EVALUACIÓN DEL APRENDIZAJE

I n the VA,'( 8800 C PU , the de output cou l d . under fau lt con d i tions, producc approxi matl' l y 4 0 0 a m p e res . Thus the i nterconnection must han d l e this high fau l t currem This i n terconnec­ tion was accompl ished by bolt i ng the j u nction n o d e of t h e co m b i n e d d e - p o w e r a n d l o g i c

rl'turn ro the chassis for the ent i re length o f the

horizontal bus bar. This porr ion of the c hassis was choscn as the connection poin t becausc it was nor used as a conductor for any other h igh­

frequency currents .

I n summary, the ground i ng approach we used for t h e 8 8 0 0 fe a t u red t h e fo l l o w i n g dc s i g n points:

• The l o g i c a n d de r e t u r n a n d t h c c h a s s i s gro u n d a r e connected together a t t h e hori ­ zontal power-return bus.

• 'T'hc power-system o u t p u ts and the c ha�sis

gro u nd arc isola ted fro m gro u nd at RF fre­ quencies by high i mpedances using lossy fer­ r i t e i n d u c t ors . DC c u r re n t s a n d l i n e - frc ­ quency ( '; Oj60 H z ) fa u l t c u rren ts may thus flow uni mpeded .

• Part i cu l ar care was taken to m i n i m ize t hc

now of logi c-rem rn currents through the sys­ t e m chass i s , t h u s isol a t i n g the p e r i p h era l boxes ( C I 7 5 0 , BA l l AW , e re . ) from t h e sys­ tem chassis grou n d . I ns u lated chassis sl ides. s h u n ted by l ossy ferrite i n d u cto rs , acco m ­ p l ished that isolation . Although there arc sti l l c o m m o n - m o d e c u r re n ts w i t h t h e fer r i t e i ndu ctors, t hey reduce unwanted common­ mode n oise voltages that can couple int o cir­ c u i ts through parasitic inductances . That is a far worse problem , as we demonstrated to our own c hagrin .

• T h e l j O pa n e l b u l k head a n d t h e log i c and

power returns for the VAXBI bus and memory backpl anes are tightly bonded tO the s i ngle­ point ground at the CPU power-return bus.

• The e l i mi nation of circulat i ng noi se and l ogic

curren ts t h rough the c hassis wil l max i m i ze the effectiveness of the shielded cabinet as an atren uaror of radi ated energy.

T h e i m p l e m e n t a t i o n of t h i s a p p r o a c h i s shown i n Figure 6 .

ljO and Expansion of Grounding

O nce the m a i n processor's grou nding had been defi ned, we had to deal w ith grounds between the externa l c l ements , such as the 1/0 s ubsys­ tem . The VAX 8800 system can accommodate a large array of r;o devi ces by uti l izing the VA.,'<:Bl archi tecture . The H96 5 2 EC- ED cab has provi­ si ons for two expansion boxes, the C I 7 5 0 and the BA I l AW. These boxes arc self contai ned and have i ntegra l power supplies, logi c backplanes, and int ercon nects . In keeping with our ground­ i ng arc h i tcctur e . we isolated these boxes from the chassis ground by using low-Q i nductances . The si gnaljlogic ground was then established by means of cables ro the VA.XBI -to-CPU backplane . This scheme ensures that the chassis is not used as a signaljlogic return .

System to System Grounding

Grouping systems togcther or netwo rk i ng them has a large impact on systcm noise and the sub­ sequent grounding techniques tO e l i m i nate it. I n terms of t h e signa l-to-noise ra t i o a n d from the aspect of grounding. a networked system can be divided into two cases : the dense network, and t ill' dispersed nctwork.

Dense Network

A dense nc t\vork is a group of compu ters or sys­ tems w i t h associa ted su pport hardware rhat i s l ocated w i t h i n one area , e i t her an offi ce or a c o m p u ter roo m . This area i s l ikely tO conta i n systems fro m different vendors as well as phone­ swi tc h i ng networks, cxperi me n tal equi pmen t , o r i ndustrial control lers and monitors. Al l these devices sbarc a common ground that could be a grid or simply a bra nch ground as part of the i r safety ground. This connection also provides a s i g n a l r e fe r e n c e b e t w e e n i n t e r c o n n e c t i n g d e v i c es i n t h e a rea t h ro u g h t h e c ha s s i s a n d

Digital Technical journal

AC FRONT -END CABINET r - - - -r-- - -, I Cl750 I I I I I I I I I I I I I I I I L - - ---L _ _ _ _ ...J AC CORD GROUNDS - - , +5 V +5 V I MAIN CABINET :�� IBACKUP I I II I _{1 "-}

�

L __ _ _ _ L

!�

_ _ _ _ J

w

-MPS CHASSIS (ISOLATED)

w

-CABINET FRAME

w

- ISOLATED BOX FRAME

\!f

-PERIPHERAL FRAME

w

-H7170 FRAME

w

-CSP FRAME Figure 6 MEMORY 876 POWER CONTROL (PRO 380) CONSOLE

1/0

LASO PRINTER I I I I I 1 . I I L---J r - - 1 I I I I I I I I "­ l o , a I ­ I I I L-- ---, I I I I I I I I I I I I I I _ _ J MPS AC/DC CONVERTERS ,--- ---, ,---l I _{t 1} I : _{: :} : : _: I : I o I

NOTES: 1 . The return of the power bus lor the CPU backplane is the common connection (single­ POint ground) for the logic. power. and chassis (RF) grounds.

2. The 1 200-pF capacitance is the typical parasitic capacitance between the isolated MPS power-supply rack and the main chassis.

3. The indicated common-mode indicators. or baluns, are composed of lossy ferrite cores surrounding the bus supply and return conductors.

4. The inductors shown between the ··e·· ground or chassis and the subassemblies are lossy inductors designed to increase the RF impedance and to prevent the circulalion of noise currents in the chassis.

5. The indicated inductance is the stray. or leakage. inductance in the power cord isolating the cabinets from ac. or utility. ground.

A L ogical Grounding Scheme for the VA X 8800 Processor

power line ground in a complex way . Al l these devices can generate h i gh-frequency cur rents that flow into the groun d . These currents must fl ow through the complex i m peda nce of t h e g r i d w h e r e , cons e q u e n t l y , R F vo l t a ges c a n deve l o p . Under those cond i t i o n s the gro u n d wou ld act a s a noise i njection poi nt rather than a stable reference.

Dispersed Network

The dispersed network is an interconnection of computers or systems spread over a wide area, perhaps resid i ng on different floors of a b u i l d ­ i ng o r i n d i fferent build i ngs altogether. Commu­ n i c a t i o n on t h i s sca l e c a n n o t d e p e nd on a mutual RF ground because i t cannot be reason­ abl y esta b l ished . In this case , com m u n i cation must be accompl ished by means of either trans­ former-cou pled circuits, opti cal l i nks , or di ffe r­ ential driver/receiver logic.

Both types of networks i l l ustrate the fact that system network i n g cannot, and i n some cases shoul d not, be accom plished by attempting ro create an absolute ground reference to the net­ work .

System to Peripheral Grounding

As a system expands with the addition of periph­ era l devices , such as disk drives, pri nters , and LANs, the ground system must be vi ewed as a large hybrid arrange ment. Intercon necting these devices must be predicated on the groun d- cur­ rent characteristics (si gnature) and the I/0 con­ nections of these devices to the syste m .

This signature is particularly i m portant when connecting devices that were designe d co be used as sm a l l , standalone appl ications . T h e i r designs may have i n volved decreased l i ne-fi l ter­ ing capa bi l i ti es and m i n i ma l ly sized chokes for grou n d is o l a t i on or perh aps none a t a U . I t i s i mperative that such factors b e consi dered when connecting peri pheral devices to a large syste m .

Summary

We now offe r some conclusions based on our re c e n t e xp e r i e n c e s w i t h the VAX 8 8 0 0 a n d ot her new syste ms. These concl usions rake the for m of recommendations for m i ni mizing noise­ related problems in any compurer syste m .

Ground Noise Current Signature

I t is i m porta n t to i d e n t i fy t h e s p e c t r u m o f ground -conducted noise for each subsystem ele-

98

ment . This noise depends on paras itic ele ments i n the circu i ts and el ectromechanica l structure . Th erefo r e , t h i s i nfo rmat i o n is best obta i n ed e m p i ri c a l l y by m e a s u re m e n ts on t h e act u a l ha rdwa re . The noise current a m p l i t udes a n d fu ndamenta l frequenc i es should b e m easured on cable s h i e l ds , c h assis grou n d s , 1 / 0 logic returns, and power inputs.

Segregation of System Gro u nd Networks

A ground system schematic shoul d be deve loped for each particular su bsystem. The in tercon nec­ t i o n of g r o u n d types w i I I b e b a s e d on t h e i ntended system application . As a general rule, the g ro u n d types s h o u l d b e s e g r e g a t e d to accou n t fo r t h e fi n i t e a m p l i t u d e s and oft e n u npredi c table paths o f the noise currents . This segrega t i o n of grounds (e . g . , powe r , chass i s , a n d safety grounds) c a n be acc o m p l ished by carefu l ly c hoos i n g t h e freq u en cy-depen d e n t impedances. These i m pedan ces are lossy ferrite inductors p l aced in series with the appropriate ground connecti o n .

Appropriate Signal and Power In terco nnect

T h e o p t i m a l s i g n a l i n t e r c o n n e c t i o n s a r e designed a s controlled-i mpedance tra nsmission l i nes with each signal and its return path closely cou p l ed a n d havi n g e q u a l i m p e d a n c e co t h e chassis ground . Depen d i ng o n t h e noise sensitiv­ i t y , d a t a r a t e , a n d i n t e rc o n n e c t l e n g t h , t h e i mplementation can ran ge from coaxial cables w i t h o vera l l s h i e l d s co grou n d - p l a n e ri bbon cables co r i bbon cables with a l ternate ground/ signa l p a i rs . Even t h e cru dest , s l owest s i gn a l J i n e that re l i cs o n chassis grou nd for a signal return is doomed co fa i l ure if i t is sensitive co noise .

H igh- performance data l ines shoul d certainly b e d es i gn ed w i th l ow - i m pe d a nce d i ffe ren t i a l l i ne drivers a nd rece ivers , e i ther d i rectly cou­ pled or transformer cou pled. Single-ended l i ne drivers and receivers may be acceptable with i n a subsystem i n which the noise between grounds is low and control l e d . Communication through u n b u ffe red TTL outputs a n d i n p u ts are never accepta b l e w h e n l e a v i n g a su bsystem back­ plan e .

The i n itial cost o f a n d board space needed for proper li ne drivers and receivers are more than justified in today's distributed comput i ng envi -

Digital TecbnicalJournal No. 4 February 1987

ron m e n t . The i r use i ncreases re l i a b i l i ty a n d decreases start- u p probl ems. T h e power i nter­ connects s h o u l d be designed w i t h m i n i mu m inductance and t he lowest h igh-frequency char­ acteristic i m pedance that is reasonable. The cir­ culating path of supply and return power cur­ ren ts s h o u l d be kept as low a s poss i b l e . This design al lows better power-system transient per­ formance and ensures the existence of m i n i m a l rad iated magnetic fields.

Notes

1 . A short c i rcuit between the h igh-voltage p r i m a ry and t he low-vo ltage secondary could produce lethal voltages referenced to the chassis ground at accessible poi nts w i t h i n the computer. W i t h t h i s s h i e l d , however , t he s hort wi l l produce a h i gh fault current to the chassis. That current w i l l open va r i ous p rotect i ve d e v i c e s , s u c h a s fuses a n d c i rc u i t breakers, t h a t render t h e system safe i n t h e event of a fau l t .

Appendix

Determin ing Skin Depth

To calcu late the i mpedance of a given conduc­ tor, t he depth of curren t penetration - or skin depth - in a con d u c tor m u st be ca l c u l a ted fi rst . To do t h a t , a designer must perform the following steps:

1 . Determine the type of metal of which the condu ctor is made ( i . e . , copper, z i n c , etc . ) .

2 . Look up i n a reference table the magnetic suscepti b i l i ty of the materia l . (The CRC Ha n dbook of Chemistry a n d Physics

contains tab les of this nature . ) Two types of l isti ngs of susceptibi l i ty are commonly u s e d . T h e f i rst t y p e g i ve s v a l u e s o f speci fic suscepti b il i ty t hat must b e con­ verted by m u ltiplying the value by 4 X 1r

X density of materia l , called P . For cop­ per, this value wou l d be - 0 .086 X I Q -(,

In document REFORMA CURRICULAR DEL PROGRAMA DE TRABAJO SOCIAL (página 103-107)