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1、ProgrammemoryCPUProgrammemoryCPUInput&Output unitDatamemorymemoryCPUInput&Output unitFig.3-5A-2.A conventional Princeton computerExternalTiming componentsSystemclockTimer/CounterSerial I/OResetROMPrarallelI/OInterruptsRAMCPUPowerThe single-chip microcomp uter is the culmination of both the developme

2、 nt of the digital comp uter and the integrated circuit arguably the tow most significant inventions of the 20th centuryThese tow types of architecture are found in single-chip microcomputer. Some employ the split program/data memo ry of the Harvard architecture, shown in Fig. 3-5A-1, others follow

3、the philosophy, widely adapted for general-purpose comp uters and microprocessors, of making n o logical d istinction b etween p rogram a nd d ata memo ry as in the Princeton architecture, shown in Fig. 3-5A-2.In general terms a single-chip microcomputer is characterized by the incorporation o f a l

4、l the u nits o f a c omputer into a s ingle d evice, a s shown in Fig3-5A-3.Fig.3-5A-1A Harvard typeFig3-5A-3. Principal features of a microcomputerRead only memory (ROM)ROM i s u sually f or t he p ermanent, n on-volatile s torage o f a n a pplications program .Many microcomputers and microcontroll

5、ers are intended for high-volume applications and hence the economical manufacture of the devices requires that the contents of the program memory be committed permanently during the manufacture of chips . Clearly, this implies a rigorous approach to ROM code development since changes cannot be made

6、 a fter manufacture . This d evelopment p rocess m ay i nvolve e mulation using a sophisticated development system with a hardware emulation capability as well as the use of powerful software tools.Some manufacturers provide additional ROM options by including in their range devices with ( or intend

7、ed for use with ) user programmable memory. The simplest of these is usually device which can operate in a microprocessor mode by using some of the input/output lines as an address and data bus for accessing external memory. This type of device can behave functionally a s the single c hip microcompu

8、ter f rom w hich i t is derived a lbeit w ith r estricted I /O a nd a mo dified e xternal c ircuit. T he u se of these ROM less devices is common even in production circuits where the volume does not justify the development costs of custom onchip ROM2there can still be a significant saving in I/O an

9、d other chips comp ared to a conventional microprocessor based circuit. More exact replacement for ROM d evices can be obtained in the f orm o f v ariants w ith piggybackEPROMErasable programmable ROMsockets or devices with EPROM instead of ROM 。 These devices are naturally more expensive than equiv

10、alent ROM device , but do provide comp lete circuit equivalents. EPROM based devices are also extremely attractive for lowvolume applications where they provide the advantages of a singlechip devicein terms of onchip I/Oetc. , with the convenience of flexible user programmability.Random access memor

11、y (RAM).RAM is for the storage of working variables and data used during program execution. The size of this memo ry varies with device type but it has the same characteristic width (4,8,16 bits etc. ) as the processor ,Special function registers, such as stack pointer or timer register are often lo

12、gically incorporated into the RAM area. It is also common in Hard type microcomputers to treat the RAM area as a collection of register; it is unnecessary to make distinction between RAM and processor register as is done in the case of a microprocessor system since RAM and registers are not usually

13、physically separated in a microcomputer .Central processing unit he CPU is much like that of any microprocessor. Many applications of microcomputers and microcontrollers involve the handling o f b inary-coded d ecimal ( BCD) d ata ( for n umerical d isplays, f or example) ,hence it is common to find

14、 that the CPU is well adapted to handling this type of data . It is also comm on to find good facilities fortesting, s etting a nd r esetting i ndividual b its o f m emory o r I /O s ince m any controllerapplicationsinvolvetheturningonandoffofsingleoutput lines or the reading the single line.These l

15、ines are readily interfaced to two-state devices s uch a s s witches, thermo stats, s olid-state relays, v alves, motor, etc.Parallel input/output.Parallel input and output schemes vary somewhat in different microcomputer; in mo st a mechanism is provided to at least allow some flexibility of choosi

16、ng which pins are outputs and which are inputs. This may apply to all or some of the ports. Some I/O lines are suitable for direct interfacing to, for example, fluorescent displays, or can provide sufficient current to make interfacing other components straightforward. Some devices allow an I/O port

17、 to be configured as a system bus to allow off-chip memory and I/O expansion. This facility is potentially useful as a p roduct r ange d evelops, s ince s uccessive e nhancements m ay b ecome t oo big for on-chip memory and it is undesirable not to build on the existing software base.Serial input/ou

18、tput .Serial communication with terminal devices is common means of providing a link using a small number of lines. This sort of communication can also be exploited for interfacing special function chips or linking several microcomputers together .Both the common asynchronous synchronous communicati

19、on scheme s require protocols that provide framing (start and stop) information . This can be implemented as a hardware facility or U(S) ART(Universal(synchronous) asynchronous receiver/transmitter) relieving the processor (and the applications programmer) of this low-level, time -consuming, detail.

20、 t is merely necessary to selected a baud-rate and possibly other options (numb er of stop bits, parity, etc. ) and load (or read from) the serial transmitter (or receiver) buffer. Serialization of the data in the appropriate format is then handled by the hardware circuit.Timing/counter facilities.M

21、any application of single-chip microcomp uters require accurate evaluation of elapsed real time . This can be determined by carefula s s e s s m e n t o f t h e e x e c u t i o n t i m e o f e a c h b r a n c h i n a p r o g r a m b u t t h i srapidly becomes ineff icient for all but simplest progra

22、ms . The preferred approach is t o u se t imer c ircuit that c an independently count p recise time increments and generate an interrupt after a preset time has elapsed . This type of timer is usually arranged to be reloadable with the requiredc o u n t . T h e t i m e r t h e n d e c r e m e n t s

23、t h i s v a l u e p r o d u c i n g a n i n t e r r u p t o rsetting a flag when the counter reaches zero.Better timers then have the ability to automa tically reload the initial count value. T his relieves the programmer of the responsibility of reloading the counter and assessing elapsed time befo

24、re the timer restarted ,which otherwise wound benecessary if continuous precisely timed interrupts were required (as in a clock ,for examp le). Sometimes associated with time r is an event counter. Wi th this facility there is usually a special input pin , that can drive the counter directly.Timingc

25、omponents.The clock circuitry of most microcomputers requires only simple timing comp onents. If maximum performa nce is required, a crystal must be used to ensure the maximum clock frequency is approached but not exceeded. Many clock circuits also work with a resistor and capacitor as low-cost timi

26、ng components or can be driven from an external source. This latter arrangement is useful is external synchronization of the microcomputer is required.B:PLC1PLC s (programmable logical controller) face ever mo re complex challenges these days . Where once they quietly replaced relays and gave an occ

27、asional report to a corporate mainframe, they are now grouped into cells, given new job and new languages, and are forced to compete against a growing array of control products. For this years annual PLC technology update ,we queried PLC makers on these topics and more .Programming languagesHigher l

28、evel P LC p rogramming languages h ave been a round f or s ome time ,but lately their popularity h as mushrooming. As Raymond Leveille, vice president & general manager, Sieme ns Energy &Automation .inc; Programmable controls are being used for more and mo re sophisticated operations, languages othe

29、r than ladder logic become mo re practical, efficient, and powerful. For example, its very difficult to write a trigonometric f unction u sing ladder logic . Languages g aining acceptance include Boolean, control system flowcharting, and such function chart languages a s Graphcet a nd i ts variation

30、 . And theres increasing interest in languages like C and BASIC.PLCs in process controlThus far, PLC s have not been used extensively for continuous process control . Will this continue? The feeling that Ive gotten, says Ken Jannotta, manger, product planning, series One and Series Six product ,at G

31、E Fanuc North Ame rica ,is that PLC s will be used in the process industry but not necessarily for process control. Several vendors -obviously betting that the opposite will happen-have introduced PLC s optimized for process application .Rich Ryan, manger, commercial marketing, Allen-bradley Program

32、mable Controls Div., cites PLC ss increasing use such industries as food ,chemicals ,and petroleum. Ry an f eels t here a re t wo t ypes o f a pplications i n which t heyre appropriate. one, he says, is where the size of the process control system thats being automated doesnt justify DCSdistributed

33、control system. Wi th the starting price tags of chose products being relativelyhigh, a programmable controller makes sense for small, low loop count application . The second is where you have to integrate the loop closely with the sequential logical . Batch controllers are prime example ,where the

34、sequence and maintaining the process variable are intertwined so closely that the benefits of having a programmable controller to do the sequential logical outweighs some of the disadvantages of not having a distributed control system. Bill Barkovitz, president of Tr iconex, predicts that all future

35、 controllers that come out in the process control system business will embrace a lot of more PLC technology and a lot more PLC functionality than they ever did before . Communications and MAPCommunications are vital to an individual automation cell and to be automated factory a s a whole. We ve hear

36、d a lot about MAP i n the last few yea rs, andalotofcompanieshavejumpedonthebandwagon. 2Many, however,w eredisappointedw henafully-definedandcomp letedM A P specification d idnt a ppear i mmediately . Says L arry K omarek: Right n ow, MAP i s s till a moving targ et for the manufacturers, a s pecifi

37、cation that is not f inal . Presently, for e xample. p eople a re i ntroducing p roducts to meet theMAP2. 1standard.Yet2. 1-basedproductsw illbeobsoletew hent h e new standard for MAP3. 0 is introduced. Because of this, many PLC vendors are holding off on full MAP implementations. O mron, f or e xam

38、p le, h as a n o ngoing M AP-compatibility program;3but Frank Newburn, vice president of Omrons Industrial Division ,reports that because of the lack of a firm definition ,Omrons PLC s dont yet talk to MAP.Since its unlikely that an individual PLC would talk to broad MAP anyway, makers are concentra

39、ting on proprietary networks. According to Sal Provanzano, users fear that if they do get on board and vendors withdraw from MAP, theyll be the ones left holding a communications structure thats not supported.Universal I/OWhile there are concerns about the lack of compatible communications between P

40、LC s from different vendors, the connection at the other end-the I/O-is even mo re fragmented . Wi th rare exceptions, I/O is still proprietary . Yet there are those who feel that I/O will eventually become more universal .GE Fanuc is hoping to do that with its Genius smart I/O line. The independent

41、 I/O makers are pulling in the same direction.Many say that I/O is such a high-value item that PLC makers will always want to keep it proprietary .As Ke n Jannotta, says: The I/O is going to be a disproportionate amount of the hardware sale. Certainly each P LC vendor i s going to try to p rotect t

42、hat. For that reason, h e s ays, PLC makers wont begin selling universal I/O system from other vendor.if we start selling that kind of product, says jannotta, what do we manufacture?Wi th more intelligent I/O appearing, Sal Provanzano feels this will lead t o mo re d ifferentiation a mong I /O f rom

43、 d ifferent m akers. Where t he I/O becomes extremely intelligent and becomes part of the system, he says, it really is hard to define which is the I/O and which is CPU. It really CPU, if you will, is equally integrated into the system as the I/O. Connecting PLC I/O to PCsWhile different PLC s proba

44、bly will continue to use proprietary I/O, several vendors make it possible to connect5 their I/O to IBM PC-compatibleequipment. Alle-bradeley, Could, and Cincinnati Milacron already have, and rumor has it that GE is planning something along these same lines . 4Bill Ketelhut, manage of product planning at GE Fanuc North America ,sees this sort of thing as alternative to universal I/O. I think the trend ,instead of toward universal I/O, will be multiple host interface , he says .Jodie Glore ,director of marking, Square D Automatio