Patent GB-1446609-A (GB)
Title: Time division multiplexed digital switching apparatus
IPC: G05B19/05
Filed: 1973-11-13
Abstract: 1446609 Automatic exchange systems L M ERICSSON PTY Ltd 13 Nov 1973 [13 Nov 1972] 52730/73 Heading H4K A T.D.M. digital switching arrangement includes a multi-inlet, multi-outlet switch, a store having a location for each switch outlet, means to simultaneously address a switch outlet and the corresponding store location, read out of the latter providing the address of a switch inlet to be connected to that outlet and means to monitor the condition of each inlet and to control operation of the arrangement. In the PABX shown the switch has 32 inlets comprising 23 extensions, 6 junctions, two inlets for the supply of dial and busy tone and a reset inlet used also for a common alarm system to which all free substations are connected. The outlets comprise the 23 extensions, the 6 junctions, a code receiver 18 and an unused outlet. A counter or shift register 16 cyclically addresses the outlets or demuliplexer 14 of the switch and corresponding locations in a random access store 15. Each word read from the store gives a five bit extension address M<SP>I</SP>, a three bit junction address M J  and three additional bits M M  which are a marking bit used during dialling and two bits which indicate if M I  or M J  should be used to address a switch inlet, if ringing tone should be sent to the line or if a hold tone should be applied to the junction, this arrangement allowing an outside call to be held during an enquiry to another extension. For each call two separate paths are set up for the two directions of transmission. When the switch outlet connected to code receiver 18 is addressed one inlet of the switch is scanned for its calling or signalling condition. The same inlet is repeatedly scanned until the code receiver (which is the subject of Specification 1,446,608 recognizes a code. Overall control is effected by a processor 17 having a program in a read only memory and which has access to the store 15 which is also used as a last look store and a register store on a time shared basis. An address register 23 alternately supplies to the store 15 an address from 16 and an address from processor 17, and a buffer 24 and recoder (net shown) derive from the data read from 15 in response to an address from 17 the address of the next instruction to be performed. The switch is in the form of a multiplexer 11 and demuliplexer 14 connected by a T.D.M. highway 13 on to which tones other than dial and busy tone are applied by a tone inserter 19. Two digit dialling is effected, the first digit dialled being stored in the M I  portion of memory location corresponding to that extension with simultaneous setting of the marking bit in that location until the second digit has been detected. In the preferred form all extensions are digital telephones or data stations and dialling and other signals are also digital (delta modulation). All tones are also preferably digitally generated by division of the system clock frequency. To interface with analog exchanges the junctors have interface units controlled by a common control processor as described in Specification 1,446,610 and this processor can search for a relay set which is idle, has a new call or has a call which has timed out.