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