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The '''Master Test Frame (MTF)''' is part of the maintenance center in a No. 5 Crossbar switch. It consists of bays of equipment that are used to test the various parts of its associated switching machine. Generally, there is one MTF per marker group, so in an office with several marker groups, there would be several MTFs.

In a No. 5 central office, practically all the controls for maintenance equipment are concentrated in one maintenance center called the master test frame. The equipment includes primarily '''a''' [[Trouble Recorder|trouble recorder]], a master test circuit, and a jack bay. In addition, there is available on an optional basis an automatic monitor circuit for testing pulsing equipment. Here also are means for extending the alarms to a distant office during un-attended periods.

The master test circuit provides for simulating service calls under controlled conditions. This aids the maintenance personnel in the final diagnosis of an indicated trouble condition, and also permits insurance tests to be made of those parts of the central office equipment that do not have access to the trouble recorder. It also permits tests to be made of subscribers' line and outgoing trunk conductors, and associated incoming trunk circuits in distant offices. This includes a rapid test for continuity and polarity of out-going trunks.

The automatic monitor checks on a sampling basis the performances of the pulse-receiving equipment of all register circuits, and the pulse-sending equipment of all senders, and causes the trouble recorder to make records of irregularities disclosed. The alarm extension facilities can keep an attend.ant at a remote central point informed of the occurrence of each trouble, and indicate to him its classification as to urgency of corrective action.