Synchronised Digital Clock Systems

Synchronised digital clocks allow a number of individual time clock displays to show identical accurate time. Individual stand-alone digital clocks each drift at different rates eventually all display differing times. This article discusses the methods utilised to achieve synchronised digital clock time displays throughout an organisation.

Radio-Controlled Digital Clock

Many digital clocks are available with radio receivers that synchronise to radio time broadcasts that are freely available in many areas. Time and frequency broadcasts such as the DCF-77 broadcast from Germany, the MSF-60 broadcast from the UK and the WWVB broadcast from the US. These time and frequency broadcasts provide an accurate timing reference for digital clocks to synchronise. 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. Also, radio transmissions are local to the transmitter and do not extend beyond a finite range.

Wired Digital Clock Systems

Many synchronised digital clock systems operate on a bespoke serial network. They utilise a multi-drop RS422 or RS485 wired serial network. Each clock is generally mains powered at the point of installation. The digital 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 located single external timing receiver is required to synchronise all the clocks. The advantage of such systems is that the equipment costs are fairly low. However, this can often be outweighed by the installation costs of a dedicated network for the clock system.

Ethernet NTP Digital Clock Systems

Ethernet NTP digital wall clock time displays utilise 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 Server.

Ethernet NTP digital clock time displays are available with four-inch high numerals with four or six digits. The large time display makes the clocks easily visible from a distance of up to 150 feet. The large wall clock obtains time from an intranet or Internet based NTP server, to provide an accurate time display, synchronised to one-fifth of a second. Ethernet NTP digital wall clocks operate with any NTP or SNTP time resource such as a dedicated NTP Time Server. Additionally, the units can utilise Internet based time sources, such as NIST.

For simplicity of operation, synchronised digital clocks are Power-over-Ethernet (PoE) devices (IEEE 802.af) using the same network components as IP telephones. The large digital wall clocks plug into an Ethernet jack on a local area network. No mains socket is required; the device is powered from the network jack plug. Installation is therefore very straightforward, and it's easy to move units around.

A synchronised Ethernet NTP digital clock system has a number of advantages. Firstly, installation costs are reduced due to the use ‘common' network components, such as CAT5 and Ethernet hubs and switches. In many cases the network infrastructure may already be present. Additionally, no mains spur is required at the clock location, reducing installation costs. The clocks can also reference multiple time sources in the event of failure of a time reference.

The devices are powered over standard CAT5 cabling using Power-over-Ethernet (IEEE 802.af), the same Ethernet standard as Voice-over-IP telephones. If your network is not PoE enabled, single or multi-port mid-span converters (power injectors) are available at very reasonable cost.

Ethernet NTP Digital Clock Configuration

Networked digital clocks can easily be configured from any PC on the same network segment as the clocks. Configuration simply consists of opening a telnet session to a specified clock and entering password information. Once a connection is established there are a number of configuration commands available to the administrator. All digital clocks can therefore be configured from a single point.

Time Zone Configuration

NTP Ethernet digital clocks can be configured to the display local time in any time zone. There are a number of pre-configured common time zones that can easily be selected. Alternatively, any time zone and daylight saving configuration can be specified by supplying offsets and daylight saving start and end times.