3 times timeX
time3are simple "industrial" PCs running Linux. They are mounted on a common DIN rail in a 19" rack and connected to the same power supply (right) and network switch.
These small PC boxes each have an AMD LX800 500 MHz processor, 256 MB DDR RAM, 4 GB flash disk, and a 100 MBit NIC. Initially installed software was Linux 2.6.26 with NTP 4.2.4.
Because there are no moving parts in these PCs, neither hard disks nor cooling fans, we hope for decent reliability and a long service life.
Even though the PCs have PS/2 keybord and VGA display connectors, they are managed completely remote with telnet and ftp.
These boxes are 10, 15, or even 20 times as fast as our old NTP hosts, hence NTP service quality is much better than before. It now depends almost entirely on network quality.
As pictured above, we started without any radio clocks, which would be plugged into the RS232 or USB ports. At that time (2012) our Internet connection was congested frequently what ruined NTP service quality. That's why we dug out the old radio clock receivers again and even bought a new GPS receiver.
Now each of the three servers has its own fallback time source. These cheap receivers give not nearly the NTP service quality as a well working network, but if the network is congested the receivers give still better quality.
This modern (2012) GPS device is made by Gude Analog- und Digitalsysteme GmbH as well as the two old DCF77 devices from 1992 (below).
Nowadays a GPS device is cheap but yet is potentially a very accurate time source. Unfortunately, this accuracy doesn't make it to the NTP hosts, probably due to processing latencies and transmitting huge NMEA records. In practice it's not more accurate than the DCF77 devices but is much more delicate as to radio signal.
Probably the IGEL:clock is as well made by Gude, as is the Expert mouseCLOCK, both in 1992. It's still made today and of course still a good fallback time source.
All Gude "clocks", GPS or DCF77, new or old, are supported by the NTP software. The Reference Clock Support page lists all available drivers. The GPSclock is treated as generic NMEA device (type 20) while the new USB mouseCLOCK has its own mode (19) in the generic parse driver (type 8) as has the IGEL:clock with adapter (mode 8). The old RS232 mouseCLOCK can be seen as a Conrad DCF77 receiver (mode 5) or as a generic "raw" DCF77 receiver with the DTR line at high voltage and the RTS line at low voltage for power supply (mode 14).