Note: Descriptions are shown in the official language in which they were submitted.
~196407
1 The present invention relates in general to a duplex
communication system and in particular to a duplex transmission
system employing a two-wire link between a main station and a
local station, the main station generating a clock signal where-
by the coded data signal is transmitted during one half period
of the main clock signal from the main station to the local sta-
tion and during the other half period the coded data signal is
transmitted from the local station to the main station.
A data transmission system of this type is known from
the German patent 2,453,628. Such systems are employed for sig-
nal transmission over short distances and for a short duration
of transmission. The two-wire link transmits in rapid direction
exchange alternately from the main station to the local one and
vice versa informat on contained in a single signal step such as
one bit, one dibit, or one tribut. The data transmission code
is selected such that in the local station the rate of period of
the main clock signal can be detected so that the local station
is synchronized with the main station.
; In the prior system according to the German patent
25 53 628 no active clock generator is present in the local sta-
tion. As a consequence in the event of an interruption in the
link, the local station is without clock signals.
Moreover, in the prior art system, the main station
~transmits to the local station a bipolar RZ (return to zero)
signal and from the local station to the main station a uni-
polar RZ signal is transmitted. Most transmitted signals are
scanned or sensed as to their timing by means of NRZ (non-return
to zero) signals. In the RZ signals thexe is no possibility to
achieve a transmission without direct current components.
A general ob~ect of the present invention is to overcome
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1 the disadvantages of prior-art systems of this kind.
More particularly, it is an object of this inv~ntion
to provide an improved duplex transmission system of the afore-
described kind in which the local station generates clock pulses
independently from the main station whereby the rate or period
of the received signals is promptly synchronized with the clock
signals in the main station.
In keeping with these objects and others which will be-
come apparent hereinafter, one feature of the invention resides
in providing the local station with a separate local clock pulse
generator which is switchable to generate a first local clock
signal at a period corresponding to that of the main clock sig-
nal, and a second local clock signal the period of which dif-
fers from the rate of the main signal about a fixed amount,
means for comparing the phase of the received coded data signal
with one of the local clock signals, and means for switching
over the local pulse generator when the received data signal
is out o phase with the one local clock signal.
By virtue of the active clock signal generator in
the local station it is insured that corresponding terminal
apparatus in the local station is always properly clocked.
r~uring the reception of the main data signal it is synchronized
with the main clock signal and in the case of an interruption
of the transmission link it is still timed by a clock signal
which may differ from that of the main station but which im-
mediately upon the receipt of the main signal is promptly
synchronized with the latter.
The novel features which are considered as charac-
teristic for the present invention are set forth in particular
in the appended claims. The invention itself, however, both as
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1 to its construction and its method of operation, together with
additional objects and advantages thereof, will be best under~
stood from the following description of specific embodiments
when read in connection with the accompanying drawings.
FIG. l is a block diagram of a data transmission
system according to this invention; and
FIG. 2 is a time plot of respective signals occurring
in the system of FIG. l.
Referring firstly to FIG. 1, a terminal apparatus 1
is associated with a main station 2 and the terminal apparatus
3 is connected to a local station 4. The terminal apparatus
in each station may be for example a Teletex device. The main
station 2 is connected to the local station 4 by a two wire
link 5 which is relatively short, for example within the range
of a building.
The main station2 is provided with a clock pulse gen-
erator Pl controlled by a quartz oscillator Ql to generated
a main transmission clock signal a, which is applied to a coder
Cl and to a decoder Dl. Coder Cl receives from the te;rminal
apparatus 1 data s:ignal _ (Fig. 2b) to be transmitted and con-
verts the same into a coded transmission signal c (Fig~ 2c)
which is supplied to a transformer Ul connected at one end of
the two wire links 5. The transformer Ul is also connected
to the input of the decoder Dl which decodes received signal
and supplies the decoded data to the terminal apparatus 1.
The local station 4 has also a coder C2 which
processes data signals from the terminal apparatus 3 in the
same manner as coder Cl in the main station and generatas a
local data transmission i (Fig. 2i) which is applied to the other
transformer U2 at the transforming end of the link 5. According
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to this invention, local station 4 is provided with its own
clock generator which in this example includes a quariz oscil-
lator Q2 operating at a nominal frequency of 4.6 megacycles
for instance; the output of oscillator Q2 is connected to
time clock generator T2 which includes a switchable diYider
of the incoming signal. The divider divides the frequency
by n-l or n+l depending on the signal at its switching input 6
and the resulting reception clock signal _ (Fig. 2h) is ap-
plied to the output 7. _ indicates the ratio between frequencies
of the local oscillator Q2 and the local clock signal h. The
local or reception clock signal _ is inverted in inverter J and
is applied as a local transmission clock signal g to the coder C2.
The local station 4 further includes two comparators
KOMP 1 and KOMP 2 connected in parallel to the transformer
U2. One of the comparators rectifies and limits in amplitude
the positive components, and the other comparator the negative
components of the received coded signal _. Differentiators
DIF1 and DIF2 are connected to the outputs of corresponding com-
parators and the output signals from the differentiators are ap-
pIied to inputs of an A~D-gate 8. The output of AND-gate 8 thus
produces a zero par,sage signal e (Fig. 2e) which is applied to
one input of comparator 9. The other input of comparator 9 is
supplied with comparison clock signals f (Fig. 2f) produced in a
delaying circuit 11 from reception clock signal _. The output
c\f phase comparator 9 is connected to the switching input 6
of the clock pulse generator T2. If now the received signal _ is
present or if signals e and f do not coincide in time, then the
signal generated at the ouput of comparator 9 switches the
local clock pulse generator T2 to a divlding ratio n+l or al-
ternatively when the signal e and f coincide, to a ratio n-l.
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1 The output 7 of local clock pulse generator 2 is di-
rectly connected to the terminal apparatus 3. Comparator KOMP2
together with an additional delay stage 11 and step feeler 12
from a decoder D2 of the local station 4. The step feeler 12
is controlled by the reception clock signal from the output 7
via the additional delay stage 11. The sensed step values from
the output of feeler 12 are applied to a set input of a flip-
flop 13 which is controlled by the reception clock signal _.
The operation of the duplex system of this invention
is as follows:
The main transmission clock signal a has a rate or
clock period TA and a key or step ratio of 50%. During the high
(H) or "l" signal steps of the signal a coder Cl is active and
decoder Dl is blocked. Vice versa, during the low (L) or "0"
signal steps coder l is closed and decoder l is active.
Fig. 2b shows by way of an example an NRZ-signal
"0,1,1,0,1" representing information to be transmitted. The
data b during the "1" stage of the main clock signal a are con-
~` verted in coder Cl in the coded transmission signal _ which
has no DC component. In the coded signal c, each period of
:
the signal c thus consists of a negative and a positive signal
component whereby in the case of a "0" information in signal b
the negative component trails the positive component; in the
case of "1" information in signal d the positive component is
behind the negative component (Fig. 2c).
The main coded transmission signal c after passage
through the two wire link 5 is received in the local station
as a recept signal d from which the zero passage signals e
are derived in the manner described before. As it will be
seen from Fig. 2e the period of signals e is the same as that
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1 of the main clock signal a. The reception clock signal h the
momentary rate or clock period of which is indicated as PH is
to be synchronized with the zero passage signals ~. For this
purpose a comparison clock signal f is derived from the recep-
tion clock signal _ by means of the delay stage 10 and the
phase of the falling flank of the signal r is compared with
the rising flank of the zero passage signal e. The rising
flank of the signal e is delayed about a quarter of clock
period TA with respect to the rising flank of the main clock
signal a. In order to compensate for this phase difference
the falling flank of the comparison signal f is delayed about
the same amount in delay stage 10 so that the proper relation-
ship with respect to the zero passage signal e be established.
If signal f overlaps the signal e, then the divider T2 is
switched over to divide by the ratio n-l whereby n is 384 for
example. If no overlap is present between signals _ and f,
` then the dividing ratio of clock generator T2 is switched over
to n+l. Consequently, the falling flank of the comparison
signal f oscillates about the rising flank of the signal e
until the clock signal of the local station 4 at the output
7 catches the main clock signal a of the main station 2. The
local transmission signal g is inverted relative to the re-
ception clock signal _ and thus to the main transmission clock
signal a. Accordingly, if coded signal 1 to be transmitted
is coded with information derived from the terminal apparatus
3 which may be for example "1,0,0,1,1" then the signal is trans-
mitted when coder Cl in the main station is blocked and decoder
Dl is active.
The delay stage 11 delays the reception clock signal
_ about ~ of the clock pulse period TH so as to insure an optimum
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1 step sensing operation of the sensor 12, that means at larger
time gaps.
As long as no received signal d is present at the
local station 4, the local clock signal generator ~2 operates
at a dividing ratio n+l. As a consequence, a relatively large
deviation of the clock period TH relative to the main clock
period TA is produced. The phase of the comparison clock pulse
signal f for a very short time is out of phase as to the per-
iod TA. As soon as a signal d is received and the zero passage
signals e are generated, a time point is promptly reached at
which the falling flank of the signal s coincides with the ris-
ing flank of the signal e and the desired synchronization between
the main and local clock signals is achieved.
It will be understood that each of the elements de-
scribed above, or two or more together, may also find a useful
application in other types of constructions differiny from the
types described above.
While the invention has been illustrated and described
~ as embodied in a specific example of a full duplex communication
ZO system, it is not intended to be limited to the details shown,
since various modi~Eications and structural changes may be made
without departing in any way ~rom the spirit of the present in-
vention.
Without further analysis, the foregoing will so fully
reveal the gist of the present invention that others can, by ap-
plying current knowledge, readily adapt it for various applica-
tions without omitting features that, fxom the standpoint of
prior art, fairly constitute essential characteristics of the
generic or specific aspects of the present invention.
