Real-Time D/A Sequencing Abstract An adaptive Radar system was developed requiring the generation of bursts of Radar pulse waveforms. These waveforms are generated in real-time to allow the Radar system to adapt to changes. A VXIbus system networked to a mainframe computer is used to provide the throughput required to download new waveforms and update the signal generator in real-time. After the baseband Radar pulse pattern has been generated it is up-converted to a Radar frequency and signal-conditioned for use with a Radar system. This example illustrates throughput breakthroughs in real-time signal generation that can be useful in many open and closed loop applications. Introduction: Utilizing VXIbus to Attain Real-Time Performance The VXIbus specification states that one objective for VXIbus is "to provide higher system throughput." But several years after the original issue of the specifications there were still no products available that would allow a system integrator to put a high-speed waveform application together that could operate in real-time. The Racal Instruments Model 6062 D/A Converter (DAC) breaks this trend by generating signals at a rate of 500k points per second using a memory buffer that may be freely modified while the 6062 is generating waveforms. Unlike other cards that require the control program to modify the DAC directly, the 64k-point (standard) to 576k-point (extended) memory buffer of the 6062 provides true real-time performance. The technology which allows this performance involves "shared" access into a memory buffer that may be read from and written to simultaneously. VXIbus interrupts synchronize this process by indicating that a section of buffer is available for memory update. This insures that buffer sections currently being transferred to the DAC are not disturbed. Hardware Setup The hardware setup includes a mainframe computer (to analyze and calculate the updated Radar waveforms on-the-fly), an Ethernet link (to transmit the data to the VXIbus) and a VXIbus system. The VXIbus system consists of a mainframe, an embedded Slot 0 controller with Ethernet, the 6062B-4 four channel D/A converter module, an up-converter and Radar subsystem. The high throughput required to download these real-time Radar waveforms is achieved via the Ethernet link to the embedded controller and the VXIbus backplane transfers into shared memory. The Model 6062 Waveform Generator is updated on-the-fly allowing the generation of Radar pulses with a bandwidth of 200kHz. Next, the up-converter outputs the waveform to the Radar subsystem. Waveform Memory Configuration The real-time stream of Radar pulses requires approximately 52,500 waveform memory points. This number may be varied on the fly if the number of points changes. Software Setup The software required to operate the high speed transfer can be accomplished as a C' program written under the LabWindows/CVI environment. The Model 6062 is set to interrupt the system every time the memory buffer halfway point is reached (at 32k) and every time the pointer resets back to the first point in the buffer. An interrupt handler is then activated loading either the first 32k-points of memory or the last approximately 20.5k-points. The number of points in the waveform may be modified concurrently with a refresh of the second half of the buffer (see Figure 2). The interrupt handler uses the Model 6062's shared memory to transfer the real-time data from the embedded controller (see code fragments in Figure 3). At a transfer rate of over five Mb/s, it takes less than 13ms to update half of a single analog channel's memory (a 6062 can have up to 8 channels). Real-time is possible since the recycling period with the 6062 running at 500k-points/s is ~105ms. In general, the buffer can be replaced at up to five times the maximum sample rate. Summary This example demonstrates that the VXIbus Consortium's goal of providing "higher system throughput" is one step closer to being achieved. For the first time, real-time waveform updating has been demonstrated using the Racal Instruments' Model 6062. Figure 3. Code Fragment for Real-Time Trace Updates TOC 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13