Patent GB-1494415-A (GB)
Title: Alpha-numeric terminal for a communications system
IPC: H04L1/00
Filed: 1975-08-19
Abstract: 1494415 Data transmission MOTOROLA Inc 19 Aug 1975 [21 Oct 1974] 34423/75 Heading H4P A communications system having transmit and receive terminals capable of sending both predetermined and insertable text messages comprises a keyboard providing text signals which are temporarily stored in a memory, a fixed multi message memory with messages selected by operation of dedicated keys both memories feeding into a logic circuit which also receives address codes and assembles inputs into a determined sequence. Before transmission parity bits are generated and inserted. An interleaving circuit develops a duplicate message at least including the text signals, fixed message(s), address and parity bits, the original and duplicate messages being partially interleaved to form a composite message which may be preceded by a pseudo-random code. A keyboard (20) has a series of text keys 20A connected through a temporary store 25 to circuitry 26 also receiving a number, e.g. 10 of fixed message blocks from 27 selectable by keys 20B; circuitry 26 also receives addresses from memory 28. Before transmission the content of 26, arranged in a sequence first address, duplicate first or a second address, fixed status message fixed message and text (if any) are passed to a parity generator 29 where every six bits has an even parity bit added, the whole message being duplicated and after a delay interleaved with the original into a composite message which is then passed for transmission through an optional encoder and modulator 30; a pseudo random header code may be added at this stage which code may be selected from a memory or produced from a generator which serially receives the composite message. There may be several pseudo random codes each of which is an indication of a type of message to follow. Any form of coding or modulation may be used differential encoding, i.e. 1 bit type providing a change in the data and the other no change, being particularly mentioned. The end of each message is signalled by 6 Î 0 bits. In operation a text message will be inserted in 25 which will appear on a display 53, and fixed messages selected from 27; if there is no text 2 Î 0's are inserted; transmission is initiated automatically by pressing an appropriate key which arranges the message parts which are passed through circuits 29, 30 provided that a busy detector 55 connected to receiver portion 15B has not temporarily inhibited transmitter 15A. The original message is interleaved with blank bits until the delayed duplicate message catches up when the two messages are interleaved with each other. When the original message runs out, the duplicate message is interleaved with blanks until this too runs out. At a remote receiver the fixed message portion is transmitted back (with two acknowledge bits) which is compared with the fixed portion transmitted, which if identity is detected a previously energized acknowledge signal lamp is extinguished. If an acknowledge signal is not received by the transmitter the original message is transmitted 7 times and failing as a suitable return acknowledge signal the acknowledge signal lamp in display 53 flashes thus indicating to an operator he must reinitiate the transmission. The receiver (Fig. 5, not shown) has, at its input, biased threshold amplifers (55, 56) which "strain out" interference before the message is passed to 64 bit registers (80, 83) and simultaneously through a one bit delay (84) so that both message parts enter registers (85, 86) simultaneously which parts are compared in a mismatch detector 91. Parity of each message part is also checked and if at least one is correct this will be passed to memory 51 and be displayed. If neither message part is sufficiently accurate this will be indicated by an asterisk. The arrangement is stated to be suitable for use in a police communication system whereby remote transceivers are carried by vehicles in communication with a central base station. In the embodiments described a speech channel may be incorporated, utilizing the same radio frequency, by arranging one of the fixed messages to be transmitted which causes necessary change over at the receiver, e.g. de-muting the respectively remote speaker and causing a suitable indication to be given.