Patent GB-1315425-A (GB)
Title: Digital communication system
IPC: G06F13/38
Filed: 1971-11-12
Abstract: 1315425 Digital transmission; dual rate systems INTERNATIONAL BUSINESS MACHINES CORP 12 Nov 1971 [23 Dec 1970] 52562/71 Heading H4P In a first mode character codes may be transmitted in a continuous sequence to a receiving station having an identifying arrangement to channel the signals to a first receiver. In a second mode, character codes are spaced at intervals, which may contain filler, which are gated at the receiving end to a receiver operating at a slower rate than the first mode, which gating device prevents first mode character codes from being received by the second receiver. The first mode receiver may be a magnetic recorder and the second mode receiver may be a printer but in this mode received characters may also be recorded. An error detecting device may cause the transmitter to switch to the second mode, if in the first, when more than a determined number of errors have been detected. At the transmitter an input device 11, which may comprise a magnetic card reader/recordertypewriter/printer of known type, supplies data through a buffer 91 to a register 13 which is gated out to line 25 through AND 17, OR 19, serializer 21, and modulator 23 under the control of unit 15 which also develops control codes through generator 25 gated into the system through OR 27. A check character is developed at 29 responsive to serializer 21 output. A clock 31 times 21, 23 &c. also an idle character counter 33. A mode switch 77 selects which receiver output device will be operative. A latch circuit 79 is set by a first negative acknowledge signal from the receiver which prime an AND 81 which on receipt of a second acknowledgment signal resets latch 79 and causes unit 15 to transmit a character selecting a printer 63 at the receiver. At the receiver a demodulator 51 supplies a de-serializer 55 also a clock extraction circuit 53, output being received by a decoder 57 which feeds through AND 59 a data register 61 the output from which is channelled through AND 137 and/or 141 to the printer 63 and/or magnetic reader/recorder 65. A register 67 accumulates check characters and at the end of each piece of transmission this register is tested by a unit 69 which is also responsive to an error detection circuit 71 for the reader/ recorder. If errors are not detected an ACK signal is sent to the transmitter but if an error is detected a negative signal NAK is sent. This causes retransmission at the original rate but a second successive NAK signal causes further retransmission in second (print) mode. In the second mode of transmission the counter 33, which may have five states is advanced by one with the first character and is then clocked through the remaining four to supply signals to unit 15 which generates idle codes to generator 25 at the end of which, unit 15 gates out through 17 the next data character thereafter four idle codes are again generated et seq. The last character of a part transmission, e.g. at carriage return or the advancement of a card reader/recorder to a different track causes the generation of an end of transmission ETX code also check character codes which are derived in unit 29. The arrangement may operate synchronously or asynchronously. It is envisaged in the Specification that communication may be alternatively between a computer and a remote terminal station: also other recording units such as paper and/or magnetic tape may be utilized. In addition, devices having more than two different data rates may be employed by the use of suitable character codes and select codes.