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AN-550 Datasheet(PDF) 2 Page - Holt Integrated Circuits |
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AN-550 Datasheet(HTML) 2 Page - Holt Integrated Circuits |
2 / 11 page HOLT INTEGRATED CIRCUITS 2 AN-550 applications are transformer-coupled. Comprehensive information on direct- and transformer-coupled terminals is found in U. S. Dept. of Defense MIL-HDBK-1553A. For clarity, Figure 1 shows just a single MIL-STD-1553 bus. Terminal isolation transformers and bus interconnect circuitry are repeated for the redundant bus. Figure 1 shows separate blocks for "Protocol Logic" and "Bus Transceiver" within the terminals. Sometimes the bus transceiver is contained within the protocol device; the Holt HI-6110 integrated message processor is an example. as part of the terminal hardware. The turns ratio for this transformer often differs for direct- and transformer-coupled operation, as well as for different MIL- STD-1553 transceiver part numbers and available power supply voltage. Dual-ratio isolation transformers are available from Holt to support either direct- or transformer- coupled applications. Hardware designs are often configurable within the terminal for either coupling type. transformer- coupled (long stub) terminals use an additional "coupling transformer" and a pair of series "isolation resistors" . In this case, the coupling transformer and resistors are generally inside an off-the- shelf "bus coupler" assembly. In any digital circuit, resistive and inductive conductor impedance causes voltage drops and resulting noise when high currents are conveyed from point to point on the board. Digital circuits clocked at high speed “gulp” current from the power supply at the clock frequency. Fast rise/fall times and distributed inductance contribute to common mode (L di/dt) noise on power supply wiring. Usual measures for noise control involve use of dedicated circuit board ground and power planes, and distributed use of decoupling capacitors to provide local, low impedance “charge reservoirs” to satisfy each integrated circuit's brief, increased demand for power supply current when clock edges occur. Similarly, MIL-STD-1553 bus transceivers require local decoupling capacitors to satisfy current demand when transmitting. Transceivers work best when planar conductors are used for both power and ground. A monolithic ceramic capacitor (100nF) should be connected from each transceiver VDD pin to ground, using short, direct connections. Any terminal that transmits also requires a low ESR bulk storage capacitor for each transceiver VDD pin. A 22uF tantalum chip capacitor should be sufficient. If the application guarantees that concurrent transmission will not occur on both buses (a safe assumption in most MIL-STD-1553 designs) a single tantalum capacitor can serve the needs of both transmitters on a dual transceiver IC by bridging the device's VDD power pins together where the tantalum capacitor is connected. Each IC VDD pin should retain its own ceramic capacitor nearby. As a precautionary measure to avoid crosstalk, avoid routing digital signals parallel to the MIL-STD-1553 bus analog board conductors, or on other layers in close proximity to the analog bus conductors. Crosstalk can couple digital noise onto the 1553 receiver circuit, reducing noise rejection performance. The bus isolation transformer (and isolation resistors, if a direct-coupled terminal) should be placed close to the board's bus interface boundary, where the bus enters the circuit board. In addition, the transceiver should be located as physically close as possible to the isolation transformer. Close physical proximity reduces crosstalk and minimizes conductor voltage drops when transmitting, caused by wiring inductance. Because transceiver transmit currents can exceed 500mA, it is recommended to use minimum conductor trace widths of 0.04” (2.5mm) and use short, direct paths for transformer signal and center tap conductors. When transmitting, the transformer center tap (on the transceiver IC side) conducts the large ground return currents, not the transceiver IC’s ground pin. Thus the ground conductor width for the center tap should at least match the analog drive conductor width, but ground plane can also be used for returning center tap currents to transceiver ground pin and power supply ground. Bus side conductors from the transformer usually connect to 78 ohm twisted pair shielded cable. The decision of whether to isolate or make connections between the bus- side winding center tap, chassis ground and bus cable shielding should be made on a system basis after careful consideration of ESD, lightning and EMI concerns. In most cases, the bus side center tap is not connected for MIL- STD-1553 terminal applications. While ground and power planes are recommended for the terminal's digital circuitry, it is important to avoid ground and power planes underneath the analog bus signal traces and under the bus isolation transformer. This is essential for meeting the MIL-STD-1553B requirement for input impedance. A ground or power plane under analog bus circuitry reduces input impedance at bus signaling frequencies. MIL-STD-1553B requires 1000 ohm minimum input impedance for transformer-coupled terminals, and 2000 ohm minimum input impedance for direct-coupled Both configurations require a n " isolation transformer" Direct-coupled (short stub) terminals use a pair of isolation resistors within the terminal itself. In contrast, at the physical connection where the terminal's stub cable meets the MIL-STD-1553 bus cabling Decoupling Capacitors Circuit Board Layout Ground and Power Planes |
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