Meinberg TCR51USB - Manual (Page 12)

12
Input impedance
The IRIG-specification doesn’t define values for the output impedance of generators or
the input impedance of receivers. This fact led to incompatibility of some modules,
because the manufacturers could choose the impedances freely. For example: if the
output impedance of the generator is high and the input impedance of the receiver low,
the signal level at the receiver input might be too low for correct decoding. Therefore the
board TCR51USB contains a impedance of 50
Ω.
Photocoupler input
Pulse width modulated (DC Level Shift) codes are insulated by an onboard photocoup-
ler.
The internal series resistance allows direct connection of input signals with a maximum
high level of +12 V (TTL or RS-422 for example). If signals with a higher amplitude are
used, an additional external series resistance must be applied for not exceeding the limit
of the forward current of the input diode (50 mA). The forward current should not be
limited to a value of less than 10 mA to ensure save switching of the photocoupler.
Configuration of TCR51USB
The selection of the IRIG code and a possible offset of the received IRIG time to UTC
must be set up by the monitor software via the USB. In contrast to AFNOR NFS 87-500
the IRIG telegram containes only the day of year (1...366) instead of a complete date. To
ensure correct function of TCR51USB, the date stored in the realtime clock of the board
must be set when using IRIG codes therefore. This setting can be done by a terminal
software also.
If the time zone of the received IRIG code is not UTC, the local offset to UTC must
be configured to ensure correct function of the driver software. If the local time
zone is MEZ for example, the board must be set to a local offset of ‘+60min’ (MEZ
= UTC + 1 h).