prompt alarm would be sent out. In TXE2s, a call which terminated within the same exchange went through seven switching stages, whereas a call going out to another exchange went through just three switching stages. The switches were designated as A, B, C and D (the paths were A-B-C for outgoing, A-B-C-D-C-B-A for internal and D-C-B-A for incoming). The common control equipment consisted of B- and C-switch selectors, supervisory selectors (a supervisory relay set stayed in-circuit throughout each call), Register Selectors, Registers and Call Control. The most characteristic feature of the exchange's central control unit design was that calls were processed serially. Consequently, call set-up had to be fast. In particular, Call Control had to become free in less than the time of the inter-digital pause on calls incoming to the exchange. This time could be as low as 60 milliseconds. As the TXE2 call-set-up time was some 50 milliseconds, this design requirement was just met, but even so, the overall capacity of the system was determined by the probability of an incoming call being delayed too long in its initial connection to a register. The grade of service in a TXE2 was dependent on the number of customers in an A-switch group, with access to just 25 A-B trunks. The normal standard on the earlier exchanges was 125 customers per A-switch group. If the A-switch group contained many busy PBX lines, then the number of customers might be reduced to 75. The earlier (Mark I and Mark II – the differences being slight) exchanges could handle up to 2,000 customers. Later on, Mark III TXE2s were able to handle up to 4,000 customers and on these exchanges, where the average calling rate was sufficiently low, up to 250 customers would be in an A-switch group, still with access to just 25 A-B trunks. The choice of the main type of memory used in the TXE2 (and the TXE4) was particularly characteristic of the general design philosophy, that the components used had to be of a technology that had been tested over many years. The choice thus went to the Dimond ring type of memory, named after T. L. Dimond of Bell Laboratories, who invented in 1945. They were large-diameter magnetic ferrite toroidal rings with solenoid windings, through which are threaded writing and reading wires. These racks gave the ability to convert a subscriber's directory number into an equipment location