AN/B-27: ELECTRICAL AND LAYOUT CONSIDERATIONS FOR 1553 TERMINAL...

ELECTRICAL AND LAYOUTCONSIDERATIONS

FOR 1553 TERMINAL DESIGN

AN/B-27

© 1999 Data Device Corporation

INTRODUCTION

MIL-STD-1553B defines a terminal as"The electronic module necessary tointerface the data bus with the subsystemand the subsystem to the data bus…" Bydefinition, the terminal includes the isola-tion transformer as well as the analogtransmitter/receiver and the digital proto-col section.

Design of the terminal, therefore,includes selection and interconnection ofthe isolation transformer. In order toensure proper terminal operation in com-pliance with the 1553 standard, there area number of issues that need to be con-sidered in this area.

Since most current terminal designs useintegrated components such as a SmallTerminal Interface Circuit (STIC) orAdvanced Communications Engine(ACE), this application note will concen-trate on the electrical and layout issuesbetween the hybrid transceiver pins, theisolation transformers, and the systemconnector. Other matters discussedinvolve power distribution, layout strategy,

grounding, decoupling capacitors, andtransceiver-related issues.

ISOLATION TRANSFORMERS

Figure 1 illustrates the interface betweenthe various versions of the ACE and STICseries hybrids to a 1553 data bus.Connections for both direct (short stub)and transformer (long stub) coupling, aswell as the peak-to-peak voltage levelsthat appear at various points (whentransmitting) are indicated in the figure.

Table 1 lists the characteristics of therequired isolation transformers for thevarious ACE hybrids and lists the DDCand Beta Transformer TechnologyCorporation (BTTC) corresponding partnumbers, as well as the MIL (DESC)drawing number (if applicable). BTTC is adirect subsidiary of Data DeviceCorporation.

For both coupling configurations, thetransformer that interfaces directly to the

ACE component is called the isolationtransformer. As stated above, this isdefined to be part of the terminal. For thetransformer (long stub) coupling configu-ration, the transformer that interfaces thestub to the bus is the coupling trans-former.

The turns ratio of the isolation trans-former varies, depending upon the peak-to-peak output voltage of the specificACE or STIC terminal. MIL-STD-1553Bspecifies that the turns ratio of the cou-pling transformer be 1.0 to 1.4.

The transmitter voltage of each model ofthe BU-65170/61580 or BUS-65153varies directly as a function of the powersupply voltage. The turns ratios of therespective transformers will yield a sec-ondary voltage of approximately 28V p-pon the outer taps (used for direct cou-pling) and 20V p-p on the inner taps(used for stub coupling).

It should be noted that for the 15V or 12VACE hybrids (BU-65170/61580X1[2]) or

ELECTRICAL AND LAYOUT CONSIDERATIONS FOR 1553 TERMINAL DESIGN

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AN/B-2704/04-0

STIC (BUS-65153[/54]), the isolationtransformer has a step-down turns ratioin going from the ACE (STIC) to the stub.For the 5V ACE hybrids (BU-65170/61580X3), the isolation trans-former has a step-up turns ratio in goingfrom the ACE to the stub.

ISOLATION RESISTORS

For both coupling configurations, an iso-lation resistor is required to be in serieswith each leg connecting to the 1553 bus.This protects the bus against short circuitconditions in the transformers, stubs orterminal components.

For the direct coupled configuration, notethat there is a 55 ohm isolation resistor inseries with each transformer leg on thestub side. MIL-STD-1553B requires theisolation resistors to protect the bus froma short circuit condition in the stub path,

isolation transformer, or transceiver. Thisallows the 1553 bus to continue operatingin the event of a short circuit in the termi-nal.

As stated in MIL-STD-1553B Notice 2,only the Navy permits the use of directcoupling. Both the Army and the AirForce permit only stub coupling to beused. For almost all system applications,transformer (stub) coupling is preferredover direct coupling.

ADVANTAGES OF TRANSFORMER COUPLING

Some of the advantages of transformercoupling are:

(1) Looking from the 1553 bus towardsthe stub, the effect of the 1.4 to 1.0stepdown ratio of the coupling trans-

former will be to double the imped-ance of the stub/terminal combina-tion, as seen by the bus. Since thestub impedance decreases as afunction of stub length due to distrib-uted cable capacitance, this doublingeffect serves to reduce the amountof impedance loading on the bus byindividual terminals. Heavy stubloading can degrade bus perfor-mance by increasing reflections andreducing signal voltages. Stub cou-pled terminals may be located up to20 feet from the bus; the distance fordirect coupled terminals is limited to12 inches.

(2) For a stub-coupled terminal, theimpedance seen looking into thestub side of the bus coupling trans-former is Z0, assuming that theimpedance of the coupling trans-former is much higher than the busimpedance Z0 (70 to 85 ohm).

Notes:(1) The turns ratio for the direct coupled taps for the B-2204, B-2388, and B-2344 transformers varies slightly from that of the BUS-29854 transformer. They do, however,

have the same turns ratios for transformer coupling. For transformer coupled applications, any of the transformers may be used.(2) The transceiver in the BUS-65164(64), BU-65170X2, BU-61580X2 was designed to work with a 1:0.83 turns ratio for direct coupled applications.(3) For direct coupled applications, the 1:0.83 turns ratio is recommended, but the 1.25:1.0 ratio may also be used. The 1.25:1.0 turns ratio will result in a slightly lower

transmitter amplitude (approximately 4% lower) and a corresponding 4% decrease in the ACE's or STIC's receiver.

Surface Mount

BU-65170/61580X1,BUS-65153

B-2387, M21038/27-12,B-2343, M21038/27-17,LPB-5002, LPB-5009,HLP-6002, HLP-6009

BU-65170/61580X2,BUS-65154

BU-2388, M21038/27-13,B-2344, M21038/27-18,LPB-5004, LPB-5011,HLP-6004, HLP-6011

BU-65170/61580X3(6) B-3072

BU-61590, BUS-63149

B-2386, B-2342, DESC M21038/27-11,DESC M21038/27-16,LPB-5003, LPB-5010,HLP-6003, HLP-6010

ACE, STIC PART NUMBERSDirect Coupled

1.41:1

1:0.83

1:2.5

1:1

Transformer Coupled

2:1

1:0.60

1:1.79

1.41:1

Plug-In

BUS-25679, B-2203,M21038/27-02

BUS-29854, B-2204,M21038/27-03

B-3067

BUS-27765, B-2202, DESC M21038/27-01

Recommended TransformerTurns Ratio

TABLE 1. RECOMMENDED ISOLATION TRANSFORMERS



AN/B-2704/04-0

BU-61580X1BU-61582X1

55 Ω1

2

3

8

4

1.4:1

39 VPP 28 VPP

1 FT MAX

1

2

3

7

5

2:1

20 VPP

18

3

1:1.4

COUPLINGTRANSFORMER

ISOLATIONTRANSFORMER


TRANSFORMER COUPLED (LONG STUB)

20 FT MAX

28 VPP

DIRECT COUPLED (SHORT STUB)

OR

COUPLINGTRANSFORMER




BU-61580X2BU-61582X2

1

2

3

8

4

1:0.83

28 VPP

1 FT MAX

1

2

3

7

5

1:0.6

33 VPP 20 VPP

18

34

1:1.4


20 FT MAX

28 VPP

OR

COUPLINGTRANSFORMER




BU-61580X3BU-61580X6

1

2

3

8

4

1:2.5

11.6 VPP 28 VPP

1

2

3

7

5

1:1.79

11.6 VPP 20 VPP

18

34

1:1.4


20 FT MAX

28 VPP

OR

55 Ω

Z0 (70 to 85Ω)

39 VPP

4

0.75 Z0

0.75 Z0

33 VPP

55 Ω

55 Ω

55 Ω

55 Ω

Z0 (70 to 85Ω)

0.75 Z0

0.75 Z0

0.75 Z0

0.75 Z0

1 FT MAX

FIGURE 1. ACE/STIC, BUS-66549 INTERFACES TO 1553 BUS (CONTINUED ON NEXT PAGE)



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1553B requires that the couplingtransformer have a minimum imped-ance of 3k ohm, looking from the busside. Therefore, the characteristicimpedance of the stub cabling (78ohm nominal) matches the stub'sload impedance, minimizing reflec-tions back toward the transmitter.

(3) In a direct coupled terminal, the mainbus is not protected against a shortcircuit in the stub cabling. For thetransformer coupled case, the bus isprotected against such a fault.

(4) A transformer coupled terminal pro-vides improved dc and commonmode isolation over a direct coupledterminal.

TRANSFORMER CONNECTIONS

With the exception of the BU-61590Universal ACE and the BUS-65149 RT, itis important to note that the transformercenter tap on the "ACE" or "STIC" side,pin 2, must be grounded. The reason forthis is that, at any point in time only onetransformer leg is actively driven whenthe ACE (STIC) is transmitting. There isno instantaneous current in the alternateleg. Autotransformer action will result inan equal voltage excursion of the oppo-site polarity on the alternate leg of thetransformer.

Using the BU-61580X1 as an example,during the first half of a Command/Statussync pulse, no current flows from TX/RX,

while TX/RX is driven to approximately-10V. Due to the autotransformer induc-tance TX/RX will swing to about +10V.The resulting primary voltage is nominal-ly 20V peak, or about 40Vp-p.

For the BU-61590 Universal ACE and theBUS-65149RT hybrid, the transformercenter tap, pin 2, MUST NOT be ground-ed. The transmitters in these hybridsinclude complimentary outputs that driveboth transformer legs simultaneously(with the opposite signal polarity).

If the transformer center tap is grounded,there is a strong possibility that there willbe a "dynamic offset" voltage (residualvoltage) following the end of transmissionby the BU-61590 or BUS-65149.

FIGURE 1. ACE/STIC, BUS-66549 INTERFACES TO 1553 BUS (CONTINUED FROM PREVIOUS PAGE)

BU-61590X5

55 Ω1

2

3

8

4

1:1

30 VPP 30 VPP

1 FT MAX

1

2

3

7

5

1.4:1

20 VPP

18

3

1:1.4

COUPLINGTRANSFORMER




20 FT MAX

28 VPP


OR

55 Ω

Z0 (70 to 85Ω)

30 VPP

4

0.75 Z0

0.75 Z0

Z0 (70 to 85Ω)

Notes:(1) Shown for one of two redundant buses that interface to the BU-65170/61580 or BUS-65163.(2) Transmitted voltage level on 1553 bus is 6 Vp-p min, 7 Vp-p nominal, 9 Vp-p max.(3) Required tolerance on isolation resistors is ±2%. Instantaneous power dissipation (when transmitting) is approximately 0.5 W (typ), 0.8 W (max).(4) Transformer pin numbering is correct for the DDC (e.g., BUS-25679) transmitters. For the Beta transformers (e.g., B-2203), or the QPL-21038-31 transformers (e.g.,

M21038/27-03), the winding sense and turns ratio are mechanically the same, but the pin numbering is reversed, therefore, it is necessary to reverse pins 8 and 4 orpins 7 and 5 for the Beta or QPL transformers. (Note: DDC part numbers begin with a BUS- prefix, while Beta transformer part numbers begin with a B- prefix.)



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LAYOUT CONSIDERATIONS

Figure 2 illustrates the suggested PCboard layout for the BU-61580D1 version(+5/-15V) of the ACE hybrid, two BUS-25679 (or equivalent) transformers, theclock oscillator (16 or 12 MHz), anddecoupling capacitors.

With regard to the suggested layout ofFigure 2, there are a number of importantfactors to consider relating to componentplacement, circuit routing, power distribu-tion, grounding, and decoupling capaci-tors.

ISOLATION TRANSFORMERSThe isolation transformers should belocated as physically close as possible tothe respective TX/RX pins of the hybrid.When transmitting, the typical peak cur-rents in the primary legs are 150 to 200mA for a 15V or 12V transceiver or 450 to

500 mA for a 5V transceiver. Resistiveand inductive voltage drops are mini-mized by providing widened traces andminimizing the length of the traces. Thisis particularly important for the 5V (only)ACE.

CROSSTALKIn addition to limiting the inductive andresistive voltage drops in the analog sig-nal traces when transmitting, reducingthe hybrid-to-transformer spacing servesto minimize crosstalk to the terminal'sreceivers from other signals on the board.Severe crosstalk can increase the termi-nal's word error rate above the maximumlevel of 10-7 allowed by MIL-STD-1553B.Another important precaution regardingcrosstalk is to avoid running other analogand digital signal traces (particularlyhigh-speed digital signals) in close prox-imity to the 1553 analog signal traces.

This applies to the signal traces on bothsides of the transformer.

It is most critical to avoid routing othersignals on layers of the PC board thatare adjacent to and in parallel with the1553 analog signals. Such signals canresult in the worst case crosstalk.

GROUND PLANESAs is the rule in all high-speed digital cir-cuits, it is a good practice to use groundand power supply planes under the ACEas well as the host processor and anydigital "glue" logic.

However, it is very important that therebe no ground and/or power supplyplanes underneath the analog bus sig-nal traces. This applies to the TX/RX sig-nals running between the hybrid and theisolation transformer as well as the traces

FIGURE 2. SUGGESTED PC BOARD LAYOUT

3 2 1

BUS-25679

3 2 1

BUS-25679

35 34

12

3736

69 70

BU-61580D1

(TOP VIEW)

19

12/16 MHzCLOCK

OSCILLATOR

LOGICDECOUPLINGCAPACITOR

CHAN. BDECOUPLINGCAPACITOR

CHAN. ADECOUPLINGCAPACITOR

Notes:(1) Physical spacing between ACE (STIC) transformers must be kept to a minimum.(2) There must be NO GROUND OR POWER SUPPLY PLANES underneath the signal traces running to or from the transformers.(3) The grounds for the analog and digital sections of the ACE (STIC) are not connected internally.



AN/B-2704/04-0

between the transformers to any connec-tors or cables leaving the board.

The reason for avoiding running supply orground planes under the analog signaltraces is that the effect of the distributedcapacitance will be to lower the inputimpedance of the terminal as seen fromthe MIL-STD-1553 bus. MIL-STD-1553Brequires a minimum of 2k ohm inputimpedance for direct coupled terminalsand 1k ohm for transformer (stub) cou-pled terminals. If there are ground planesunder the analog signal traces, it is likelythat the terminal will not meet thisrequirement. It has been found that plac-ing a ground plane under the isolationtransformers only slightly effects the inputimpedance. A ground and/or power planemay be placed under the transformers, ifdesired.

POWER AND GROUNDDISTRIBUTIONAnother important consideration for 1553transceiver operation is power andground distribution. Refer to Figure 3.

For the ACE (STIC) hybrid/transformercombination, the high current path whenthe ACE is transmitting will be from the-15V (or -12V or +5V) power supply,through the ACE's (STIC's) transmitteroutput stage, through one leg of the iso-lation transformer to the transformer cen-ter tap.

It is important to realize that the high cur-rent path is through the transformercenter tap and not through the ACE's(STIC's) GNDA and GNDB pins.

Two exceptions to the operationdescribed above are the BU-61590 uni-versal terminal and the BUS-65149 RThybrid. With these two units, it is impor-tant to note that the transmitter providesa differential output stage, actively drivingboth transformer legs with opposingpolarity signals.

For the BU-61590 or BUS-65149, thetransformer center tap must not begrounded. In this case, the heavy trans-mitter supply current runs from the

+15/12V (-15/-12V) supply, through thetransformer primary and back to the -15/-12V (+15/-15V) supply. Like the otherACE and STIC units, there will not be alarge transmitter return current flowingthrough GNDA and GNDB.

A worst-case system design shouldensure that with minimum supply voltageand calculated voltage drops, the trans-ceiver voltage provided between theACE's (STIC's) transceiver supply pinsand the center tap of the respective isola-tion transformer will be no less (inabsolute value) than the specified mini-mum (-14.25V, -11.6V, or +4.75V).

In some cases, the voltage drop may bereduced by means of large decouplingcapacitors, but the best practice is to min-imize voltage drops in the power supplydistribution.

ANALOG AND DIGITAL GROUNDSIt is important to note that the logicground and transceiver grounds are con-nected together internally in the STIC or

FIGURE 3. POWER/GROUND CURRENT DISTRIBUTION

LOGIC

RX

TRANSCEIVER

TX

1

2

3

6

8

7

5

4

BUS-25679

HIGH LEVELCURRENTS

LOW LEVELCURRENTS

LOW LEVELCURRENTS

LOGICGND

GND A/B

+5VLOGIC +5V A/B

-15V/-12V/+5V (A/B)



AN/B-2704/04-0

ACE hybrids. These grounds must beconnected together externally.

As far as the ACE (STIC) and its associ-ated isolation transformer are concerned,the optimal circuit layout would entail asingle ground plane for both the digitaland analog (transceiver) circuits. Whilethis is sometimes possible, in many appli-cations, there are system requirementsfor separate analog (-15/-12/+5 (analog))and digital (+5V) power supply returns.

In this case, the transformer centertap should be connected through alow impedance path to the analogreturn, not the digital return.

This provides the advantage of separat-ing the analog and digital ground cur-rents. It is assumed that the two returnpaths are ultimately bonded with lowimpedance connections to the systemground near the power supply.

In order to minimize the possibility ofground noise corrupting theprotocol/transceiver interface within theACE (STIC), it is best that both theLOGIC GND pin as well as the GNDAand GNDB pins be connected to the logicground as close as possible to the hybrid.

DECOUPLING CAPACITORSWhen the ACE (or STIC) is transmitting,it is drawing relatively large pulsating cur-rents from the active transceiver power

supply. For the -15V or -12V unit, this cur-rent will generally be in the range of 150to 200 mA peak and can be as high as300 mA. For a +5V unit, the peak currentcan be as high as 800 mA. The frequen-cy content of the power supply currentcan include components in the ranges ofDC to 30 kHz on the low end, and 500kHz to 2 MHz (and above) on the highend.

Resistive and inductive voltage drops inthe power distribution network can resultin ripple voltages on the ACE's (STIC's)power supply inputs. It is strongly recom-mended that decoupling capacitors beused to reduce the level of the ripple volt-age.

For the transceiver power inputs, it isgenerally necessary to use small decou-pling capacitors to eliminate the high fre-quency (500 kHz to 2 MHz) power supplyripple. For -15V and -12V power inputs,2.2 µF low ESR/ESL capacitors shouldbe sufficient. For the 5V only ACE, 6.8 µFlow ESR/ESL capacitors should be used.

If good power distribution practices aremaintained, the small capacitors speci-fied in the preceding paragraph should besufficient. If, however, the trough of theripple falls more than 5% below the nom-inal supply voltage, the transmitter outputmay fall below the minimum level of 6 Vp-p for a direct coupled output, 18 Vp-p fora MIL-STD-1553B transformer coupled

output, or 20 Vp-p for a MIL-STD-1760Btransformer coupled output. In this case,a larger capacitor may be needed to sus-tain the minimum voltage level.

For the +5V LOGIC input and +5VA/Binputs (for a -15 or -12V ACE (STICunit)), 0.01 µF is generally sufficient.

Avoid Switching Power Supplies andDC-to-DC Converters.

The use of switching power supplies orDC-to-DC converters on the same boardas the ACE or STIC hybrid should beavoided. The switching noise may resultin poor performance of the ACE's(STIC's) transceiver. Some of the poten-tial problems include high output noise aswell as degraded zero-crossing toleranceand word error rate.

LOCATION OF CLOCKOSCILLATORAs illustrated in Figure 2, the 16 MHz or12 MHz clock oscillator should be locatedas close as possible to the ACE's (orSTIC's) CLK IN input pin in order to min-imize attenuation, distortion, and corrupt-ing crosstalk of the clock signal. The dutycycle of the clock input should bebetween 40% and 60% in order to opti-mize operation of the Manchesterdecoders and processor/memory inter-face control logic. Some of these circuitsutilize both edges of the clock input.

FIGURE 4. ISOLATION TRANSFORMER-TO-SYSTEM CONNECTOR IMPEDANCES

ZIN/XCVR = 11 k / ohms, 10pf

ZXFMR (@ 1MHz ) = 3000 ohms capacitive

CCABLE = 30pf/ft. MAX

STUB CABLEISOLATION

TRANSFORMERSYSTEMCONNECTOR

ZIN

TOTERMINAL

ACEor

STIChybrid


ISOLATION TRANSFORMERINTERFACE TO SYSTEM CON-NECTOR; INPUT IMPEDANCECONSIDERATIONS

The general practice in connecting thestub side of a transformer (or direct) cou-pled terminal is to make use of 78 ohmtwisted-pair shielded cable. This servesto minimize impedance discontinuities.

The decision of whether to isolate ormake connections between the centertap of the isolation transformer's sec-ondary, the stub shield, the bus shield,and/or chassis ground must be made ona system basis, as determined by ananalysis of ESD, EM/RFI, and lightningconsiderations.

There is some controversy within the1553 community as to where a terminalends and a stub begins. The issue is notpurely academic, since it has a profoundeffect on the testing of terminal inputimpedance. In order to pass the 1553 RTValidation Test Plan, a transformer (stub)coupled terminal must have an inputimpedance of at least 1k ohm over thefrequency range from 75 kHz to 1 MHz.Paragraph 3.10 of MIL-STD-1553Basserts the definition of a terminal as"The electronic module necessary tointerface the data bus with the subsystemand the subsystem to the data bus.Terminals may exist as separate linereplaceable units (LRU's) or be containedwithin the elements of the subsystem."

For a transformer (stub) coupled termi-nal, it is the author's interpretation ofParagraph 3.10 (particularly the first sen-tence) that the terminal ends at the pinsof the isolation transformer. This interpre-tation is consistent with Figures 9 and 10in the 1553B standard. However, thearmed services, faced with the practicaltask of testing and maintaining equip-ment, take a different interpretation. They

contend that the terminal includes thewiring path up to and including the sys-tem connector. As a result, the distributedcapacitance of PC board and backplanetraces, cabling and connectors areincluded in the impedance measurement.

For the -15V/+5V ACE and STIC hybridsthe transceiver input impedance is speci-fied as follows: 11k ohm min, 10 pF max.Reference Figure 4. This characterizes adifferential measurement taken betweenthe TX/RXA(B) and TX/RXA(B) pins ofthe hybrid. It includes both the transmitterand receiver (tied together internally), isapplicable to both powered and unpow-ered conditions with power and groundpins connected, and assumes a 2 Vrmsbalanced, differential, sinusoidal input formaking the impedance measurement.

At 1 MHz, 11k ohm and 10 pF representa composite impedance of 9.05k ohms at-34.6°. Assuming an ideal (Zin = ) 2:1isolation transformer for a transformercoupled terminal, this reflects an imped-ance of 2.26k ohms at -34.6° on the stubside. Note this is "worst case." Typicalnumbers are about twice as high.

The -34.6° phase angle indicates areflected ACE (STIC) input impedancethat is somewhat more resistive thancapacitive. The impedances of all theother elements in the path — the isolationtransformer, PC board and backplanetraces, cable, and connector — arecapacitive. These capacitances add inparallel to form the "terminal's" inputimpedance (as defined by the armed ser-vices).

Assuming the worst case parameters forthe ACE (STIC), these elements musthave a minimum parallel impedance of1.47k ohm (capacitive) to keep the paral-lel impedance of the "terminal" above 1kohm. At 1 MHz, the highest frequencyrequired for the 1553 impedance test, thistranslates to a maximum capacitance of

108 pF. Since most 1553 isolation trans-formers specify an input impedance of atleast 3k ohms (some are higher), repre-senting about 53 pF, this allows about 55pF for the rest of the signal path. MIL-STD-1553B specifies that the cablecapacitance of the bus cable be less than30 pF/ft. 55 pF represents 1.8 feet ofsuch "worst case" cable.

In order to ensure that the "terminal" inputimpedance complies with MIL-STD-1553B, it is important to try to minimize thedistributed capacitance between the 1553signals and ground. To achieve this, thelength of internal stub wires must be keptto the shortest length possible. If this is notpossible, approaches that can be usedinclude not grounding the cable shield orusing unshielded twisted pair cable.

Another approach is to use a pair ofwires, rather than a twisted/shielded pair.In this case, the effect of the bus signalscoupled from the unshielded wires on therest of the system performance must beconsidered as a tradeoff against the ben-efit of the increased terminal impedance.

"SIMULATED BUS" (LAB BENCH)INTERCONNECTIONSFor purposes of software developmentand systems integration, it is generallynot necessary to integrate all of therequired couplers, terminators, etc. thatcomprise a complete MIL-STD-1553Bbus. In most instances, a simplified elec-trical configuration will suffice.

The three connection methods illustratedin Figure 5 allow an ACE (or STIC) to beinterfaced over a "simulated bus" to sim-ulation and test equipment, such as aBUS-6517II board. It is important to notethat if a compliant 1553 bus configurationis not used (as in Figure 1), the resistorsshown in the diagram are necessary inorder to ensure reliable communicationsbetween the ACE or STIC and the simu-lation/test equipment.


AN/B-2704/04-0


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1

2

3

8

7

5

4

ACEOR

SP'ACEHYBRID

TEST/SIMULATIONEQUIPMENT

STUBCOUPLING

STUBCOUPLING


78Ω1.5W

(A)

1

2

3

8

4

ACEOR

SP'ACEHYBRID


DIRECTCOUPLING

DIRECTCOUPLING


39Ω0.5W

(B)

55Ω1W

55Ω1W

55Ω

55Ω

1

2

3

8

4

ACEOR

SP'ACEHYBRID


STUBCOUPLING

DIRECTCOUPLING


39Ω0.5W

(C)

55Ω1W

55Ω1W

20Ω0.5W

20Ω0.5W

FIGURE 5. “SIMULATED BUS” INTERCONNECTIONS

(A) Direct-Coupled-To-Direct-Coupled

(B) Transformer-Coupled-To-Transformer-Coupled

(C) Direct-Coupled-To-Transparent-Coupled



AN/B-2704/04-0

TRANSCEIVER ISSUES

This application note considers a numberof transceiver related issues that arecommonly asked about. These include:voltage vs. current drivers, the effect ofreceiver threshold on MIL-STD-1553Bvalidation testing, and consumed vs. dis-sipated power.

VOLTAGE VS. CURRENT DRIVERSThe transmitter in the -15/+5V and–12/+5V ACE and STIC terminals arevoltage type transmitters. The transceiverin the +5V only ACE is a current type ofdriver. For both of these transceivers,only one leg of the isolation transformerprimary is actively driven at any point intime. For the BUS-65149 and BU-61590universal terminals, a true differentialtype of transmitter output stage is provid-ed: for these hybrids, both legs are drivensimultaneously.

Voltage source transmitters providesuperior line driving capability for drivinglong cables and heavy amounts ofstub/terminal loading. On the other hand,current source drivers, due to their sim-pler design, tend to consume and dissi-pate lower levels of power.

The transmitter requirements are a bit dif-ferent for MIL-STD-1760B. In MIL-STD-

1553B, the output of a transformer cou-pled terminal is required to be in therange of 18 to 27Vp-p. MIL-STD-1760B ismore stringent, requiring a transmitterstub voltage of 20 to 27Vp-p. This highervoltage ensures robust operation over arange of network topologies (stub andbus lengths, separation distances) forvarious aircraft. It is also recommendedthat a voltage, rather than a currentsource transmitter, be used in 1760 appli-cations. A voltage source transmitter ismore robust on a stores bus with itsdetaching stubs and the resulting varia-tions in load impedance.

The transmitter in the -15/+5V versions ofthe STIC and ACE terminals provides avoltage source with a minimum stub volt-age of 20Vp-p, making these productssuitable for 1760 applications.

MlL-STD-1553B specifies that a trans-former coupled terminal must be able tooperate with an incoming line-to-line sig-nal in the range of 0.86 to 14.0Vp-p. Thestandard also requires that a terminal notrespond to a signal in the range of 0.0 to0.20Vp-p. In DDC's terminals, the receiv-er threshold is generally trimmed forbetween 0.550 and 0.700 Vp-p. Thisrange provides optimal performance forthe Manchester decoders in the ACE andSTIC terminals.

RECEIVER THRESHOLDIf receiver threshold is sufficiently below0.550Vp-p, there is an increased possibil-ity of failing the word error rate (noise)test of the RT Validation Test plan, even ifthe threshold is in spec. A 1553 terminal'sword error rate is primarily a function ofthe threshold-to-noise ratio. Going to theother extreme, a receiver threshold that istoo high (too close to 860 mV p-p) maycause problems in passing the zerocrossing deviation and/or common moderejection tests.

In the zero-crossing deviation test, theterminal must accept as valid a zerocrossing-to-zero crossing time of 350 ns(500-150). If the receiver's threshold istoo high, the pulse width (as seen by theterminals' Manchester decoder) will bereduced to an unacceptably short inter-val.

The validation test for common moderejection involves superimposing a com-mon mode voltage on a stub voltage of0.86 Vp-p. Since the terminal's trans-former has a finite common mode rejec-tion ratio, this is tantamount to loweringthe received signal level below 0.86V.Therefore, a threshold too close to 0.86Vp-p may result in a failure of the valida-tion common mode test.

FIGURE 6. STUB COUPLED TERMINAL TO DATA BUS

TRANSCEIVER SUPPLY VOLTAGE

+5V OR -15V OR -12V

STICor

ACEHYBRID

1

2

3

7

5ISOLATION

TRANSFORMER

20.4V peak-to-peak 28.8V peak-to-peak= 14.4V peak

ZDRIVING POINT= 2 • Zo

STUB CABLE 1.4 : 1.0 0.75 • Zo

Zo

1553BUS

Zo

0.75 • ZoBUSCOUPLING

TRANSFORMER



AN/B-2704/04-0

POWER CONSUMPTION VS.DISSIPATIONAnother common point of misunderstand-ing regarding transceivers involves powerconsumption and power dissipation.Figure 6 illustrates a typical configurationof a stub coupled terminal and a 1553bus. When the terminal is not transmit-ting, the power consumption and dissipa-tion are essentially equal.

However, when the terminal is transmit-ting, there will be a difference betweenthe consumed and dissipated power. Thedifference in the two power numbers isthe power dissipated in external isolationand termination resistors.

As shown in Figure 6, the voltage on the''bus" side of the coupling transformer isapproximately 28.8Vp-p, or about14.4Vpk.

That is, Vpk = 14.4.

The load resistance, as seen from thispoint, consists of the two termination

resistors (in parallel), in series with thetwo isolation resistors.

This is equal to

(0.75•Z0) + (0.75•Z0) + (Z0/2) = 2•Z0.

If the transmitter waveform is assumed,for the moment to be a square wave, itsRMS value will be equal to its peak value,Vpk. With this assumption, the combinedinstantaneous dissipation of the isolationand termination resistors, when the ter-minal is transmitting, is calculated as fol-lows:

PLOAD = (Vpk)2/(2•Z0)

Since the 1553 waveform is a trapezoidalsignal, rather than a square wave, theactual PLOAD will be slightly less than thatcalculated by the equation given above.That is,

PLOAD = K•(Vpk)2/(2•Z0).

The exact value of K is a function of thetransmitter's rise and fall times as well as

the actual transmitted data pattern. Kdecreases with an increase in rise/falltimes and the number of signal transi-tions in the transmitted waveform. As afirst approximation, a value of K = 0.9may be used.

Consider a transformer coupled terminalwith a transmitter stub voltage of 20.4Vp-p and a bus with a characteristic imped-ance of Z0 = 78 ohm. These are typicalvalues. The voltage on the "bus" side ofthe coupling transformer will be:

2•20.4 = 28.85 Vp-p, therefore,

Vpk will be 28.85/2 = 14.425V. As aresult,

PLOAD = (0.9)•(14.425)2/(2•78) = 1.2W

1.2 W represents the instantaneous dissi-pation of the isolation and terminationresistors when the terminal is transmit-ting.

FIGURE 7. BU-65620 TO FIBER OPTIC TRANSCEIVER INTERFACE

16 MHzCLKOSC

CLK_INA09

CLK_BELB08

+5V

R07

R08

R10

R09

B07

C06

A06

A07

P07 SINGL—ENA

RX—B

RX—B

TX—B

TX—B

RX—A

RX—A

TX—A

TX—A F.O.TXTTTR

F.O.RCUR

N/C

F.O.TXTTR

N/C

F.O.RCUR

BU—65620P0

HOSTPROCESSOR

DATA

ADDRESS

CONTROL



AN/B-2704/04-0

For a typical terminal, the actual transmit-ter duty cycle (portion of time transmit-ting) is typically between 1% and 10%.The average external dissipation is givenby:

PLOAD /avg = D•1.2W,

where D = transmitter duty cycle.

Both power supply current and terminalpower dissipation vary linearly as func-tions of the transmitter duty cycle.

For example, consider a BU-61580D1ACE terminal operating at 25% dutycycle. At 25% duty cycle, the averageload dissipation is (0.25)•(1.2) = 0.3W.Based on the "max" spec numbers, thetotal consumption:

= (5V)•(190 mA) + (15V)•(108 mA)

= 0.95 + 1.62 = 2.57W.

This is consistent with the "total hybrid"power dissipation spec, listed as 2.25Wmax, at 25% duty cycle.

BU-65620-TO-FIBER-OPTIC-TRANSCEIVER INTERFACE

For MIL-STD-1773 applications involvinga fiber optic transceiver, the BU-65620digital monolithic version of the ACE maybe easily interfaced to a fiber optic trans-ceiver. To facilitate this interface, theManchester decoders in the BU-65620provide a pin-programmable option allow-ing them to accept the single-ended inputsignals from a fiber optic receiver.

As shown in Figure 7, this option is acti-vated by strapping the input signalSNGL_ENA to ground.

13AN/B-2704/04-0 PRINTED IN THE U.S.A.

The information in this application note is believed to be accurate; however, no responsibility isassumed by Data Device Corporation for its use, and no license or rights are

granted by implication or otherwise in connection therewith.Specifications are subject to change without notice.

Please visit our web site at www.ddc-web.com for the latest information.

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For Technical Support - 1-800-DDC-5757 ext. 7771

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