What is the Strobe or Signal Common, Pin 6?

The strobe function turns the encoder data on and off. If this pin is grounded the data is "on" or enabled. If pin 6 is open or high then the data is disabled.

Any transponder with a strobe or encoder enable connection (i.e. NARCO) should be connected to the encoder signal common pin 6. If it is not controlled by the transponder it must be grounded to enable the parallel altitude data.

Note: The Narco AT-5A, AT-6A, AT-50 and AT-50A require a modification to the SSD120-XXA Altitude Digitizer. The decoupling capacitor network must be removed to enable proper operation. See install manuals 881000 or 930005 for the modification requirements.

On altitude encoders with serial ports, does the strobe function affect the serial data?

No. The serial data is independent of the parallel data. Enabling or disabling the parallel data will not affect the serial data.

Must the parallel data be connected to use the serial data?

No. The serial data is independent of the parallel data. However, power to the unit must be supplied through the parallel data connector.

How many devices may be driven off of the parallel altitude data port?

The number of devices which may be connected is a function of the current required and any possible cross talk sources. The altitude encoder parallel outputs are "uncommitted" collectors of a transistor array which are pulled-up through a resistive load by the transponder to some voltage about 3 to 50VDC. Each data output (i.e. A1, A2, A4 etc.) is capable of providing 35 milliamperes when it is "sinking" current in the "on" condition. Typical modern applications call for 1 milliampere or less per data line per device.

Cross talk may occur when two devices are connected in parallel to the encoder at the same time, and are pulling up to a slightly different voltage level causing current to flow in an incorrect direction. This condition will result in a miscode on one or both receiving devices. Diode isolation is required to correct this condition. Most modern devices are diode isolated and should be immune from this condition.

How many devices may be driven off of the RS232 port?

The RS232 specification was written in the 1960's and specifies one receiver per transmitter. However, Trans-Cal altitude encoders use modern drivers and under the worst case loading conditions of 3kS and 2500pf, the output of the altitude encoder serial port is guaranteed to be ±5V, which is consistent with the RS-232 standard specifications. Typically, two or three devices may be connected as long as the impedance remains above 3kS.

Can I transmit two different data protocols at the same time?

Yes. The digitizer must be configured via the serial port and an IBM compatible PC to set the protocols for each port. Reference manual 930005 or 930006 or 930007 for a complete description of the configuration procedure.

What is the maximum length of an RS232C wiring harness?

           25 feet.

What is the difference between RS232 and RS485?

RS232 operates in an unbalanced (single-ended) transmission method, where the receiving device monitors the difference between the signal voltage and a common ground.

RS485 is a balanced (differential) transmission method, where each signal has a dedicated pair of wires, with the voltage on one wire equal to the complement of the voltage on the other. The receiving device monitors the difference between these voltages to determine the signal.

In general, RS232 is used for slower data rates and covers shorter distances. RS485 is used for faster data rates, longer distances and has a higher immunity to noise.

How many devices may be driven off of the RS485 port?

32.

What is the maximum length of an RS485 wiring harness?

4000 feet.

How can the serial data ports be tested?

1) A PC with serial data capture software may be used to view the serial data stream.

Recommended serial data capture software: HYPER TERMINAL(Windows®95 & 98), SOFTWARE WEDGE™, PROCOMM™, or VERSATERM™.

2) A dedicated serial data test box such as the BLACK     BOX™ RS232 MONITOR.

3) Use an oscilloscope to view the 9VDC square wave transmitted about once a second.

4) To test the RS485 port consult Trans-Cal.

Can the pilots altimeter be exactly matched to the Altitude Digitizer?

Yes. Any Trans-Cal altitude digitizer with a serial port may be matched via an IBM compatible PC at 1000 foot intervals throughout its operating range. This allows for greater accuracy when matching to the pilots altimeter. Reference manual 930005 or 930006 or 930007 for a complete description of the calibration procedure.

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