5. The computer sets the IDA line, indicating that
it has sampled the data word on the ID lines.
6. The external equipment detects the setting of the
IDA line. The external equipment may clear the
IDR line anytime after detecting the setting of
the IDA line, but will clear the IDR before the
computer will recognize the next IDR.
7. The computer clears the IDA line before reading
the next word on the ID lines.
OUTPUT DATA (OD) SEQUENCE OF
EVENTS. —We begin from the point that an OD has
been established for a channel. The computer and the
external equipment on that channel will do the
following to transfer data:
1.
2.
3.
4.
5.
6.
7.
When the external equipment is ready to accept
data, it sets the ODR line (this may already have
happened before the OD buffer was
established).
The computer detects the setting of the ODR
line in accordance with internal priorities.
The computer places a word of data on the OD
lines.
The computer sets the ODA line to indicate that
a word of data is on the OD lines.
The external equipment detects the setting of the
ODA lines. (The external equipment may clear
the ODR line anytime after detecting the setting
of the ODA, but clears the ODR line before the
computer will recognize the next ODR).
The external equipment samples the data word
that is on the OD lines.
The computer clears the ODA line before
placing the next word on the OD lines.
EXTERNAL FUNCTION (EF) SEQUENCE
OF EVENTS (NORMAL). —We begin from the point
that an EF has been established for a channel. The
computer and the external equipment on that channel
will do the following to transfer:
1. When the external equipment is ready to accept
an EF code word, it sets the EFR line (this may
have already happened before the EF buffer was
established).
2. The computer detects the setting of the EFR line
in accordance with internal priorities.
3. The computer places an EF code word on the
OD lines.
4.
5.
6.
7.
The computer sets the EFA line to indicate that
the EF codeword is on the OD lines.
The external equipment detects the setting of the
EFA line. The external equipment may clear the
EFR line anytime after detecting the setting of
the EFA line, but clears the EFR line before the
computer will recognize the next EFR.
The external equipment samples the EF code
word that is on the OD lines.
The computer clears the EFA line before placing
the next word on the OD lines.
Forced external functions are the same as normal
external functions except the computer does not require
an external function ready signal from the external
equipment, so the computer will not be delayed by steps
1 and 2.
EXTERNAL INTERRUPT (EI) SEQUENCE
OF EVENTS. —The computer and the external
equipment do the following to transfer an EI code word:
1 .
2.
3.
4.
5.
6.
7.
8.
9.
10.
The computer, under program control, sets the
EIE line when ready to accept an EI.
The external equipment detects the state of the
EIE line.
When the status requires that the computer be
interrupted, the external equipment places an
EI code word on the ID lines.
The external equipment sets the EIR line to
indicate that the EI code word is on the ID lines.
The computer detects the setting of the EIR line
in accordance with internal priorities.
The computer samples the EI codeword that is
on the ID lines.
The computer clears the EIE line.
The computer sets the IDA line.
The external equipment detects step (8) or both
steps (7) and (8). The external equipment may
clear the EIR line anytime after detecting the
setting of the IDA line, but clears EIR line
before the computer will recognize the next
EIR.
The computer clears the IDA line before
sampling the next word on the ID lines.
NOTE: Not all computers have the EIE
lines; consult your computer’s technical
manual.
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