here to indicate an error when a DDI is not received from the
processor within 19 seconds.
7
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