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3. 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.
DI GI TI ZER PARALLEL DATA OUTPUT
4. My transponder does not have a D2 or D4 input. What do I do with these signals from the Digitizer?
Leave unused data bits unconnected.
5. What is the Strobe or Signal Common or Output Enable function on the ICAO altitude data port?
This is a control signal for the ICAO parallel altitude data. On devices manufactured by Trans-Cal
this function is always on pin 6 of the ICAO altitude port. A "high" or "open" on this pin will disable
the ICAO altitude data. A "low" or "ground" on this line will enable the altitude data. Some
interconnecting devices may use this signal to control the flow of data from the Digitizer. Be
aware that when using this signal and connecting multiple devices to the Digitizer, interruptions of
the ICAO data will occur when the controlling device "strobes" the Digitizer.
6. On Altitude Digitizers with serial ports, does the strobe function control the serial data?
No, the serial data is independent of the parallel ICAO altitude data. Transmission of the serial
data is asynchronous. Enabling or disabling the parallel data will not affect the serial data
transmission.
TYPI CAL BLOCKI NG
DI ODE 1 OF 10
D4 THRU C4
CI RCUI T 1 OF 10
D4 THRU C4
COLLECTOR
BASE
LOGI C
EMITTER
PIN 6
ENABLE
TYPI CAL TRANSPONDER PA
DATA I NPUT CI RCUI T 1 OF
D4 THRU C4
+5Vdc
4.7K
TYPI CAL I CAO PARALLEL D
I NPUT CI RCUI T 1 OF 10
D4 THRU C4
+9Vdc
10K
STROBE/OUTPUT
46
RALLEL
10
LOGI C
ATA
LOGI C
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