Meinberg GPS169PCI - Manual (Page 16)

16
Timecodes
The transmission of coded timing signals began to take on widespread importance
in the early 1950´s. Especially the US missile and space programs were the forces
behind the development of these time codes, which were used for the correlation of
data. The definition of time code formats was completely arbitrary and left to the
individual ideas of each design engineer. Hundreds of different time codes were
formed, some of which were standardized by the „Inter Range Instrumantation
Group“ (IRIG) in the early 60´s.
Except these „IRIG Time Codes“ other formats, like NASA36, XR3 or 2137, are
still in use. The board GPS169PCI however generates the IRIG-B, AFNOR NFS 87-
500 code as well as IEEE1344 code which is an IRIG-B123 coded extended by
information for time zone, leap second and date. If desired other formats are
available.
The timecode generator
The board GPS169PCI generates modulated and unmodulated timecodes. Modula-
ted signals are transmitting the information by varying the amplitude of a sinewave
carrier, unmodulated timecodes are transmitted by pulse duration modulation of a
DC-signal (TTL in case of GPS169PCI), see chapter „IRIG standard format“ for
details.
The sinewave carrier needed for modulated signals is generated in a digital way by
a programmable logic device on the board. The frequency of this signal is derived
from the main oscillator of GPS169PCI, which is disciplined by the GPS-system.
This leads to a sinewave carrier with high accuracy. Transmission of date is synchro-
nized by the PPS (pulse per second) derived from the GPS-system.
The modulated timecode has an amplitude of 3V
pp
(MARK) and 1V
pp
(SPACE)
into 50
Ω. The number of MARK-amplitudes within ten periods of the carrier defines
the coding:
a) binary „0“ : 2 MARK-amplitudes, 8 SPACE-amplitudes
b) binary „1“ : 5 MARK-amplitudes, 5 SPACE-amplitudes
c) position-identifier : 8 MARK-amplitudes, 2 SPACE-amplitudes
The DC-signal has the following pulse durations accordingly:
a) binary „0“ : 2 msec
b) binary „1“ : 5 msec
c) position-identifier : 8 msec