NTDS Input/Output (MIL-STD-1397)
The NTDS input/output interface is probably one
of the most versatile of formats because it is designed
to handle either parallel or serial formatted information,
depending on the type of computer and its I/O
requirements. This interface specifies three I/O control
and data signal categories. We cover the first two under
parallel and serial operations later in this topic. The
categories include:
Category I —Computer to external device
Category II —Computer to computer,
intercomputer (IC)
Category III —External device to external device
Within this standard, there are nine types of formats
(A through H and J). They include both serial and
parallel formats as described in the following
paragraphs.
TYPE A (NTDS) SLOW. —Type A transfers
parallel data of up to 41,667 words per second on one
cable. This type interface uses 0 vdc (logical 1) and -15
vdc (logical 0) to transmit bit groupings of 16,30, or 32
bits, depending on the type of computer. The relatively
large voltage change between logic states, with its
inherent time delays, limits the speed of data
transmission. Type A can transmit digital signals up to
1000 feet. It is most frequently used in large mainframe
and some minicomputers to interface with equipment
found in the data processing, display, and
communication subsystems. Type A uses a request and
acknowledge protocol process. It transfers control and
data words using two cables: one input and one output
for the same channel. You may, however, encounter a
few devices that use input only or output only portions
of an NTDS slow channel. Type A signal designations
for input and output include the following:
EIE —External interrupt enable
IDR —Input data request
EIR —Extemal interrupt request
IDA —Input data acknowledge
EFR —Extemal function request
EFA —External function acknowledge
ODR —Output data request
ODA —Output data acknowledge
TYPE B (NTDS) FAST. —Type B transfers
parallel data of up to 250,000 words per second on one
cable. This type interface uses 0 vdc (logical 1) and -3
vdc (logical 0) to transmit bit groupings of 16, 30, or 32
bits depending on the type of computer. Type B can
transmit digital signals up to 300 feet depending on the
type of cable used. It is most frequently used in large
mainframe or some minicomputers to interface with
equipment found in the data processing, display, and
communication subsystems. Type B uses a request and
acknowledge protocol process. It transfers control and
data words using two cables: one input and one output
for the same channel. You may, however, encounter a
few devices that use input only or output only portions
of an NTDS fast channel. Type B uses the same input
and output signal designations as type A.
TYPE C (ANEW). —Type C transfers parallel data
of up to 250,000 words per second on one cable. This
type of interface uses 0 vdc (logical 1) and +3.5 vdc
(logical 0) to transmit bit groupings of 16, 30, or 32 bits,
depending on the type of computer. Type C can
transmit digital signals up to 300 feet depending on the
type of cable used. It is most frequently used in large
mainframe or some minicomputers to interface with
equipment found in the data processing, display, and
communication subsystems. Type C uses a request and
acknowledge protocol process. It transfers control and
data words using two cables: one input and one output
for the same channel. You may, however, encounter a
few devices that use input only or output only portions
of an NTDS ANEW channel. Type C uses the same
input and output signal designations as type A.
TYPE D (NTDS SERIAL). —Type D
asynchronously transfers serial data using a 10
megabits per second (Mb/s) clock rate over a single
coaxial cable. Two cables are required for bidirectional
communications, a source line (computer to peripheral)
and a sink line (peripheral to computer). The source
line is used to transmit data and external functions,
while the sink line is used to transmit input data and
external interrupt codes. Type D transfers are
accomplished using two types of bipolar pulse trains:
(1) control frames and (2) control and data words. The
actual input or output data is transmitted in 32-bit
information frames. Control frames are three bits in
length, a sync bit followed by two control bits. The
signals required for input transfer will occur on the input
channel (input request, input enable, and not ready) and
the signals required for output transfer will occur on the
output channel (output request, output enable, and not
ready). A binary 1 will be a pulse of phase zero degrees
and will be a high polarity followed by a low polarity.
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