Slave clock

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In telecommunication, a slave clock is a clock that is coordinated with a master clock. Slave clock coordination is usually achieved by phase-locking the slave clock signal to a signal received from the master clock. To adjust for the transit time of the signal from the master clock to the slave clock, the phase of the slave clock may be adjusted with respect to the signal from the master clock so that both clocks are in phase. Thus, the time markers of both clocks, at the output of the clocks, occur simultaneously.[1]

In other areas, the term refers to satellite electrical clocks that operate remotely from an electrical pulse issued by a master clock. In the 19th and early 20th centuries, slave clocks were widely used throughout public buildings and business offices, and their remote operation was regulated by electrical signals sent by a centralized master clock.

These older styles of slave clocks either keep time by themselves, and are corrected by the master clock, or require impulses from the master clock to advance. Many slave clocks of these types remain in operation, most commonly in schools.


IBM/Simplex Synchronous Correction cycle

Synchronous Systems

This type system was developed by IBM in the late 1940s. IBM sold their time division to the Simplex Time Recorder Company in 1958. The methods & clock mechanisms have remained basically unchanged, other than appearance & style over the years. Today, this same protocol is used by several manufacturers, including Latham, Dukane, Cincinnati & Standard Electric Time. Digital master clocks have replaced older mechanical types.

All consist of a master clock & a number of secondary clocks in remote locations. These secondary clocks appear to run like an ordinary plug-in electric clock & run on their own synchronous electric motor and will maintain correct time unless there is an interruption in electric service, depending on the master clock only for the periodic synchronizing impulse. These clocks differ from the ‘impulse’ type, which are electrically advanced each minute by the master clock. Impulse type clocks are usually called ‘slave’ clocks since they cannot function at all without the master clock.

There are two basic variations in synchronous systems, ’wired’ and ‘electronic’.

In a wired system, the secondary clocks are ‘hard-wired’ to the master clock with their own network of wiring. This dedicated wiring consists of 3 wires & requires that all secondary clocks be connected to this network in order to receive corrections from the master clock. These systems are usually 110 volts but some are 24 volts. The master clock energizes the third wire to apply voltage to the correction solenoids (called ‘clutch magnets’ by the manufacturer) in the secondary clocks.

In an electronic’ system, the correction impulse is applied to the normal building wiring, in the form of a high-frequency audio signal (typically 3510 Hz) applied to the normal 60 Hz frequency of the electrical system. Special receivers in the secondary clocks receive this impulse & apply voltage to the correction solenoid. In this type system, secondary clocks need no special wiring & may be plugged into any 110 volt outlet in the building.

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