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.