Patent GB-1469465-A (GB)
Title: Detection of errors in digital information transmission systems
IPC: G06F11/10
Filed: 1974-09-18
Abstract: 1469465 Data transmission; error detection INTERNATIONAL BUSINESS MACHINES CORP 18 Sept 1974 [23 Nov 1973] 40594/74 Heading H4P An error prone transmitting system employing information blocks separated by one or more unique flag signals and which utilizes polynomial check characters R<SP>1</SP>(X) (defined in claim 1), the sequence comprising a flag signal, information blocks, a check block and one or more ending signals flag, requires at the receiver, flag signals to be detected and deleted, and the arrangement disclosed is directed to this function particularly where errors may occur in the vicinity of flag signals, together with identification of polynomial remainder to indicate correctness of transmission. At the receiver.-From a modem (not shown) input bits are received into AND 101 also inverted to AND 102 both sampled at clock 1 time and controlling latch 104, a 1 input bit = positive output which is led to a counter 105- 107 seeking the initial flag sequence as well as zeros inserted at a transmitter; AND gates 108 (109) connected thereto detect 6 (5) successive 1's; 5 x 1's followed by a 0 produces a delete 0 signal, the 0 resetting the counter. Latch 104 output is also led to AND 110 and through inverter 111 to AND's 112, 115, latch 113 being controlled by 114, 115. If a flag sequence of say 01111110 is detected the first 0 resets latch 104 which gives a negative output, inverted at 111 and supplied to gates 112, 115. At clock 8 time 112 sends a positive pulse through OR 116 resetting counter 105-107 to zero. At clock 9 time, gate 115 sets 113 to provide a positive output to gate 110. Successive 1's then increment the counter; at count 6, gate 108 gives positive output to 114, resetting latch 113 which blocks 110, also resets the counter. Gate 117 also receives the output of 108 and inverter 118 thus if next bit=0 gate 117 provides an output at clock 2 time to set latch 119 thereby providing a FLAG DETECT signal; latch 119 is reset at clock 10 time. Only when the next bit is 0, at clock 2 time a gate 120 provides an output which sets latch 121 which is reset at clock 8 time, hence a delete 0 signal will not be produced during a flag sequence but at such times as required to delete an inserted 0 bit. The arrangement of Fig. 6 (not shown) counts the 8 bits following the detection of a flag sequence and it is desired to learn whether a second flag sequence is present. The first FLAG DETECT signal is supplied through OR (129) to reset counter stages (125-128) also to AND (122) which at clock 6 time sets latch (123) which primes (124) to pass shift pulses to the counter which occur each bit time, other than when an inserted 0 is received. At a count of 8, output of stage 128 will be positive and hence prime AND (130) to reset the counter and latch (123). If a second flag sequence is present FLAG DETECT resets the counter before it reaches 8 thus a BIT CTR = 8 signal only occurs when the 8 bits following a first flag sequence is not a second flag sequence, FLAG DETECT + BIT CTR are applied to decoder Fig. 8, comprising a 16 stage feedback register 142-157 connected for an identical polynome as the transmitter. FLAG DETECT primes AND 161 and at clock 9 time register 142-157 is set to an initial non- zero number, e.g. all 1's. Input data from Fig. 7 arrangement (not shown) is led into exclusive OR 160 and is shifted by clock signals 5 except under delete 0 bit conditions. Once a flag sequence has been received and the next 8 bits are not the flag sequence, the terminal, e.g. operational machine, can be enabled to receive bits hence the first information bits are led to AND 169 and through inverter 170 to AND 171. If first information bit is 0 (1) gate 171 (169) resets latch 172 at clock 2 time, hence 172 provides an enabling signal for the terminal (i.e. receiving machine). Having reached an ending flag sequence, stages 142-157 should contain a predetermined number; to check, the stages are connected to AND 174 which through inverter 175 and AND 176 gives an error signal when appropriate, thus an error will be indicated. In Fig. 7, data bits are delayed in register (132-139) hence an ending flag sequence is detected as the last bit of the frame is received in 142-157. While the arrangement disclosed only indicates detection of errors it is envisaged that this may cause a transmitter to retransmit from the last correct frame but there is no particular disclosure of this.