4. In accordance with internal priorities, computer
B detects the setting of the EFA line of computer
A (which will be recognized as the EIR line) and
samples the ID lines.
5. Computer B clears the EIE line.
6. Computer B sets the IDA line.
7. Computer A detects the setting of the IDA line
of computer B (which will be recognized as the
resume line).
8. Computer A clears the EFA line before placing
the next word on the OD lines, and computer B
clears the IDA line before reading the next word
on the ID lines.
NOTE: Whenever the transmitting
computer does not have an EFR line, or the
receiving computer does not have an EIE line,
a command will be transferred with force. For
forced transfers, step 3 and step 7 are not
used.
Computer A and computer B repeat this sequence
for each successive command word until they have
transferred the block of command words specified by
computer B’s EF buffer control words.
INTERCOMPUTER DATA TRANSFER.—
Whenever an OD buffer has been established in
computer A and an ID buffer has been established in
computer B for the same channel, computer A and
computer B transfer data. Again refer to figure 7-29
with computer A as the sending computer and
computer B as the receiving computer. The sequence
is performed as follows:
1.
2.
3.
4.
5
Computer A places a word of data on the OD
lines. The OD word is held on the data lines
until computer B sets the resume line, or until
computer A’s program intervenes to resolve the
no resume condition.
Computer A sets the ready line to indicate that
a word of data is on the OD lines.
In accordance with internal priorities, computer
B detects the setting of the ready line of
computer A (which will be recognized as the
IDR line).
Computer B samples the ID lines.
Computer B sets the IDA line.
6.
7.
Computer A detects the setting of the IDA line
of computer B (which will be recognized as the
resume line).
Computer A clears the ready line before placing
the next word of data on- the OD lines, and
computer B clears the IDA line before sampling
the next word of data on the ID lines.
Computer A and computer B repeat this sequence
until they have transferred the block of words specified
by the buffer control words. Buffer lengths specified
by each computer are the same.
SUMMARY—INPUT/OUTPUT (I/O)
AND INTERFACING
This chapter has introduced you to how computers
communicate with and control other computers and
external devices.
The following information
summarizes important points you should have learned:
I/O ORGANIZATION— All computers are
capable of I/O operations. Some rely on the CPU to
handle I/O operations. Others have an I/O processor
(IOC). An I/O processor enables the computer to
perform other operations while still performing I/O
operations.
I/O PROCESSOR— An I/O processor (IOC)
controls the transfer of information between the
computer’s main memory and the external equipments.
IOCs are packaged in (1) IOC/IOA modules or multiple
IOC/IOA pcb’s, and (2) I/O pcb’s. The IOC relieves the
CPU of the necessity to perform the time consuming
functions of establishing, directing, and monitoring
transfers with external equipments. Data and control
signals are exchanged with external equipments via the
IOA. The IOA changes the input and output control and
data signal voltages to the voltage requirements of the
computer or external equipments. Communication
between the IOC and the IOA is by means of a
bidirectional bus.
I/O DATA ARRANGEMENTS— The types of
information exchanged between the computer and the
external equipments fall into two basic categories: data
words and control words. Data words represent the
alphabetic and numeric information exchanged.
Control words specify an action to be accomplished by
an external equipment.
I/O DATA FORMATS— Computers exchange
data in either parallel or serial format. When the
computer uses a parallel configuration, all bits of
information represented by a byte or word are input or
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