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Page Title:
Computer/Peripheral Equipment Communications |
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|  MIL-D-81347C(AS)
Figure 1-1 shows the Avionics
Computer receiving input from
Equipment I and sending output to Equipment II. Figure 1-2 shows a
computer to computer interface.
Note the direction of information flow. The Data Request signals are
always sent from the peripheral
equipment to the computer. The Acknowledge signals are always sent
from the computer to the pe-
ripheral equipment.
2.1.4
Sequence of Events
2.1.4.1
Computer/Peripheral Equipment Communications
2.1.4.1.1
Input - The sequence of events for the two methods in which
inputs can be made to the computer are as follows:
Interrupt Input - A peripheral equipment shall be able to input
2.1.4.1.1.1
one 30 bit word to the computer by following the control line sequence below, as illustrated in
Figure 1-3.
(1) Computer sets the Interrupt Enable when it is ready to
accept an External Interrupt.
(2) Peripheral equipment detects the Interrupt Enable.
(3) Peripheral equipment places the Interrupt word on the 30
data lines.
(4)
Peripheral equipment sets the Interrupt line to indicate
that the External Interrupt word is on the data lines.
(5) Computer detects the Interrupt signal, and at its conven-
ience, accepts the Interrupt word.
(6) Computer drops the Interrupt Enable.
(7)
Peripheral equipment detects the drop of the Interrupt
Enable and clears the Interrupt line and data lines.
The Input Acknowledge of an interrupt will be initiated at the
same time that the Interrupt Enable is cleared. The simultaneous occurrence of these conditions
should be used by peripheral equipment to differentiate between an Interrupt Acknowledge and a Data
Acknowledge. If a peripheral equipment is not designed to sense an Interrupt Enable, then the se-
quence would include only steps (3), (4), (5), and the setting of the Input Acknowledge. An interrupt
transfer would occur by Method II as shown in Figure 1-3.
Data Inputs - The computer shall be able to accept 30 bit data
2.1.4.1.1.2
words from peripheral equipments by following the control line sequence below, as illustrated in
Figure 1-4.
(1) Computer initiates input buffer for given channel.
Peripheral equipment places data on the 30 data lines.
(2)
(3) Peripheral equipment sets the Input Data Request line to
indicate that it has data ready for transmission.
(4) Computer detects the Input Data Request.
(5) Computer samples the 30 data lines, at its convenience.
(6) Computer sets the Input Acknowledge line, indicating that
it has sampled the data.
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