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Most time displays, such as wall clocks, are independent devices whose time gradually drifts away from each other at different rates after being set. This can be a real headache for organisations that require a reliable time display. For instance, shifts may be starting, and knocking off at slightly different times. Also, time displays may not be synchronised to clocking machines with inaccurate early or late penalties being applied to personnel records. Synchronised clocks solve all these problems by allowing any number of individual time displays to show identical accurate time. This article describes the methods that can be utilised to achieve synchronised time displays throughout an organisation. A common solution to the synchronised clock problem is to provide small, integrated radio receivers that can synchronise to radio time and frequency broadcasts which are freely available in many countries. A number of broadcasts are available, generally provided by Government scientific establishments. The DCF-77 time signal, broadcast from Germany, the MSF-60 signal broadcast from the UK and the WWVB signal broadcast from the US are all examples of radio time transmissions. These broadcasts provide a very accurate timing reference that can be used by clocks to provide synchronisation. However, often installation problems are encountered due to the fact that in particular locations the radio signal quality may be very low or even non-existent. Mounting radio-controlled digital clocks in basements or inside metal structures or too close to electrically noisy equipment can all cause radio reception difficulties. Radio transmissions also have a finite range; therefore many areas do not have access to such transmissions. An alternative solution is to have a synchronised clock systems operating on a bespoke serial network. Often, these devices utilise a multi-drop RS422 serial network. Each clock is generally mains powered at the point of installation. The clocks synchronise to a master clock on the wired network that may be synchronised to a GPS or radio external time reference. Only an optimally positioned single external timing receiver is then required to synchronise all the clocks on a network. The advantage of such systems is that the equipment costs are fairly low. However, this can quite often be outweighed by the installation costs of a dedicated bespoke network and mains spurs. A third alternative is an Ethernet NTP synchronised clock system. Time displays utilise the Network Time Protocol (NTP) over Ethernet to display a continuously accurate synchronised time. The digital clocks connect directly to an Ethernet network and periodically synchronise using SNTP to a NTP time reference or server. Costs can be reduced by utilising an Internet based NTP server for synchronisation, such as NIST. An additional benefit of is the possibility of utilising Power-over-Ethernet (PoE, IEEE 802.af). This means of powering clocks uses the same network components as IP telephones. No mains socket or spur is required; the clock is powered from the network jack. This greatly simplifies installation and makes it much easier to relocate clocks on the network. A synchronised Ethernet NTP digital clock system has a number of advantages. Firstly, installation costs are reduced due to the use of common, low-cost network components, such as CAT5 and Ethernet hubs and switches. In many cases the network infrastructure may already be present or require little expansion. Additionally, no mains socket or spur is required at the clock location, vastly reducing installation and cabling costs. Often in the event of failure, clocks can reference multiple time sources providing inherent redundancy. Networked digital wall clocks can easily be configured from any PC on the same network segment. Generally, configuration simply requires opening a telnet session to a specified clock and entering password information and a series of configuration commands. All digital clocks on the network can therefore be simply and straightforwardly configured from a single point.
Article Source: http://blogticles.com
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