Why do altitude encoding errors occur when connecting a second or third device to the altitude encoder, but not when only one device is connected?
This is a symptom of “Cross-Talk.” This condition typically occurs when the devices connected to the altitude encoder are “pulling-up” to different voltages without diode isolation. When the altitude encoder is in the “off” state the data line electrical current may flow in undesired directions due to this pull-up voltage imbalance. Most modern avionics devices are diode isolated, but in applications where older equipment is mixed with new devices, blocking diodes may be required to isolate the older device. Germanium or Schottky blocking diodes are the preferred devices to install due to the low forward voltage drop across the device. Connect as detailed in the illustration below. Use of general purpose silicon diodes are NOT recommended, as the larger voltage drop may interfere with the logic threshold detection in the equipment.
