METHOD AND APPARATUS FOR EVALUATING REPETITIVELY TRANSMITTED SIGNALS
First Claim
1. A method for evaluating data words transmitted several times from a transmitter to a receiver for the purposes of error correction or at least detection, comprising, at the receiver, the steps of:
- transmitting a given word several times;
subjecting the data word received during a first transmission to a parity test;
storing such word;
identifying a word having a parity error;
subjecting such word received during a second transmission to a parity test;
comparing the word received during the first transmission with the word received during the second transmission, replacing the stored word from the first transmission with the word received during the second transmission whenever the word received during the second transmission has proper parity;
producing a difference indication when the word received during the first transmission differs from that received during the second transmission; and
replacing the stored word with the word received during a third transmission only when such a difference indication is produced and when the word received during the third transmission has proper parity.
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Abstract
A method and apparatus for correcting or at least detecting, at a receiver, errors in a sequence of data words each sent together with an associated parity test signal over a transmission channel between a transmitter and the receiver, the error correction or at least detection being performed by subjecting each received word to a parity test and delivering the received words, together with a signal indicating the result of their individual parity tests, into a memory, sending the words and their associated parity test signals a second time over the transmission channel, subjecting the words received during the second transmission to a parity test and comparing each word with the word received during the first transmission, replacing, in the memory, a word received during the first transmission by the word received during the second transmission only when the word received during the second transmission does not have a parity error, causing the words to be sent over the transmission channel a third time, subjecting the words received during the third transmission to a parity test, and replacing a word then in the memory with the word received during the third transmission only when the versions received during the first and second transmissions differed from one another and the word received during the third transmission has correct parity.
15 Citations
8 Claims
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1. A method for evaluating data words transmitted several times from a transmitter to a receiver for the purposes of error correction or at least detection, comprising, at the receiver, the steps of:
- transmitting a given word several times;
subjecting the data word received during a first transmission to a parity test;
storing such word;
identifying a word having a parity error;
subjecting such word received during a second transmission to a parity test;
comparing the word received during the first transmission with the word received during the second transmission, replacing the stored word from the first transmission with the word received during the second transmission whenever the word received during the second transmission has proper parity;
producing a difference indication when the word received during the first transmission differs from that received during the second transmission; and
replacing the stored word with the word received during a third transmission only when such a difference indication is produced and when the word received during the third transmission has proper parity.
- transmitting a given word several times;
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2. A method as defined in claim 1 wherein said step of identifying produces an identifying signal different from the signal representing the difference indication.
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3. A method as defined in claim 1 for evaluating a block of data words transmitted with a block parity signal, further comprising selectively preventing at least one of the second and third transmissions on the basis of the results of the parity tests performed.
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4. Apparatus for evaluating data words transmitted several times together with a parity test signal from a transmitter to a receiver for purposes of error correction or at least detection, comprising, in combination, at the receiver:
- memory means for storing the data words and associated identifying signals;
a first register connected for holding a word received during a transmission together with its associated received parity test signal;
parity test means connected for subjecting the data word in said first register to a parity test and producing a parity error signal when a word contains a parity error;
transfer gate means connected for transferring a received word, without its parity test signal, from said first register to a selected location in said memory means;
identity testing means connected to compare a word in said first register with the corresponding word received during a previous transmission and stored at the selected memory means location, for producing a difference indication when the compared words differ;
a transmission counter connected for producing a cOunt signal identifying the number of times a word has been transmitted; and
logic circuit means connected to said transfer gate means, said transmission counter and said parity test means for causing said difference indication to be introduced into the selected memory means location upon the production by said transmission counter of a count signal indicating completion of a predetermined number of transmissions and for enabling said gate means in dependence on the relation between the count signal, the output produced by said parity test means and the value of the indication in said selected memory means location.
- memory means for storing the data words and associated identifying signals;
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5. An arrangement as defined in claim 4 wherein said transfer gate means are further connected for placing a signal representing the output of said parity test means into the selected location of said memory means.
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6. An arrangement as defined in claim 4 wherein said logic circuit causes a word stored at a location of said memory means after the first transmission to be replaced by the corresponding word received during the second transmission only when that word received during the second transmission does not produce a parity error signal, and causes a word stored at a location of said memory means after the second transmission to be replaced by the corresponding word received during the third transmission only when the comparison of the stored word produced a difference signal in said identity testing means and that word received during the third transmission does not produce a parity error signal.
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7. An arrangement as defined in claim 6 for evaluating blocks of data words transmitted with a block parity test signal, further comprising:
- block parity test means connected for subjecting each received block to a parity test and for producing an output representing the result of that test; and
means connected to said parity test means and said block parity test means for indicating the occurrence of a word parity error or a block parity error to control the initiation of a second or third transmission only when at least one of these evaluation signals indicates the presence of an error.
- block parity test means connected for subjecting each received block to a parity test and for producing an output representing the result of that test; and
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8. A method for evaluating data words transmitted several times from a transmitter to a receiver for the purposes of error correction or at least detection in a system including an image memory connected to a display at the receiver, comprising, at the receiver, the steps of:
- transmitting a given word several times;
subjecting the data word received during a first transmission to a parity test;
storing such word;
identifying a word having a parity error;
subjecting such word received during a second transmission to a parity test;
comparing the word received during the first transmission with the word received during the second transmission, replacing the stored word from the first transmission with the word received during the second transmission whenever the word received during the second transmission has proper parity;
producing a difference indication when the word received during the first transmission differs from that received during the second transmission, said step of identifying producing an identifying signal different from the signal representing the difference indication;
placing a stored word received during the first transmission with a parity error on the display in the form of a special error symbol; and
providing a stored word received during the second transmission and differing from the word received during the first transmission with a special identifying bit which effects a display of the signal which differs visibly from the display produced when there is no such difference.
- transmitting a given word several times;
Specification