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Description  |
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The invention relates generally to modular communications systems and more
particularly to telephone systems wherein each module of the peripheral
units thereof are adapted to generate self-identifying type-code
information.
In the last few years, most communication systems that have been developed
have used a modular construction in their implementation. In such a
system, the majority of sub-systems are packaged on respective printed
circuit boards which are inserted into card racks mounted on a frame. For
example, in the contemporary telephone systems which use pulse code
modulation and time division multiplexing techniques, the architectural
layout of the systems is modular. A central control unit usually comprises
a switching network and a central processor which is connected by data
links to a large number of peripheral units which provide the interface to
the outside world. These peripheral units consist mainly of large
quantitities of trunk and line circuits. A full size office may have as
many as sixty thousand trunk circuits and one hundred thousand line
circuits. These circuits are conventionally divided into small groups each
of which has common control circuitry for controlling the operation of the
circuits as well as the interface to the central control unit of the
office and the outside world. The size of each group of circuits is
usually chosen such that it may be completely mounted on a single shelf of
printed circuit boards. As is well known, the peripheral circuits of any
one office may comprise a mixture of a large number of different types of
circuits. For example, any one office may comprise incoming and outgoing
trunk circuits, a variety of signalling trunk circuits as well as a large
variety of special function trunk circuits such as CAMA and announcement
trunks. In addition, any one trunk board position may be replaced with
special function boards such as conference circuits and MF and/or digitone
receiver boards. A similar variety of types of boards exists when the
peripheral units comprise line circuits.
Because of the large variety of types of printed circuit boards that are
used in a telephone switching office, the use of modular construction in
such a system has created a very onerous problem, that of maintaining
system integrity through the manufacturing checkout of the system as well
as through the installation and maintenance procedures of the office.
As a system reaches the checkout stage of manufacture, the equipment bays
are loaded with the printed circuit cards and the system is usually
checked automatically. A digital computer program for emulating the input
and output loading of the system is activated and it exercises a full
range of operations of the system. It is often found that a substantial
proportion of faults uncovered at the initial stages of the checkout
procedures are due to improperly located printed circuit cards in the
shelves. This type of mistake is easily made when it is considered that
such a system comprises a large number of frames of equipment each having
a large number of printed circuit cards and that the only noticeable
difference in the majority of the faceplates for these cards is usually in
the code number. In addition, many cards having the same number are
sometimes located in different slots on different shelves. In order to
verify the physical integrity of a frame of equipment it is necessary to
verify the location of the cards against a chart. That in itself is a
tedious operation which further contributes to the problem.
The same problem occurs when the system is being installed or is undergoing
a maintenance procedure. Many of the cards are pulled out, put aside for a
period of time, and then reinserted in what is hoped are the correct
positions. Quite often, this is not the case and the original problem
cause of the maintenance procedure becomes compounded.
The attempts at solving this problem have usually been directed at a visual
differentiation between cards; that is, different types of cards have been
provided with different colour faceplates so that they may be easily
recognized. This solution may be effective in a small system, but, in a
system wherein the peripheral frames of the system may have thousands of
cards and shelves, the different colour faceplates do not contribute
significantly to the solution of the problem of misplaced cards.
Applicant has found that the physical system integrity of a contemporary
electronic modular system may be achieved and maintained through its
working life very simply and economically by providing each individual
printed circuit board of the peripheral units with the means of generating
type-code information individual to the type thereof.
The invention also provides a system for surveying the integrity of an
electronic modular system having a plurality of types of modules each one
of which is located at a respective addressable location accessible by a
control unit which has a data storage means containing type-code
information associated with respective ones of the modules of the system.
Each module of the system has a respective memory means containing
type-code information associated therewith. Each of the modules comprises
means associated therewith and responsive to a polling signal from the
central control unit for transferring its type-code information thereto.
The central control unit is provided with means for comparing the received
type code information with the type-code information contained in its data
storage means at the location associated with the polling signal. The
central control unit is also provided with means for generating an error
signal upon non-correlation between the received type-code information and
the data storage means type-code information.
From another aspect of the invention, there is provided a method for
surveying the integrity of an electronic modular system having a central
control unit having data storage means containing type-code information
associated with respective ones of the modules of the system and wherein
each module is located at a respective addressable location accessible by
the central control unit. Each of the modules is polled to cause it to
transfer the type-code information stored in its respective memory means
to the central control unit. The type-code information received from the
modules is compared with the typecode information contained in the data
storage means of the central control unit, and upon non-correlation
between the received type-code information and the data storage means
type-code information an error signal is generated.
In addition to providing an electronic modular system with the capability
of surveying its physical integrity, the invention provides a number of
other important advantages. Since each type of printed circuit board is
provided with a means of identifying itself to the outside world, the
boards are adapted for use in a fully automatic test set which exercises
all the functions of the boards. Upon insertion of a board into the test
set, the latter reads the type-code information associated with the board
and is thereby able to recognize which test sequence is relevant for that
type of board. This obviates the need for the test set to be provided with
an extensive and usually complex setting-up procedure for different types
of board. It also obviates or at least greatly minimizes the occurrence of
human error during these test procedures. In the case where the invention
is used in an electronic modular system which uses pulse code modulation
and time division multiplexing techniques, the type-code information of
each type of board may be generated thereon with an active circuit which
comprises a source of analogue signal converted to a digital signal. A
further advantage of using the invention in such a system is that the
operation of the analogue to digital converter associated with the boards
may be verified by requesting the board to output its type-code
information.
Further features and advantages of the invention will become evident from
the following description of an example embodiment of the invention in
which:
FIG. 1 is a block diagram of a modular electronic telephone system using
the invention; and
FIG. 2 is a block schematic diagram of a peripheral unit shown in FIG. 1.
FIG. 1 is a block diagram of a modular electronic telephone system using
pulse code modulation and time division multiplexing techniques. Only
those portions of the system necessary to the understanding of the
invention are shown. A plurality of peripheral units 10 are connected on
one side to a source of analogue signals (e.g. speech) and on the other
side to a central control unit 20 of a switching system via data links 30.
Each peripheral unit 10 represents a shelf of printed circuit cards or
modules such as may be found in a contemporary switching office.
As is conventional in systems using distributed processing, a peripheral
unit performs the majority of repetitive or routine real time significant
tasks associated therewith, under control of a peripheral processor. The
functions of a peripheral unit 10, in this case a trunk unit, include
scanning, converting analogue to digital signals and digital to analogue
signals, and multiplexing the digital signals into a serial digital signal
for transmission to the central control unit of the office. The trunk unit
10 is also responsible for generating digital tones as well as sending
signalling and control information to the central control unit. In order
to provide these functions, the trunk unit 10 is provided with a
peripheral unit control circuit 12 (including a peripheral processor), a
tone and memory circuit board 13, a transmission interface circuit board
14, and a converter circuit board 15. Each trunk circuit 11 represents a
printed circuit card or module, and since as explained earlier, a variety
of functions have to be performed by the trunk circuits, a corresponding
variety of types of trunk circuits are required. In any one central
office, up to about twenty-five different types of trunk circuits may be
in use. Therefore, in accordance with the invention, each trunk circuit 11
is provided with a type circuit 16. Each type circuit 11 provides
type-code information individual to that board. In response to a request
or polling signal from the central control unit each trunk is adapted to
output its type-code information which is transmitted via the converter
circuit 15 and the transmission interface circuit board 14 to the central
control unit 20.
As is conventional, the central control unit is provided with a processor
circuit 21, a data memory 22, and an interface transmission circuit 23. In
addition, the central control unit is shown to include a comparator
circuit 24 and an alarm circuit 25 responsive thereto. The comparator
circuit 24 may be of any well-known type such as is available
commercially, and the alarm circuit may be of the audible or visual type,
or of the type which generates a signal to cause a printout such as on a
teletypewriter.
Briefly, the operation of the system of FIG. 1 is as follows. Whenever the
central control unit 20 is directed (automatically, or through directions
from a maintenance operator), to verify or survey the physical integrity
of the switching system, it issues polling signals, each one associated
with a respective trunk circuit 11. In response to the polling signal,
each trunk circuit 11 outputs its type-code information which is
transmitted to the central control unit 20 via the transmission interface
circuit 14, a data link 30, and the transmission interface circuit 23.
When the type-code information is received from a particular trunk
circuit, the processor causes the memory 22 to output the type-code
information related to the polling signal that caused the received
type-code information from the trunk circuit. The received type-code
information and the central control unit memory type-code information are
compared in the comparator circuit 24 to determine the correlation of the
two signals. Upon non-correlation of the signals, the alarm circuit 25 is
responsive to the output signal of the comparator circuit for providing an
indication of a problem which may then be acted upon.
The central control unit was only described in block diagram form since all
of the circuit blocks shown are conventional in the art and may be
realized employing a variety of well-known circuits.
FIG. 2 is a block schematic diagram of the trunk circuit 10 illustrated in
FIG. 1. Only those portions of the trunk unit necessary to the
understanding and implementation of the invention are described.
FIG. 2 shows a converter module 15, a peripheral unit control module 12,
and a transmission interface module 14. The tone and memory module 13
shown in FIG. 1 is omitted from the diagram of FIG. 2 because it is not
relevant to the invention.
The control module 12 serves as the central control unit for the trunk unit
and it controls the internal operation thereof. It is shown to comprise a
peripheral processor 120, timing control circuits 121, and a type-code
register 122 the function of which will become evident below.
The transmission interface module 14 is of conventional design and
comprises multiplexing circuitry as well as various transmission circuits
and buffers. It serves to receive and transmit digital signals to the
central control unit 20 (FIG. 1) via the data link 30 and to interface
these signals to the remainder of the trunk unit 10.
The module 15 is a successive approximation converter circuit and comprises
a companding (.mu.255) A/D converter circuit 150, a differential
comparator 151, encoder logic circuit 152, and a sample, hold, and stretch
circuit including gates 153 connected to a differential pulse amplitude
modulation (PAM) bus, capacitors 154 and amplifiers 155. The functions
provided by modules 12, 14 and 15 are well known and may be realized using
conventional circuits and components. In addition, the converter module 15
includes an amplifier 156 having its input connected to the source of
reference voltage of the converter circuit 150. The amplifier 156 provides
an output voltage in balanced configuration to a balanced power supply bus
and it may simply consist of an operational amplifier in balanced output
configuration.
Both the power supply bus and the differential PAM bus extend to each trunk
circuit 11. The differential nature of the PAM bus insures that a minimum
of noise is created thereon.
FIG. 2 shows only one of the plurality of trunk circuits 11 comprising the
trunk unit. In some cases, a trunk unit may have as many as twenty-four or
even thirty trunk circuits and similar trunk circuits may be packaged two
per printed circuit card. As is conventional, an input port 110 of the
trunk circuit 11 is connected to a source of analogue signals which are
filtered in a lowpass filter 111 and appear at the input of a differential
pulse amplitude modulation (PAM) gate 112 which is controlled by sampling
signals - speech enable signals - from the peripheral unit control board
12 for generating PAM signals on the differential PAM bus. Also included
in each trunk module 11 is a type circuit 16 which comprises a resistor
bridge R.sub.1 R.sub.2, capacitor C.sub.1 connected across resistor
R.sub.1 and a differential PAM gate 160 controlled by sampling signals on
the type-code enable lead from the timing control circuits 121 in the
peripheral unit control module 12. Upon the occurrence of these sampling
signals, the gate 160 is responsive to provide PAM signals on the
differential PAM bus. These signals correspond to the analogue voltage
level across resistor R.sub.1.
The resistor bridge R.sub.1 R.sub.2 is connected to the power supply bus
via a pair of straps 161 and 162 connected to terminals AB and CD
respectively. If the straps 161 and 162 are connected to terminals AD and
CB respectively the polarity of the analogue voltage across the resistor
bridge R.sub.1 R.sub.2 is inverted. This has the effect of doubling the
number of useful PCM codes which may be encoded. It should be noted that
the resistor bridge R.sub.1 R.sub.2 may simply be connected across a
source of voltage and the voltage reversal straps located between the
capacitor C.sub.1 and the PAM gate 160. However, the provision of a
balanced power supply source derived from the reference voltage supply of
the converter provides an important advantage. It allows the voltage
applied to the PAM gate via the resistor bridge to be controlled within
upper and lower limits which follow the variation of the converter
reference voltage supply. The range of voltage determined by the resistor
ratio defines the quantity of PCM codes available for type coding.
For example, if the reference voltage of the codec is 5 volts, it is
desirable to provide a differential voltage at the sampling gate input
varying between 0.3 and 5 volts. This ensures that a PCM word
corresponding to the analogue voltage signal across R.sub.1 corresponds to
a code derived from the segments of the converter companding curve which
are higher than the midpoint thereof. This has the advantage that the
resistors used in the bridge R.sub.1 R.sub.2 are not required to be
special tolerance resistors. This is because the .mu.255 companding curve
has a logarithmic shape and the incremented difference between the upper
and lower codes of the upper four segments vary by at least three percent.
Therefore, the use of one percent resistors in the type-coding resistor
bridge R.sub.1 R.sub.2 is well within the tolerance required for the
effective discrete encoding of various types of modules. Of course, it is
entirely possible to use the lower portion of the companding curve;
however the tolerance of the resistors must be chosen accordingly.
Increasingly lower tolerance of resistors is necessary as the origin of
the curve is approached.
BRIEF DESCRIPTION OF OPERATION
Under normal conditons the circuits of the peripheral unit of FIG. 2
perform their functions in the conventional manner. Analogue signals
appear at the input port 110, are filtered in the filter circuit 111 and
are converted to digital signals by the PAM gate 112 and the codec
circuitry 15 under control of the peripheral control unit 12. The
resulting PCM information is buffered and multiplexed in the transmission
interface circuit 14 and transmitted serially on the digital line 30 under
control of the central control unit. Of course, since there are a
plurality of trunk circuits, each one is enabled in a sequential and
orderly manner and, as is conventional in a system using PCM and time
division multiplexing, the information from the various sources (e.g.
trunk circuits) is allocated to respective channels and is timely
transferred to the central control unit. Conversely, information from the
central control unit is received at the peripheral unit in an orderly
manner thereby enabling the latter to identify the destination (e.g. trunk
circuit) of the various pieces of information. In addition to the
information channels corresponding to the individual trunk circuits, at
least one extra channel is usually provided between each peripheral unit
and the central control unit. This channel is often designated the
signalling channel and is used in the system for exchanging control
information between the peripheral unit and the central control unit.
When it is desired to survey the physical integrity of the system, such as
in a maintenance procedure, the central control unit instructs the
processor of the peripheral unit to provide type-code information related
to the modules in the unit. During the signalling channel time slot, the
peripheral processor 120 causes the timing control circuits 121 to provide
a type-code enable signal to the PAM gate 160. The speech PAM gate 112 is
not enabled during the signalling time slot. The voltage level across
resistor R.sub.1 is sampled by PAM gate 160 and this signal appears on the
differential PAM bus and is applied to the converter circuit 15 which
converts it to a companded PCM word representing the type of module. This
PCM word may be transmitted directly to the central control unit via the
interface circuit 14 or it may be stored in the register 122 for timely
transfer to the central control unit which is then in a position to
correlate the received type-code information with that contained in its
data memory. In this manner, the central control unit is able to determine
the type of module at every location in each of the peripheral units.
It should be realized that other types of type-code information generators
may be provided at every module without departing from the scope and
spirit of the invention. For example, each module may be provided with a
memory means in which is stored a PCM code word representing the type of
module; this word being retrievable on command from the central control
unit. However, the embodiment described herein provides the advantages of
economy and simplicity in addition to causing the converter circuit to be
exercised every time that the type-code information is requested thereby
verifying its operation.
It should also be noted that under some circumstances, it may be desirable
to provide a module which includes both the analogue reference voltage
source and the codec, thereby obviating the need for a multiplexed PAM
bus.
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