How many devices may be driven off of the parallel ICAO Altitude data port?
The number of devices that may be connected to the Digitizer ICAO altitude output is a function of the current and power required. The Digitizer parallel data outputs are “uncommitted” collectors of a transistor array which are “pulled-up” through a resistive load by the transponder (or other device) to some positive voltage. This voltage may range from about +3 to +40Vdc. Each Digitizer data output line (i.e.D2, D4, A1, A2 etc.) is capable of providing 35 mA (0.035 Amperes) with a “not to exceed” power rating of 100mW (0.1 Watts), when it is “sinking” current in the “on” position. Typical modern applications require about 1 milliampere or less per data line, per device. In the circuit illustrated below, the current is calculated as 0.9mA at 8.1mW. At this current and power rating, a total of 12 identical devices could be connected to the digitizer. Given the wide variety of input circuits capable of interfacing with the Digitizer and the possibility of cross-talk, careful planning of
