and A-stages to the register. The register then checked the connection to the subscriber and sent dial tone. Normally, the whole process took about one-fifth of a second, less than the time required for the subscriber to lift the handset to his ear. The MCU, having completed its immediate tasks for this call, was free to deal with other demands. It retained a record of the calling equipment number against the identity of the register and notes the stage, which had been reached in the progress of the call. The subscriber dialled the required number and as each digit was received it was stored in electronic circuits within the register, which will call for the MCU after each digit and ask for instructions. Until sufficient digits have been received to determine the outgoing route from the exchange, the instruction will be "apply again after the next digit" and the MCU returned to serving other demands. When sufficient digits had been received, the MCU would have been able to determine the required path through the exchange, the routing digits to be sent (if an outgoing call is indicated), and which of the received digits have to be repeated forward. It would advise the register accordingly and then set the paths necessary to allow the register to signal forward and finally extend the caller to the required number or junction. On calls which terminated on the exchange a transmission bridge and supervisory circuit needed to be introduced within the switching network. This was done by employing a "bridge link" in the final connection. To allow metering on own-exchange calls these links also contain local call timing elements which pulse the P-wire in the desired "X" or "Y" phase at the appropriate times. "X" and "Y" phases were only required to support the metering for shared service subscribers, which has long since disappeared thankfully. Similar procedures would be followed for any other type of call. In every case the MCU would decide, in accordance with its program instructions, what connection pattern was appropriate in the circumstances indicated and issue orders for setting the paths. Within each MCU information was handled in a "two-out-of-five" code which enabled errors to be detected, and the output of the program store was duplicated to give additional protection. The TXE 3 model gave satisfactory service and the experience gained from the model confirmed the validity of the basic