Meinberg TCR170PEX - Manual (Page 15)

15
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 modu-
les, 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 TCR170PEX contains a jumper to select the impedance (50
Ω,
600
Ω or 5 kΩ) of the input for modulated codes (BNC) to comply with the
requirements of several systems.
Meinberg IRIG-generators have an output impedance of 50
Ω Ω Ω Ω Ω, to build a mat-
ched transmission system when using a coaxial cable. If such a generator is used to
synchronize TCR170PEX, the input impedance has to be set to 50
Ω accordingly
(default on delivery).
In addition to the telegram, the AFNOR-code defines the input/output impedances
also. If TCR170PEX is synchronized by this code, an input impedance of 600
Ω Ω Ω Ω Ω must
be set.
The setting „5 k
Ω“ may be necessary if the generator has a high output impedance
(see specifications of manufacturer). The driver software shows a bar chart for
evaluation of the signal level at the receiver input.
Photocoupler input
Pulse width modulated (DC Level Shift) codes are insulated by an onboard photo-
coupler. The connection scheme is shown below:
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.