1,206,781. Automatic exchange systems. AUTOMATIC TELEPHONE & ELECTRIC CO. Ltd., and POST OFFICE. 7 March, 1968 [30 May, 1967], No. 24831/67. Heading H4K. Loop disconnect impulse digit signalling from a path set up over Strowger switches is received by register access circuits RAC of which one is provided in each of the forward paths into a cross-bar group selector, a group of registers REG fewer in number than the number of access circuits being connectible to incoming paths IP to store a digit received in loop disconnect form and having done so to obtain the use of a common high speed sender DCS which forwards the digit to a stage register in the crossbar group selector when this reverts a proceedto-send signal. The sender DCS may transmit with coded D.C. leg signals or may be such as to transmit frequency combinations. The system is that described in Specification 1,131,367 but differs in that digit code converters are not provided in all access circuits to the cross-bar group selector. As shown in Fig. 1 eight groups of twelve speech paths IP1 to IP96 each contain a register access circuit RAC and give access to corresponding access circuits AC1 to AC96 of eight twelve inlet cross-bar switches. A group of one digit registers REG1 to REG16 is available to the access circuits RAC each one of which is served by a set of five of the registers the sets of five being distributed to equalize register use. A loop disconnect impulse detected in a register access circuit RAC produces an application to a call isolator CI comprised of a one-out-of-eight and a one-out-of-twelve lock out chain such as to select only one out of competing demands for the isolator. The selected, or the only, calling access circuit RAC is singled out by the isolator CI by connecting energizing battery to one side of a set of five register connect relays in that access circuit. A register interrogator/selector is instructed by isolator CI to examine the group of five registers available to the access circuit and, in accordance with a cyclic order of choice to equalize use, allots a free register REG. When seized the register extends earth across all the access circuits RAC for which it provides service and in this way operates the register connect relay corresponding to itself in the access circuit singled out by battery from the isolator CI. A four wire path now connects the register to the access circuit RAC, two wires connecting the incoming loop for the detection of loop disconnections, one wire holding the register connect relay so that the connection survives release of the call isolator, and one wire being used to order connection of the forward speech path to the high speed code sender DCS when the time is ripe. A three wire path from the sender is connectible to any one of the register access circuits. When a digit has been received and set up on a ten stage counter the register applies to a register isolator RI which comprises two one-out-of-four lock out circuits which select one of any concurrent demands for exclusive use of the sender DCS. The locked up state of isolator RI operates a connect relay in the register selected to make a ten digit and four control wire path to the sender. The digit is transferred to the sender as a mark on the corresponding one of the ten digit wires and is registered in the sender as operated ones out of a set of three and a set of four relays to produce a corresponding choice of potential to be connected to the one and the other legs of the forward path. When such a registration is checked for validity, and a proceed-to-send signal has been received from a stage register of the forward group selector seized earlier in response to a mark from the register connect circuit when this was connected to a register REG, the digit is sent forward and results in the line being connected to an inlet of a further group selection stage. If the further group selector stage uses cross-bar switches a signal is reverted to the code sender to indicate that a further digit will be required in highspeed code. The sender accordingly instructs the register to clear its counter and stay in service for another digit, the sender and register isolator releasing to serve other calls. When the next digit is accumulated in the register a demand for the code sender is made to the isolator RI in the same manner as for the first digit to obtain use of the sender DCS. If the further group selector stage uses Strowger switches a signal is reverted to the sender which instructs the register REG to set up the access circuit RAC to provide a through path for subsequent loop disconnect impulses; the sender, isolator, and register releasing. If high speed code digits are employed for the whole of a connection the final selection stage is such as to revert a signal to release the sender, isolator, and register REG after the access circuit RAC has been cut through. Forced release conditions.-If no register REG is available to a register access circuit RAC when this detects an initial loop disconnection the access circuit, described in detail with reference to Figs. 4 and 5, not shown, connects plant busy tone to the calling path at the end of the loop disconnect impulse. If an all-outletsbusy condition is encountered in any of the group selector stages a signal to that effect is reverted to release the digit conversion equipment and to condition the access circuit to send back plant busy tone. If a register REG has not received a digit in within 30 seconds the register releases and conditions the access circuit RAC to send back busy tone. Preparatory to maintenance work in a group selection stage, or as a result of fault conditions in those stages, a busying earth is reverted which after a delay of 40 milliseconds operates a busying out relay in the access circuit RAC to apply busy earth to the incoming private wire and to disconnect the loop disconnect impulse detecting relay in case the circuit is directly accessible from an incoming junction. When such busying earth is reverted while the access circuit is switched through for a connection, or is conditioned at that moment to revert plant busy tone, the access circuit remains undisturbed and is not busied out until it is first made idle by release.