Avionics News January 2012 - 69

Inputs and outputs were set up by naming the associated parameters, specifying the bit alignment within the incoming or outgoing word and specifying the bit resolution (least-significant bit weight) by entering a double-precision floating-point scale factor. Connections between inputs and outputs were defiined by selecting from a set of operators called connectors. The parameter connector simply transfers an input to an output. If unit conversion is required, a linear transformation is possible by entering a scale factor and offset, both being double-precision floating point values. For this application, the linear conversions included feet to nautical miles, semi-circles to degrees and feet/second to feet/minute. Data from multiple input words are combined into single parameters using a double-parameter connector, which combines two inputs into a single 32-bit output value and a 32-bit remainder, using double-precision floating-point arithmetic. This same connector is used to convert seconds to hours:minutes:seconds as follows: a first double-parameter connector converts seconds to hours with a fractional hours remainder, a second double-parameter connector then converts the hours remainder to minutes with a fractional minutes remainder, and a final Parameter Connector converts the minutes remainder into the final seconds parameter. These three output parameters are then formatted for the final output of hours:minutes:seconds. Input discrete bits are processed by connectors to drive output discrete bits, to control operations, or to drive ARINC 429 sign/status matrix bits. A discrete to SSM connector reads data validity bits from the MIL-STD-1553B data and drives the SSM status of “normal operation” or “failure warning,” while taking into account the type of ARINC 429 label being transmitted, as the two-bit code for normal operation/failure warning differs among binary, BCD and discrete labels. Since the desired output rate of ARINC transmissions is often desired to be regular (20 Hz) and may differ from the input rate of the associated input parameter (25 Hz), the Lynx Converter operates in the following fashion. All inputs are immediately stored when received. If a new value is received, it over-writes the old value. ARINC outputs are transmitted per a defined schedule. When a label is due to be transmitted, the current (newest) value in memory is accessed, scaled, formatted and output. This effectively decouples the output rate from the input rate, and the system designer need only determine that the combination of input/output rates meet the required system refresh rate. When the ARINC output labels are configured, an option is presented to define a time-out value, such that if the 1553B data destined for an ARINC output label is not received for more than a specified interval, then the label transmission will be terminated until fresh data is received. In this example, the compass heading (Label 320) and radar altitude (Label 164) parameters are not available on the 1553B bus from a fixed remote terminal address, and are acquired as follows: • The required input rate of 25 Hz must be acquired from a

triplet of equally spaced 1553B messages received at 8.33 Hz. The triplet of messages switches to another triplet when the aircraft changes operating mode. To accommodate these input characteristics, the Lynx Converter is configured as follows: Connect each 1553B sub-address to transmit on the appropriate ARINC label. • Use a comparator connector in conjunction with timestamps to determine the most current input value. • Use the output of the comparator connector to control the conditional transmission of the ARINC 429 labels, so that only the most current value will be transmitted. •

TESTING

In some cases it is desirable to perform a separate integration test between the Lynx Converter and a COTS avionics unit prior to testing both items in the target end system. To achieve this integration test without benefit of the 1553B bus traffic present on the target system, a bench-test configuration of the Lynx Converter unit can be generated that exploits the ability of the Lynx Converter unit to act simultaneously as a bus monitor, remote terminal and bus controller. This capability allows the Lynx Converter unit, acting as both a bus controller and a remote terminal, to simulate a portion of the target system 1553B databus traffic, and then simultaneously act as the bus monitor to collect data from that simulated traffic and drive the ARINC output data to the COTS avionics unit. Another function of the LynxCom-Pro personal computer application that can be used during integration testing is the LynxMonitor function, which allows the user to set and/or observe values that the Lynx Converter unit is inputting and outputting. With the LynxMonitor function, an integration team can set various values and verify that they are received properly by the COTS avionics and the interpretation of the scaling and resolution is correct. This capability has allowed the quick resolution of interface issues among connected systems using the Lynx Converter unit.

CONCLUSION

The successful integration of avionics into aircraft using the PCU-100 Lynx Protocol Converter unit demonstrates that a COTS approach to MIL-STD-1553B to ARINC 429 data conversion is feasible in a military aircraft avionics architecture. The powerful feature set built into the Lynx Protocol Converter unit accommodates a wide variety of real-world situations and allows the integration of COTS avionics without time-consuming software changes and reverification efforts. The Lynx Protocol Converter unit is designed as a generic avionics device that meets aircraft standards (DO-160D and DO-178B), yet is quickly configurable for a wide variety of applications, using the LynxCom-Pro software application on a personal computer. q
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january

2012

Avionics News January 2012

Table of Contents for the Digital Edition of Avionics News January 2012