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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to digital data transmission systems and more
specifically to asynchronous digital data systems including error
correction and automatic synchronization circuits for maintaining
synchronization when the synchronization code is not detected for several
word intervals and for resynchronizing the system where synchronization is
lost.
2. Description of the Prior Art
A typical prior art system is disclosed in U.S. Pat. No. 3,805,234,
assigned to the same assignee as this patent application. This prior art
system includes circuitry for correcting errors, however if
synchronization between the sending and receiving circuits is established
and then lost due to interference in the communications path for example,
synchronization is difficult to re-establish because the receiving
circuits cannot recognize the difference between the synchronization code
comprising start and stop bits and normal information bits within a data
word. These problems are solved by the system which is the subject of this
patent application. The system disclosed in this application is a
modification and improvement of the system disclosed in U.S. Pat. No.
3,805,234 discussed above.
SUMMARY OF THE INVENTION
The invention comprises a data transmission system which includes a data
origination station and a data utilization station which communicate via a
duplex channel to transfer digital data from the data origination station
to the data utilization station. Data is transferred from the data
origination station to the data utilization station in the asynchronous
mode. The system also includes circuitry for correcting errors introduced
in the transmission channel and for maintaining synchronization between
the data origination station and the data utilization station when the
synchronizing code associated with each data word is destroyed in the
transmission channel.
The data origination station includes (a) circuits for accepting digital
data from a data source and for transmitting the digital data as a
plurality of digital data words each including a synchronizing code, a
field code and a data portion as a continuous output data stream; (b)
small memory circuits for temporarily storing a selected number of the
digital data words; (c) circuits for receiving a returned data stream from
said data utilization station; (d) circuits for detecting a synchronizing
code included in each data word comprising said returned data stream to
generate a synchronization signal; (e) circuits for generating an
automatic synchronization signal which includes a pulse substantially
coincident with each pulse of the synchronizing signal; (f) circuits for
combining said synchronization signal and the auto synchronization signal
to generate an initiate signal; (g) circuits responsive to the initiate
signal to produce a clock signal which includes a pulse positioned to
shift each bit of the data words of the returned data signal into a shift
register; (h) circuits for comparing the data words of the returned data
stream to the counterparts of these data words as stored in the memory to
generate a signal indicating which data words of the returned data stream
are identical to the same words originally transmitted; (i) circuits
responsive to the compare signal to generate a loss of synchronization
signal when a preselected successive number of the data words comprising
said returned data stream are not identical to the same words as
originally transmitted; and (j) circuits responsive to said loss of
synchronization signal to interrupt the output data stream and for
inserting a selected number of special synchronizing words into the output
data streams.
The data utilization station including (a) circuits for detecting a
synchronization code associated with each data word of the received data
stream to generate a pulsed synchronization signal; (b) circuit means for
generating an automatic synchronizing signal which includes a pulse
substantially coincident with each pulse of the synchronizing signal; (c)
circuit means for combining the pulses of the synchronizing and automatic
synchronizing signals to generate an initiate signal; (d) circuit means
responsive to the initiate signal to shift of the data words of the
received data stream into a register and for generating the returned data
stream; (e) circuits responsive to the field code portion of the words of
the received data stream which permit only those words received error free
to be outputted and which also generate a loss of synchronization signal;
(f) means responsive to the loss of synchronization signal to disable the
circuitry from generating the automatic synchronizing signal and for
resynchronizing the data utilization station in response to the special
zero synchronizing words transmitted by said data utilization station.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of the system which is the subject of this
invention;
FIG. 2 is a waveform diagram illustrating the relationship between the
individual data words comprising the data streams, the clock signal and
the automatic synchronization signal;
FIG. 3 is a chart illustrating an example of field codes used for error
detection and correction;
FIG. 4 is a diagram illustrating the transmission of data when no errors
are detected;
FIG. 5 is a diagram illustrating the transmission of data when errors are
detected;
FIG. 6 is a diagram illustrating the transmission of data when
synchronization is lost;
FIG. 7 is a block diagram of the transmission and receiving circuits
utilized by the data origination station;
FIG. 8 is a block diagram of the receiving and transmitting circuits of the
data utilization stations.
DESCRIPTION OF PREFERRED EMBODIMENT
FIG. 1 is a functional block diagram of the preferred embodiment of the
system which is the subject of this invention. The system is a
modification of the prior art system disclosed in U.S. Pat. No. 3,805,234
which is assigned to the same assignee as this application. The subject
matter of U.S. Pat. No. 3,805,234 is incorporated by reference.
The modification of the system which is the subject of U.S. Pat. No.
3,805,234, consists of the addition of circuitry to maintain
synchronization when interference in the transmission path, for example,
results in the destruction of or the failure to detect the synchronization
code of one or several consecutive words. If the interference persists and
is so severe that the said additional circuitry cannot continue to
maintain synchronization due to accumulated timing errors, circuitry is
included also for inserting two synchronizing words in the data stream to
re-establish synchronization.
The system includes a data origination (transmitting) station and a data
utilization (receiving) station, 10 and 11. At the data origination
station 10, digital data to be transmitted is coupled from a data source
12 to a transmitter 13. Data source 12 may be any source which generates
data in a digital format. During any transmission interval a data block
consisting of one or more data words may be transmitted from the data
origination station 10 to the data utilization station 11.
The transmitter 13 combines a field code (the function of the field codes
will be subsequently explained) and a synchronization code comprising a
start bit and a stop bit with digital data from the data source 12 and
couples the resulting digital data words, as a serial data stream, via an
acoustic coupler 14 to a standard telephone network 15. At the data
utilization station 11, the data words comprising the data stream are
coupled from the telephone network 15 via a second acoustic coupler 16, to
a receiver 17. The receiver 17 couples the data words as received to a
data transmitter 25 which retransmits the received data words to the data
utilization station 10 via acoustic coupler 16 and the telephone network
15. At the data origination station 10, the retransmitted data words are
coupled via the acoustic coupler 14 to a receiver 26. Alternatively,
standard modems may be used instead of the acoustic couplers 14 and 16.
Thus data transmitted via the telephone network 15 consists of two
simultaneous data streams which are identical except for possible errors
and a time delay.
At the time a data word is originally transmitted by the data origination
station 10, it is stored in a memory which is included in the transmitter
13 with storage provided for two data words. Each of the returned data
words is compared to the corresponding stored data word as transmitted to
detect errors. A signal indicating the detection of an error is generated
if the returned data word is not identical to its stored counterpart. If
an error is detected in a returned word, the two stored data words are
reinserted into the transmitted data stream while data flow from the data
source 12 is inhibited. This results in the correct data word, which had
been returned in error, and the next subsequent data word to be
retransmitted. The same two words continue to be retransmitted, if
necessary, until the first one is returned without error. When the data
word which was originally returned in error is correctly returned,
inputting of additional data words from data source 12 is resumed. The
checking and retransmitting procedure is repeated until all the data words
transmitted are received at the data utilization station error-free.
FIG. 2 is a diagram illustrating a typical data stream, a typical clock
signal, and a typical auto-synchronization signal. The data stream, the
clock signal and the auto-sync signals illustrated in FIG. 2 are
"typical". By "typical" it is meant that the data stream represents the
data stream from the data utilization station 10, as referenced on FIG.1,
to the data utilization station 11 as well as the data stream in the
opposite direction. Similarly the clock signal is typical of the clock
signal generated by both the data utilization station 11 and the data
origination station 10 and used to shift the arriving data stream into a
shift register. The auto-synch signal illustrated also represents the
signal used by both the data origination station 10 and the data
utilization station 11 to maintain synchronization when the
synchronization code is not detected.
Two typical data words are illustrated in FIG. 2. The data stream
transmitted in both directions via the telephone network 15 referenced in
FIG. 1 consists of an indefinite number of data words having the format
illustrated in FIG. 2. Each of the data words comprising the data stream
includes a data portion 28 consisting of an arbitrary number of bits, a
field code portion 29 illustrated as consisting of 4 bits, a start bit 27,
and a stop bit 30. The start and the stop bits, 27 and 30, comprise the
synchronization code previously discussed. The start and the stop bits, 27
and 30, always have opposite logic levels. In FIG. 2 the start bit 27 is
indicated as having a low level (logic "zero") while the stop bit 30 has a
high (logic "one") level. It should also be noted that the position of the
field code portion 29 can be interchanged with the data portion 28. The
second and subsequent data words are identical in structure to the first
data word. By "identical" it is meant that both of the illustrated data
words illustrated include a start bit, a data portion, a field code and a
stop bit. In the first data word these portions are respectively
identified by reference numerals 27, 28, 29 and 30, as discussed above. In
the second data word these portions are respectively identified by
reference numerals 27a, 28a, 29a and 30a.
At both of the data origination and data utilization stations, 10 and 11,
an internal clock signal 31 is generated. Each pulse of the clock signal
31 is overlapped by a bit of the data word. At the data utilization
station 11, the clock signal 31 is utilized to shift the bits of the data
words comprising the arriving data stream into a shift register. At the
data origination station 10, the clock signal is also used to shift the
bits of the returned data stream into a shift register. The overlapping
relationship between the first data bit 228 of a typical data word and the
corresponding pulse 231 of the clock signal is illustrated in FIG. 2. The
first data bit 228 of the data portion 28 is illustrated as having a high
value (logic one). It is obvious that data bit 228 could also have a low
value (logic zero).
A circuit which generates an auto-synchronization signal 32 is enabled when
synchronization is established. Synchronization is indicated as being
established by the data utilization system 11 outputting the first data
word and the data origination station 10 detecting that the first word of
the data stream has been transmitted and returned error free. The
auto-synchronization signal 32 includes a pulse 32a slightly delayed from
the falling edge of the start bit 27.
The function of the auto-synchronization signal 32 is to maintain
synchronization when the start code 27 and/or the stop code 30 is lost due
to interference in the transmission channel. Loss of either the stop or
start bit can result in a failure to detect the start bit because the
falling edge of the stop bit 30 is detected to establish synchronization.
Loss of either of these bits will prevent this edge from being detected.
The automatic synchronization signal and its function will be described in
more detail later with reference to the block diagram of the system.
Each data word includes a four bit field code, as previously discussed.
There are four unique field codes with these codes being cyclically
assigned to the data words comprising the data stream. By cyclically
assigned, it is meant that the field codes are sequentially assigned
beginning with the first through the fourth code, with this one through
four sequence being repeated for all subsequent new data words so long as
the transmission process continues.
FIG. 3 is a chart indicating one example of a set of bit patterns
representing each of the unique field codes. In the example given, the
field code contains 4 bits. The bit patterns for the field codes are
selected such that several bits of each code must be changed in order to
generate another valid code. This prohibits a single error from changing
one field code into a different code. This substantially improves the
reliability of detection of these field codes. This is important because
correct detection of the field codes is essential to the error correction
process. If one valid field code is changed into another valid field code
during transmission, the ability of the system to output error free data
is disrupted. The use of the field codes in the error detection process is
subsequently described in detail.
FIG. 4 is a diagram illustrating the output data stream from the data
origination station 10 to the data utilization station 11 in the absence
of any errors being detected in the transmission process. In the top line
(line A) of FIG. 4, the data words as sequentially transmitted are
symbolically illustrated as rectangles and identified by reference
numerals 33a-33f. The field code assigned to each of the data words is
indicated by a single digit number inside each of the rectangles which
symbolically represent the data words. For example, a single digit number
"1" is inside the first rectangle identified by reference numeral 33a
which symbolically represents the first data word transmitted. This
corresponds to a field code (1100) as indicated in FIG. 3. From examining
line A of FIG. 4 it can be seen that the field codes sequentially range
from 1-4 and then sequentially repeated. Field codes 1 through 4
respectively represent field codes having (1100), (0101), (1011) and
(0010) bit patterns as indicated in the example of FIG. 3.
Each of the data words symbolically illustrated at reference numerals
33a-33f are sequentially coupled to the telephone network 15 (FIG. 1) by
the acoustic coupler 14 (FIG. 1) to produce the output data stream from
the data origination station 11. This data stream is transmitted by
telephone network 15 (FIG. 1) to an acoustic coupler 16 (FIG. 1)
positioned at the data utilization station 11 (FIG. 1). The output signal
of acoustic coupler 16 is the data stream arriving at the data utilization
station 11 (FIG. 1) and illustrated in line B FIG. 4. Each word of the
arriving data stream is symbolically represented by a rectangle and
identified by reference numerals 34a-34f. As in line A, the field codes
assigned to each of these data words are identified by a single digit
number inside the rectangles representing the data words. It can be seen
that the field codes arriving at the data utilization station 11 range
from 1 to 4 and are then repeated.
The data stream arriving at the data utilization station 11 is coupled to
the receiver 17. Each of the data words comprising the arriving data
stream is also coupled via the receiver 17 to the transmitter 25 (FIG. 1),
through acoustic coupler 16 (FIG. 1), and then retransmitted to the data
origination station 10 (FIG. 1) through telephone network 15 (FIG. 1). At
the data origination station 10 (FIG. 1), acoustic coupler 14 (FIG. 1)
couples the retransmitted data words comprising the data stream arriving
at the data origination station 10 to a receiver 26 (FIG. 1). The
retransmitted data words arriving at receiver 26 (FIG. 1) are symbolically
35a-35f (line C) of FIG. 4.
As the data words were originally transmitted they were also stored in a
memory comprising a portion of the transmitter 13 (FIG. 1) located at the
data origination station 10 (FIG. 1). As each of the data words are
returned from data utilization station 11 (FIG. 1), it is compared to the
corresponding stored word to detect any errors that may have been
introduced in the bi-directional transmission process. The time at which
these comparisons are completed is illustrated at reference numerals
37a-37f (line E) of FIG. 4. For example, the comparison of the first word
which is symbolically illustrated as a rectangle and identified by
reference numeral 33a to this word as returned and similarly illustrated
and identified by reference numeral 35a, is completed at a point
identified by reference numeral 37a. The fact that the field code of the
data word being compared to its stored counterpart is "1" is indicated by
the single digit number "1" just below the point in time (identified by
reference numeral 37a) at which this comparison is made. The fact that
this data word was correctly transmitted is indicated by the word "good"
immediately under the number "1" identifying the field code. The time at
which the other words are compared and the results of these comparisons
are similarly indicated at reference numerals 37b-37f. Points 37a through
37f respectively correspond to the points in time when the data words
symbolically illustrated as rectangles and identified by reference number
35a through 35f in line C are respectively compared to their stored
counterparts. When a particular data word has been found to be correctly
transmitted, its stored counterpart is discarded.
The data words comprising the data stream arriving at the data utilization
station 11 (FIG. 1) are temporarily stored with storage provided for two
data words. The data words are temporarily stored until a decision can be
made that they are not being retransmitted due to detected errors. The
determination that a particular data word is not being retransmitted is
made by examining the sequence of subsequently arriving field codes with
the next two field codes being in the expected (normal) sequence
indicating that the data word is not being retransmitted to correct
detected errors. In the case illustrated in FIG. 4, it is assumed that no
transmission errors occurred; therefore, the field codes arrive in the
expected sequence. This results in the data words comprising the data
stream arriving at the data utilization station 11 (FIG. 1) being
outputted to generate an output data stream comprising data words
symbolically illustrated as rectangles and identified by reference
numerals 36a, through 36d (line D) of FIG. 4. Since this illustrates the
operation when no errors are detected, the sequence of data words in line
A and line D of FIG. 4 will be identical except for the time delay. If
errors are detected, the sequence of the field codes of the output data
stream from the data origination station 10 (line A) will be different
from the sequence of field codes of the output data stream from the data
utilization station 11 (line D) because all data words returned in error
are retransmitted. However, the output data stream (line D) will always be
identical to that from the data source, item 12, FIG. 1.
FIG. 5 illustrates the transmission of data when errors are detected and
selected data words are retransmitted to correct the detected errors. The
data words comprising the output data stream from the data origination
station 10 (FIG. 1) are symbolically illustrated as rectangles in line A
of FIG. 5. As previously discussed with reference to FIG. 4, the field
code attached to the individual words is identified by a single digit
number within the rectangles symbolically representing the data words.
Similarly the data stream arriving at the data utilization station 11
(FIG. 1) are symbolically illustrated in line B with the data stream
arriving at the data origination station 10 (FIG. 1) illustrated in line
C.
The first data word of the output data stream of the data origination
station 10 is symbolically illustrated at reference numeral 45a of line A.
This word is determind to be transmitted free of errors by comparing the
first word of the data stream arriving at the data origination station 11
(illustrated at reference number 47a of FIG. 5) to the counterpart of this
word as stored at the time when it was originally transmitted. This
comparison is completed at a point in time identified by reference numeral
48a (line E). This word is indicated as having a field code "1" by the
number "1" appearing below the point identified by reference numeral 48a
and as being transmitted correctly by the word "good" immediately below
the number "1". Similarly the second word transmitted is symbolically
illustrated by a rectangle identified by reference numeral 45b (line A).
This word as returned to the data origination station 10 (FIG. 1) is
compared to the corresponding stored data word at a point illustrated at
reference numeral 48b (line E). This comparison results in an indication
that an error was introduced in the second data word during the
transmission process as indicated by the word "bad" immediately below the
number "2" identifying the field code. However, it should be noted that
the comparison of the second word 476, as returned, to this word as
transmitted 456 is not completed until after the third word symbolically
illustrated by a rectangle and identified by reference numeral 45c (line
A) is in the process of transmission. Therefore, the third data word is
completely transmitted. After the transmission of the third data word is
completed, inputting of data from the data source 12 is inhibited, and the
second and third data words are retransmitted as indicated by these data
words appearing for a second time in the output data stream (line A) from
the data origination station. These words are also retransmitted from the
data utilization station 11 (FIG. 1) to the data origination station 10
(FIG. 1) where the second word is again compared to the corresponding
stored data word as originally transmitted. The times at which the two
retransmitted words are compared to their stored counterparts are
indicated at reference numerals 48c and 48d (line E). These comparisons
result in indications that the second and third words were properly
transmitted. These words are outputted to the data sink 18 (FIG. 1) as
part of the output data stream as indicated at reference numerals 49b and
49c (line D) of FIG. 5. Following the determination that the second data
word has been transmitted correctly and after the completion of the
transmission of the third data word, inputting of data words from the data
source 12 (FIG. 1) is resumed beginning with the data word having the
field code 4 symbolically illustrated by a rectangle identified by
reference numeral 45d (line A). The data word having a field code of 4 is
returned to the data utilization station 10 (FIG. 1) and compared to the
counterpart of this data word as transmitted at a point identified at
reference numerals 48e (line E). This comparison results in an error
indication. Inputting of data from data source 12 (FIG. 1) is again
inhibited and the words having field codes of 4 and 1 are reinserted into
the data stream as indicated by these data words appearing for a second
time in the data stream (line A). Again, an error is introduced in the
word having field code of 4 as indicated by the comparison completed at a
point in time indicated at reference numeral 48f (line D). This results in
the words having field codes of 4 and 1 being retransmitted a second time
as indicated. Comparison of the data word having a field code of 4 to its
stored counterpart for a third time occurs at the point identified at
reference numerals 48g (line D). This comparison indicates that the data
word having a field code of "4" was properly transmitted. Inputting of
data words from the data source 12 (FIG. 1) is resumed following
completion of the transmission of the data word having a field code of
"1". The data word having a field code of "1" is compared to its stored
counterpart at a time identified at reference numeral 48h. This comparison
indicates that the data word having a field codes of "1" was properly
transmitted. The data words having field codes of "4" and "1" are
outputted as part of the output data stream as symbolically indicated by
rectangles identified by reference numerals 49d and 49e (line E).
The above process illustrates how the errors are corrected. The criteria
for outputting a particular data word as a part of the output data stream
to the data sink 18 (FIG. 1) is that the field codes of the next two data
words of the data stream arriving at the data utilization station 11 must
be in the expected sequence. For example, the first data word received by
the data utilization station 11 (FIG. 1), symbolically illustrated at
reference numeral 46a (line B of FIG. 5), is indicated as correct by the
field codes for the next two data words indicated at reference numerals
46b and 46c being in the expected 2-3 sequence. This results in the first
data word being outputted to the data sink 18 (FIG. 1) as symbolically
illustrated at reference numeral 49a (line D). As illustrated, the next
two field codes following the second word, symbolically illustrated at
46b, are out of the expected 3-4 sequence indicating that the second and
third data words, symbolically illustrated at reference numerals 45b and
45c (line A), are being retransmitted because an error has been detected
in the second data word illustrated at reference numeral 45b. Since the
second data word is retransmitted correctly, the next subsequent field
code following "3" should be "4" as illustrated at reference numerals 46c
and 46d. This results in the retransmitted word having a field code of "2"
being outputted as symbolically illustrated at reference numeral 49b (line
D). Similarly, the words with field codes 3, 4 and 1 are outputted as
symbolically illustrated in line D FIG. 5. Due to the structure of the
field codes, when a data word with field code 3 is received a stored data
word with field code "1" is outputted. When word with field code 4 is
received, a stored data word with field code "2" is outputted. When a data
word with field code "1" is received a stored data word with field code
"3" is outputted; and when data word with field code "2" is received a
stored data word with field code 4 is outputted. By the same process the
system also corrects for errors introduced into the field code during
transmission. The comparison at the data origination station includes the
field code as well as the data portion of the returned word. If the field
code portion contains errors, the data word is repeated the same as if the
error had been in the data portion of the word.
FIG. 6 is a timing chart illustrating the data streams when synchronization
is lost. In FIG. 6 the one digit number inside each rectangle symbolically
representing the data words identifies the field code of the data word
represented thereby. Synchronization is independent of error correction
except that loss of synchronization always results in introduction of
errors and the loss of the field codes at the data utilization station and
in the returned data. The time intervals during which the field codes are
lost due to loss of synchronization are shown cross-matched in the data
streams illustrated in lines B and C of FIG. 6.
At the data utilization station 11 (FIG. 1) the loss of synchronization is
detected by the failure to output a data word for eight successive word
intervals. At the data origination station 10, loss of synchronization is
detected by a data word being transmitted eight times with each
transmission resulting in an error indication. Since two data words are
retransmitted each time an error is detected, loss of synchronization at
the data origination station is detected in sixteen data word intervals.
Detection of loss synchronization at the data utilization station 11 (FIG.
11) disables the circuit which generates the automatic synchronization
signal. At the data origination station 10 (FIG. 1), detection of loss of
synchronization also disables the circuit which generates the
auto-synchronization signal, interrupts the normal data sequence and
inserts two synchronizing words which contain only the normal
synchronization code (i.e. the start/stop bits) with all the data and
field code bits having logic "zero" levels. This prohibits the data
utilization station 11 from interpreting either the bits of the field code
or the data portion as a start bit.
To illustrate the operation of the system when synchronization is lost, the
first four data words symbolically illustrated at reference numbers
55a-55d in line A of FIG. 6 and several previous data words are assumed to
have been transmitted from the data origination station 10 (FIG. 1) to the
data utilization station 11 (FIG. 1) through a very noisy channel such
that the start and stop bits of these and several consecutive words have
been destroyed. The system, however, has maintained synchronization by the
use of the auto-synchronization signal even though the synchronization
code has not been detected for several word intervals. It is also assumed
for purposes of illustration that the period of the severe interference
has continued for an interval sufficiently long for cumulative timing
errors in the automatic synchronization circuits to cause a loss of
synchronization. It is also assumed that during each of the next sixteen
word intervals (illustrated at reference numeral 57) no valid start bit is
detected at data utilization station 11 (FIG. 1) and consequently no data
words are outputted as a part of the output data stream. Detection of a
valid start bit during this period will resynchronize the system; however,
for purposes of illustration, it is assumed that no valid start bit is
detected between the time when synchronization is lost and the
transmission of the two special synchronizing words.
Since it is not possible for the data utilization station 11 (FIG. 1) to
distinguish between a ZERO data bit and a legitimate start bit once
synchronization is lost, the receiver circuits are designed to assume that
if no data words are outputted during a continuous eight data word time
interval, the cause is a loss of synchronization. Thus, the
auto-synchronization circuit located at the data utilization station 11
(FIG. 1) is disabled for failure to output a data word at a point
identified by reference number 66, FIG. 6.
In the illustrative example, data words comprising the output data stream
of the data origination station 10 during the period 57 when loss of
synchronization occurs will be two words continuously repeated as a result
of errors caused by loss of synchronization. In the example, two data
words having field codes of 4 and 1 are repeated. The time interval during
which no data is outputted by the data utilization station 11 is
illustrated at reference numeral 57d, line D.
During the time interval when synchronization is lost, an error signal will
be generated at the data origination station 10 (FIG. 1) each time a
returned data word is compared to its counterpart as stored at data
origination station 10 (FIG. 1). Since one word following the word
containing the error is always retransmitted when an error is detected,
comparison of the retransmitted word in the returned data stream to its
stored counterparts is delayed for two word intervals following the
detection of the error as previously indicated in FIG. 5. This results in
every second word of the returned data stream being compared to its stored
counterpart during the interval 57c during which synchronization is lost.
Each of these detected errors are counted. When eight successive errors
are detected, the automatic synchronization circuit of the data
origination station 10 is disabled and two data words containing all
"zero" data and field code bits are inserted into the data stream. These
two data words are illustrated at reference numeral 58 (line A). Following
the two synchronizing words 58, normal transmission of data is resumed
with the first data word transmitted having a field code of "4" as
illustrated at reference numeral 55e.
The synchronizing words containing all zero data bits arrive at the data
utilization station 11 (FIG. 1). Since the only logic "one" bit in either
of these data words is the "stop bit", the data utilization station 11
(FIG. 1) will recognize a "start bit" and is prohibited from recognizing
data or field code bits as a start bit. When the start bit is recognized,
the clock signal is initialized and reception of data is resumed. When the
first data word is outputted to the output data stream, the automatic
synchronization signal of the data utilization station 11 is re-enabled.
The point in time where the automatic synchronization circuits located at
the data utilization station 11 is enabled is identified by reference
numeral 70, FIG. 6. When the data origination station 10 determines that
the data word 55e with a field code of "4" has been transmitted
error-free, the auto synchronization circuits of the data origination
station 10 are enabled and normal data transmission is resumed. The point
in time at which normal data transfer is resumed is identified by
reference numeral 72.
Conversely, if a "start bit" is not recognized due to continued
transmission channel interference, the clock signal is not initialized and
a continuous succession of errors will be detected by the data origination
10 causing the entire resynchronization process to be repeated. This cycle
will continue as long as channel interference persists, periodically
transmitting the two synchronizing words until synchroni | | |
