Computers&LogicCircuits

8/13/2019 Computers&LogicCircuits

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COMPUTERS AND LOGIC CIRCUITS

Dea l ing wi th comput ers ca n seemoverwhelming for t hose w ho are

a c cus tomed t o wor king w i th mecha n ica lsys tems. Since we cann ot a c tual ly seew ha t is going on inside the comput er orth e system it cont rols , comput ers ma y notbe as easy t o under sta nd a s mechan ica lc omponents suc h a s t r a nsmiss ions a ndengines . H ow ever , computers a re not a scomplica ted a s th ey might sound. Thischa pter w il l help demyst i fy computers.

The comput ers found on a v ehicle a rereally no dif ferent t ha n a ny othercomput er encount ered in everyda y l i fe.Vehicle comput ers rely on da ta fromsome type of input device and th en followthe ins tru ct ions in t heir progra ms t odetermine t he required output . The inputdevice ma y be a keyboa rd or a coolan ttempera tur e sensor ,

a nd t he output ma y be video displa y or afuel injector . The progra m t he comput erfollows m a y be for w ord processing or forcont ro ll ing fuel meter ing a nd engine

t iming .

Computers ca n process a great deal o fda ta very quickly a nd a ccura tely , ma kingth em very us eful for severa l jobsinc luding cont ro ll ing ma ny of t hesyst ems on a n a ut omobile. This chapt erexpla ins how a comput er func t ions ,s ta r t ing wi th th e inputs a nd outputs , thecomputer 's centra l process ing uni t (CP U )a nd memory , a nd logic ga tes an d theirsymbols.

U nders ta nding how comput ers work isessentia l beca use most vehicles havesome type of computer . Kn ow ing howcomputers operat e and f i t t ogether w ithva r ious sensors a nd a c tua tors wi l lincrea se your a bil ity t o diag nose a ndrepair problems.

COMPUTERS A ND LOGIC CIRCUITS

Pa ge 1 Toyota Motor S ales, U .S.A., Inc. All Rights R eserved.


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This cha pter is divided int o the follow ingsections:

Ana log and D i g i t a l Inpu t sAna log and D i g i ta l Outpu ts

S igna ls, in clu d i ng .

Ana l og and d i g i ta l w ave for ms

AI D conver ter s

D/A con ver ters

Microprocessor

Ra n dom Access M em ory (RAM )

Read -On ly Memor y (ROM )

Pr ogr amma ble Read -Onl y Memor y

(PROM)

Log ic C i rcu i t s

INPUTS

As demonstra ted in th e previous chapt er ,the EC U , as w ell as a ny o ther a utomobi lecomput er , depends on sensors t o monitorvar ious sys t em func t ions a nd repor t theirsta tus ba ck to the computer . Once thecomput er receives th e dat a from t hesensors , it a na lyzes i t a gains t pre-p r og r a m m e d s t a n d a r d s a n d a c t saccordingly .

One problem w ith m a ny of these inputs istha t t hey do not spea k the same langua gea s t he comput er . The comput er onlyund ersta nds d igita l s igna ls or on/offsignals. A resistive type sensor providesth e comput er

wi th a var ia ble vol ta ge, known a s ana na log s igna l . Some sensors , like th esw itch type sensors, do provide a d igita l

s ignal for the computer . In th is case , th ecomputer can in terpret the s igna lbeca use it is either on or it is off-nothin gin-between.

B eca use computers must ha ve digi ta linputs to use the dat a received, al l ana logsigna ls must be converted t o digita l . Howcomputers in terpret th e ana log s ignalsw ith a n A/D convert er w ill be coveredlater .

OUTPUTS

Computer output to most a ctua tors isdigita l . The signal tel ls the actua tor toeither t urn on for a specified lengt h oft ime or shut off . St epper motors, relaysa nd solenoids ha ve only t wo modes ofopera t ion: on or off .

Aga in , w hen a ctua to r s r equi r e a va r i ab levolt a ge, such a s th e speed cont rol for ablow er motor for a ir condit ioning, t he

computer n eeds a nother in terpreter . Inth is case, th e interpreter is a D/Aconverter , w hich w il l be covered la ter .

SIGNALS

As expla ined previously, t he t w o types ofs igna ls are an a log an d digi ta l . Thevol ta ge of these s igna ls ma y chan geslow ly or very q uickly depending on th esensor a nd w ha t i t moni tors . Whensigna ls ar e expressed a s w a ve forms ona n osci l loscope, the a na log s igna l showsup a s a f lowing l ine wi th curved pea ksa nd va l leys , indica t ing va r iable r ises anddrops in volta ge. The digita l s igna l ha sver t ica l r ises a nd drops , an d a hor izont a ll ine wit h sha rp corners. The tophorizont a l lines indicat e w hen th e volta geis high or on a nd t he bott om horizont a ll ines indicat e wh en th e volta ge is low oroff.




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When using a vol tmeter t o measuredigita l or a na log s ignals tha t chan ge veryquickly, such a s speed sensor or RP M

signals , it i s impor ta nt to remember th a tthe meter rea ding is not a t ruerepresent a tion of th e signa l . A voltmet erdisplays t he a vera ge reading of thes igna l . For ex ample, w i th a d ig it a l s igna lth e voltmeter w il l display t he avera gebetween zero volts (off) and the voltagew hen t he circuit w a s on. The comput erlooks for "on" s igna ls , not volta ge. Thevolt met er, however, is looking for volta ge,not wh ether a s igna l c omes thr ough . Avoltmeter ma y show t ha t th e volta ge is

w ithin specif ica tions even i f a pulse ismiss ing. Tha t m iss ing s igna l couldrepresent th e ca use of a n engine problem.You might not know it by t he voltmeter ,causing you to a ssume incorrect ly t heproblem is e lsewh ere and w a ste t imesear c h ing .

So if you suspect th e problem is in acert a in circuit, but t he voltm eter does notshow it , consider using a n oscilloscopefor a more accura te rea ding. At the very

least , you should be aw a re of thisvol tmeter l imita t ion w i th d igi ta l s ignals .

When dea l ing wi t h computer s igna ls i t i sa lso impor ta nt to remember tha t there is

a dif ference betw een the signa l sourcea nd t he source of the volta ge on t he signa lw ire. This is especial ly import a nt w hen a

sensor input goes t o more th a n onecomput er , such a s a speed sensor s ignal ,or i f the signa l is from one comput er toa nother . One computer ma y supply t hevolta ge to the sensor wh ich t oggles thevolta ge to ground, a nd t he oth er computerma y jus t moni tor the s igna l . I f a wire isdisconnected from the comput er th a tsupplies volta ge to the sensor , the signa lis lost t o both comput ers. Do not m ista keth is for a defective comput er.

Ana log s igna ls a lso have l imi ta t ions intha t t heir inputs are not usa ble by thecomputer unt i l t r a ns la t ed in to d igita lsigna ls . The A/D convert er ha ndles t ha tt r ans l a t ion .

This t a kes us briefly ba ck to computerla ngua ge. Digit a l on/off can berepresent ed by th e bina ry numberingsys t em of 0 (off) a nd 1 (on). Any decima lnum ber (1, 2, 3, etc.) can be representedusing O's a nd 1's so the comput er

unders ta nds . The several t housandtra ns is tors ins ide th e computer 'smicroprocessor ca n sw itch on a nd off incombinat ions tha t equa l any binarynum ber in a microsecond.




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The A/D convert er cha ng es th e a na logs ignal to th is binary la ngua ge by ta k ingsa mples of the an a log s ignal a t a

f requency known a s the sam pling ra te .The convert er measu res the w a ve anda ssigns a d igita l value to it . The higherthe sa mpling ra te , the c loser the digi ta ls ignal comes t o represent ing t he a na logone. In m ost cases each sa mple is dividedinto eight bits . E a ch bit is a ssigned eithera " 0" or a " 1" . These eight bit s a re ca l led aw ord. As i l lustra ted (below ), w henevert he A/D convert er sa mples t he sign a l, ita ss igns a b inary number to the vol tage a ttha t po int (wh ich t he computer reads a s a

ser ies of "ONs" a nd " OFFs" ), an d s l icesup the w a ve l ike a loaf of bread.

With th e signa l converted to eight-bitwords , the computer can use th e dat afrom t he sensor . The comput er t hensends out inst ructions in t he form of adigita l s igna l to a n a ctua tor . In mostcases th is w orks because most a c tuat orsa re solenoids or stepper motors w hichoperat e on digi ta l comma nds .

There a re, how ever , some component ssuch a s blow er motors or t he pow ersteering pump m otor on t he 1991 MR2,tha t require var ia ble volta ge to opera temotors a t va r iable speeds . In such ca ses ,t he comput er uses a D/A convert er t ochange t he digita l s ignal t o ana log. Theprinciples of D/A convert er opera t ion a ret he sa me a s t he A/D convert er. The

pulses of volta ge coming from th ecomputer a re converted t o var ia blevoltage.

THE MICROPROCESSORThe microprocessor is the hea rt of t hecomput er . I t is a lso cal led the centr a lprocessing un it (CP U ). Aga in, keep inmind tha t t he CP U does not per formcomplica ted opera tions. Inst ead , itperform s th ousan ds of s imple opera tionsincredibly fa st . To keep all of theopera t ions the CP U per forms f rombecoming enta ngled, i t execut es them inorder, pa ced by a clock.

The CP U can be divided int o threesections: th e contr ol section, thea r i thmetic a nd logic sect ion , a nd t heregister s ection.

The control section controls thecomput er 's ba sic opera t ions. I t ispr ogr a mmed w i th ins t r uc t ions f r om amemory t o ha ndle th ese chief operat ions:

Sendi ng dat a fr om one par t of the compu ter t o

another

Data in put an d output to and fr om the compu terAr i t hm et i c ca l cu l a t i ons

Ha l t i ng comput er opera t ions

Ju mp in g to ano ther i nst ru c t ion dur ing the

ru nn i n g of a p rogram

The a r i t hm etic an d logic section ca rr iesout the a ctua l process ing o f dat a , wh ichconsist s of ar i t hm etic opera tions an dlogical opera tions.




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The register section temporarily storesda ta or program s unt i l they are sent t oth e ar i t hmet ic a nd logic section or thecont rol section

COMPUTER MEMORY

Computers ha ve their own f i ling sys tem,k n ow n a s " m e m o r y ," w h i c h is t h ein ter na l ci r cu i t r y w her e pr ogr a ms a ndda ta a re s tored. Computer memory isdivided into sepa ra te a ddresses to w hichdat a is sent y th e CPU . The CPU thenknows w here to find tha t da ta when i t i sneeded. Computers use their ma inmemories for la rge a mounts o f dat a orprogra m informa t ion. There are t wokinds o f memory : ra ndom a ccessmem ory (RAM) a nd r ea d-only memory

(ROM).

RANDOM ACCESS MEMORY (RAM)

RAM is memory w hich the comput er ca nboth read from a nd w rite t o. This isw here the comput er stores dat a receivedfront sensors , such a s engine RP M orcoola nt t empera tu re. RAM works l iketh ousa nds o f toggle swi tches which can

be either on or off to represent 0's a nd 1's.This is how th e dat a is s tored in RAM.The sw itches w ork l ike spring loadedsw itches, therefore th ey must be held inth e on" posit ion electr ical ly . I f pow er islost , everyth ing st ored in RAM is lost .

In most of the comput ers used onToyota s, th e RAM is divided into tw osections. One section receives its powerfrom the ignit ion sw itch. This is wh ereda ta a bout opera t ing condi t ions , such a svehicle speed an d coolan t tempera tur e, isst ored. The oth er section, called K eepAlive Memory, is powered directly by th eba t t ery . In forma t ion such a s d iagnost iccodes is s tored in Keep Alive Memory sotha t i t is retained a f ter the ignit ion is off .

This is why a fuse or ba tt ery ca ble ha s tobe removed to clear dia gnost ic codes.




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READ-ONLY MEMORY (ROM)

This is w here th e basic operat inginst ructions for t he computer a re locat ed.

The inst ructions a re buil t int o the chipwhen i t i s ma nufac tur ed a nd c annot becha nged. The comput er ca n only read t heinforma t ion loca ted in ROM a nd can notwrite t o i t or use i t to store da ta . Since theinforma t ion in ROM is bui l t in dur ingma nufa cture , i t i s no t los t w hen pow er isremoved.

PROGRAMMABLE READ-ONLYMEMORY (PROM)

A P ROM is like a ROM except it ca n beprogra mmed or ha ve informa t ion wr i t t ent o it once. This is done before it isinst a l led in th e comput er . The comput ercan only read f rom the P ROM an d ca nnotwr ite to i t . The P ROM conta ins thespecif ic progra m inst ructions for thecomput er , such as the t iming a dva ncecurve for a par ticular engin e or th e shif tpoints for an a utoma t ic t ra nsmiss ion.There a re oth er ty pes of progra mm a ble

ROM being used, such a s erasa bleprogra mma ble rea d only memory(EP ROM) wh ich can be era sed byul tra violet light a nd reprogra mmed.Another t ype is electronica lly era sa bleprogra mma ble rea d only memory(EE P ROM) wh ich can be erasedelec tronica l ly a nd reprogra mmed. This isa ll done outside of th e computer by t hem a n u f a c t u r e r .

NON-VOLATILE MEMORY

Some computers use a type of RAM th a tis non-volat i le, meaning t ha t i t reta ins i tsmemory w hen t he power is removed.This t ype of memory ca n only be era sed bygoing t hrough a specif ic procedure. Thisis th e ty pe of memory u sed t o store code 41in the SRS a ir ba g sys tem on Cel ica a ndS u p r a .

LOGIC CIRCUITS

As comput ers a nd solid st a te cont rol

modules become more prevalent ona ut omobiles, some of th e logic gat esymbols tha t represent t heir

in terna l c ircuits w i l l show up more of ten .I t is necessary t o know n ot only wha t t helogic symbols sta nd for , but to understa ndth e basic opera tion of th e circuits t heyrepresent w hen you an a lyze w ir ingdiagrams dur ing t roubleshoot ing.Therefore, you should know a litt le a boutlogic circuits a nd t he symbols used to

represent th em. A logic gat e symbol iss imply a shor th a nd w a y of represent inga n electr onic circuit t ha t opera tes in acer ta in w ay . Un der sta nd ing the log icsymbols ca n ma k e under s ta nd ing theopera tion of a c ircuit mu ch quicker a ndeas ier tha n i f the c ircuit wererepresented by show ing a l l th etr a nsist ors , diodes an d resistors . Thelogic symbols shown in d ia gra ms in t heEWD a nd New C a r Fea ture book showwh a t pin vol tages must be present for a nelectr onic contr oller t o function properly.

Aga in , any th ing connec ted wi th acomput er is ba sed on t he digit a l on/offla ngu a ge. The sa me holds true for logicc ircui ts , wh ich a re ma de up oftra ns is tors combined in un i ts ca l led" ga tes. " These ga tes process tw o or mores igna ls logica l ly . In essence, they a resw itches. Depending on the input volta ge,th e ga te or swit ch will be eith er on or off .

The first th ing to lea rn a bout t he differentga tes is t heir symbols. Once you know th esymbols a nd how ea ch gat e works ,diagnosing a computer related problemw ill be easier .




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P age 7 Toyota Motor S ales, U .S.A., Inc. All Rights R eserved.


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Ta ken wit h permission from t heToyot a Adva nced E lectr ical C ourse#672




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COMPUTERS AND LOGIC CIRCUITS

ASSIGNMENT NAME:

1. Ex pla in both the pur pose and d i ff er en t t ypes of inputs used by the computer .

2. N a m e t h e t y pe of ou t pu t s ig n a l mos t of t en u sed b y t he com pu t er .

3. N a m e t h e com pon en t s t h a t a r e t y pica l ly u sed a s ou t pu t d ev ices .

4. E xpla i n t h e d if fer en ce bet w e en An a l og a n d D ig it a l S ig na l s.

5. Ex pla in both the pur pose and c omplete name of an A/D c onver ter .

6. D ra w bot h a n An a log a nd Digit a l sign a l.

7. E xpla i n th e bin a r y nu mber in g sy st em a n d w h y it i s u sed .

8. E x pla i n t h e f un ct ion of t h e M icr opr oces sor.

9. D escr ibe the pur pose of the RAM (Random Access Memor y)

10. Descr ibe the purpose of the ROM (Read Only Memory)

11. Descr ibe the purpose of the P ROM (P rogramma ble Read Only Memory)

12. Explain the bas ic funct ion and l ist the t ruth ta ble of an AND logic gat e ci rcui t .

13. Dra w the equivalent mecha nical ci rcui t of an AND logic gat e ci rcui t .

14. Explain the bas ic funct ion and l ist the t ruth ta ble of an OR logic gat e ci rcui t .

15. Dra w the equivalent mechanical ci rcui t of an OR logic gat e ci rcui t .

16. Descr ibe the bas ic funct ion a nd l is t the t ruth ta ble of a NOT logic gat e ci rcui t .

17. Descr ibe the bas ic funct ion a nd l is t the t ruth ta ble of a NAND logic gat e ci rcui t .

18. Descr ibe the bas ic funct ion a nd l is t the t ruth ta ble of a NOR logic gat e ci rcui t .

19. Descr ibe are the two bas ic components of a FLIP-FLOP logic gat e ci rcui t .

Computers&LogicCircuits

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