Hello members,
I'm trying to understand a specification,
but it is not completely clear to me. Maybe someone can help me out. I have a
current loop-a-like interface with optocouplers. This interfaces are not true
20mA current loop interface as familiar in the industry, but rather a current
sensitive serial link.
It specifies a maximum input current while in
"inactive" state of some uAmperes. Is this figure based on the leak current of
the used PNP transistor or is it based on other parameters?
For the transmitter on our side we use an
optocoupler. Opto's do have a dark current of a few nanoamps. Do I specify
such a small inactive current for our transmitter? Or do I miss something?
Below a rather simple drawing of the receiver (distance is maximum a few meters). Transmitter on our side is a optocoupler transistor output. Pull-up resistor on the other side (receiver) with an CMOS gate (series resistor before the input).
Vcc
_|_
| |
|__|
|
____|/
|\ PNP
| TX RX
'--+---------------------0-------------------------------0------, Imax = 20mA active, Imax = ??uA inactive
| |
_|_ |
| | _|_
|__| | | 270 ohm
_|_ |__|
|
_|_
_\_/_ -> Opto diode
|
_|_ Imax = 20mA active, Imax = ??uA inactive
GND GND
,---------0---------------------------------0----,
_|_ _|_
Kind regards,
Elbert Smit.
