a prompt alarm. In periods of very low traffic, there would typically be fewer than eight call set-up attempts on the exchange in eight minutes and this would have prevented the above security system from working. The exchange was therefore provided with an automatic Test Call Unit, which originated a test call every 30 seconds. As well as thus enabling Call Control to detect eight failures in less than eight minutes (if all call-attempts were failing), the Test Call would raise its own prompt alarm if it detected 31 sequential call-attempt failures, indicating that neither security side of the exchange was able to connect calls. As a further security measure, if the first attempt to set up a path to a register failed, so that, on an outgoing call, the customer did not get dial-tone, the exchange recognised the failure, stored the details of the equipment in use on the failed call and automatically made a second attempt, using different equipment. This happened so quickly (about 50 milliseconds) that, if the second attempt was successful, the customer would not have been aware of the failed first attempt to get dial-tone. Unlike the preceding rural Strowger exchanges (UAX 13s and smaller) the TXE2s were equipped with an uninterruptible power supply with auto-starting diesel generators. As a maintenance aid, the exchange was equipped with a Maintenance Data Recorder (MDR). This had a rather primitive printer, which displayed the identities of equipment in use at the time that the exchange had detected a call failure. For example, in the event of a successful repeat attempt to provide dial-tone, the MDR would print. If the repeat attempt failed, then the MDR would print twice in quick succession, giving details of the equipment in use on both the failed paths. The prints were not easy to read. All that emerged were short burn marks on the special paper in up to 45 different places in each of two rows. It was necessary to hold a plastic graticule (see picture below right, under that of an MDR) over the paper to find out what the presence of each burn mark indicated. If more than eight call failures were detected in less than 8 minutes, then the critical common control units would be forced to change from the side in service (A or B) to the other side, the automatic 8-minute changeover would be suspended and a