> 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?
If this link transmits digital signals, it is usual to define the logic levels, comparable with the TTL output levels of <0.4V for LOW and >2.8V for high, or the input levels of <0.8V for low and >2.4V for high. So all input currents below the specified amount of uA's is seen as o low-level. I expect that also for the high level a currentvalue will be specified. > > 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? It is preferable to specify a noise margin, as by TTL the difference between the input levels and the output levels of 0.4V. So if the input current is seen as low below 10uA, you can specify that a low output will deliver a current less than 5uA. In that case you will have a noise margin of 10-5uA = 5uA. Interference currents induced in the connection cable less than 5uA will not cause any harm. For the high level you need similar definitions. > > 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, BTW, sending only plain text e-mails will prevent a lot of work and illegible text and/or figures. Harry Croon -- Author: H.C. Croon INET: [EMAIL PROTECTED] Fat City Network Services -- 858-538-5051 http://www.fatcity.com San Diego, California -- Mailing list and web hosting services --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB CHIPDIR-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing).
