The Circuit Designer's Companion - diagramas.diagram...

More documents

Recommendations

Info

Digital circuits 203specified as 10Mbaud, with a trade-off against cable length; maximum cable length at100kbaud is 4000 feet. Note that unlike EIA-232F, EIA-422 does not specify functionalor mechanical parameters of the interface. These are included in other standards whichincorporate it, notably EIA-449 and EIA-530.EIA-422 achieves its high-speed and long-distance capabilities by specifying abalanced and terminated design. The balanced design reduces sensitivity to externalcommon mode noise and allows a ground differential of up to a few volts to existbetween the driver and one or more of the receivers without affecting the receiver’sthresholds. A cable termination, together with increased driver current, allows fast slewrates which in turn allows high data rates. If the cable is not terminated, serious ringingon the edges occurs which may cause spurious switching in the receiver. The specifiedtermination of 100Ω is closely matched to the characteristic impedance of typicaltwisted pair cables. Only one termination is used, at the receiver at the far end of thecable.Interface designBy far the easiest way to realise either EIA-232F or EIA-422 interfaces is to use one ofthe many specially-tailored driver and receiver chip sets that are available. The morecommon ones, such as the 1488 driver/1489 receiver for EIA-232F or the 26LS31driver/26LS32 receiver for EIA-422, are available competitively from many sourcesand in low-power CMOS versions. You can also obtain combined driver/receiver partsso that a small interface can be handled with one IC. Because the 9-pin implementationof EIA-232F is so common, a single package 3-transmitter plus 5-receiver part is alsowidely sourced. The high-voltage requirement of EIA-232F, typically ±12V supplies,is addressed by some suppliers who offer on-chip dc-to-dc converters from the +5Vrail.Figure 6.21 suggests typical interface circuits for the two standards. Note theinclusion of power supply isolating diodes, to protect the rest of the circuit against theinevitable overvoltages that will come its way. You can also construct an interface,particularly the simpler EIA-232F, using standard components such as op-amps,comparators, CMOS buffer devices or discrete components if you are prepared to spendsome time characterising the circuit against the requirements of the standard andagainst expected overload conditions. This may turn out to be marginally cheaper incomponent cost, but its overall worth is somewhat questionable.6.2.6 High performance data interface standardsThis section briefly reviews some of the newer data interface standards that have grownup for high-speed purposes around particular applications and have subsequentlybecome more widely entrenched.EIA-485EIA-485 shares many similarities with EIA-422, and is widely used as the basis for inhouseand industrial datacomm systems. For instance, one variant of the SCSI interface(HVD-SCSI: high voltage differential – small computer systems interface) uses 485 asthe basis for its electrical specification. 485-compliant devices can be used in 422systems, though the reverse is not necessarily true. The principal difference is that 485allows multiple transmitters on the same line, driving up to 32 unit loads, with halfduplex(bidirectional) communication. One Unit Load is defined as a steady-state loadallowing 1mA of current under a maximum common mode voltage of 12V or 0.8mA at
204 The Circuit Designer’s Companion+10>12V1/4 1488etcensures min. 300Ω power-off R O+5VPS protection diodes and local decoupling preferable1/4 1489etc-10>12V0Vshielded twisted pair preferablebut not essentialoutput protectioninput protectionand slew rate limiting (but see text)0Vbandwidth shapingto reduce noise(a) EIA-232F+5V1/4 26LS31etcPS protection diodes and local decoupling necessaryterminating resistor 100Ω+5V1/4 26LS32etc0Vshielded twisted pairnecessary0V-5V(b) EIA-422Figure 6.21 Typical EIA-232F and EIA-422 interface circuits–7V. ULs may consist of drivers or receivers and failsafe resistors (see below), but donot include the termination resistors. The bidirectional communication means that 485drivers must allow for line contention and for driving a line that is terminated at eachend with 120Ω. The two specifications are compared in Table 6.2.One further problem that arises in a half-duplex system is that there will be periodswhen no transmitters are driving the line, so that it becomes high impedance, and it isdesirable for the receivers to remain in a fixed state in this situation. This means that adifferential voltage of more than 200mV should be provided by a suitable passivecircuit that complies with both the termination impedance requirements and the unitload constraints. A network designed to do this is called a “failsafe” network.CANThe Controller Area Network standard was originally developed within the automotiveindustry to replace the complex electrical wiring harness with a two-wire data bus. Ithas since been standardised in ISO 11898. The specification allows signalling rates upto 1MB/s, high immunity from electrical interference, and an ability to self-diagnoseand repair errors. It is now widespread in many sectors, including factory automation,medical, marine, aerospace and of course automotive. It is particularly suited toapplications requiring many short messages in a short period of time with highreliability in noisy operating environments.The ISO 11898 architecture defines the lowest two layers of the OSI/ISO sevenlayer model, that is, the data-link layer and the physical layer. The communicationprotocol is carrier-sense multiple access, with collision detection and arbitration onmessage priority (CSMA/CD+AMP). The first version of CAN was defined in ISO11519 and allowed applications up to 125kB/s with an 11-bit message identifier. The
Page 2:
The Circuit Designer’s Companion
Page 5 and 6:
NewnesAn imprint of ElsevierLinacre
Page 7 and 8:
vi Contents2.2 Design rules 452.2.1
Page 9 and 10:
viii ContentsChapter 5Analogue inte
Page 11 and 12:
x Contents7.5.3 Secondary cells 256
Page 13 and 14:
xii Contents
Page 15 and 16:
2 The Circuit Designer’s Companio
Page 17 and 18:
Page 19 and 20:
Page 21 and 22:
Page 23 and 24:
10 The Circuit Designer’s Compani
Page 25 and 26:
Page 27 and 28:
Page 29 and 30:
Page 31 and 32:
Page 33 and 34:
Page 35:
Page 38 and 39:
Grounding and wiring 25Cable type R
Page 40 and 41:
Grounding and wiring 27Shielding an
Page 42 and 43:
Grounding and wiring 29successive h
Page 44 and 45:
Grounding and wiring 31(a) Two audi
Page 46 and 47:
Grounding and wiring 331. Side-by-s
Page 48 and 49:
Grounding and wiring 35ABV pZ out =
Page 50 and 51:
Grounding and wiring 37times will d
Page 52 and 53:
Grounding and wiring 39of Z o . Thu
Page 54 and 55:
Printed circuits 41material. The co
Page 56 and 57:
Printed circuits 43board cost you s
Page 58 and 59:
Printed circuits 45Board ABoard ABo
Page 60:
Printed circuits 47which they can w
Page 63 and 64:
Page 65 and 66:
Page 67 and 68:
Page 69 and 70:
Page 71 and 72:
Page 73 and 74:
Page 75 and 76:
Page 77 and 78:
Page 79 and 80:
Page 81 and 82:
Page 83 and 84:
Page 85 and 86:
Page 87 and 88:
Page 89 and 90:
Page 91 and 92:
Page 93 and 94:
Page 95 and 96:
Page 97 and 98:
Page 99 and 100:
Table 3.3 Survey of capacitor types
Page 101 and 102:
Page 103 and 104:
Page 105 and 106:
Page 107 and 108:
Page 109 and 110:
Page 111 and 112:
Page 113 and 114:
100 The Circuit Designer’s Compan
Page 115 and 116:
Page 117 and 118:
Page 119 and 120:
Page 121 and 122:
Page 123 and 124:
Page 125 and 126:
Page 127 and 128:
Page 129 and 130:
Page 131 and 132:
Page 133 and 134:
Page 135 and 136:
Page 137 and 138:
Page 139 and 140:
Page 141 and 142:
Page 143 and 144:
Page 145 and 146:
Page 147 and 148:
Page 149 and 150:
Page 151 and 152:
Page 153 and 154:
Page 155 and 156:
Page 157 and 158:
Page 159 and 160:
Page 161 and 162:
Page 163 and 164:
Page 165 and 166: 152 The Circuit Designer’s Compan
Page 215: 202 The Circuit Designer’s Compan
Page 267 and 268:
Page 269 and 270:
Page 271 and 272:
Page 273 and 274:
Page 275 and 276:
Page 277 and 278:
Page 279 and 280:
Page 281 and 282:
Page 283 and 284:
Page 285 and 286:
Page 287 and 288:
Page 289 and 290:
Page 291 and 292:
Page 293 and 294:
Page 295 and 296:
Page 297 and 298:
Page 299 and 300:
Page 301 and 302:
Page 303 and 304:
Page 305 and 306:
Page 307 and 308:
Page 309 and 310:
Page 311 and 312:
Page 313 and 314:
Page 315 and 316:
Page 317 and 318:
Page 319 and 320:
Page 321 and 322:
Page 323 and 324:
Page 325 and 326:
Page 327 and 328:
Page 329 and 330:
Page 331 and 332:
Page 333 and 334:
Page 335 and 336:
Page 337 and 338:
Page 339 and 340:
Page 341 and 342:
Page 343 and 344:
Page 345 and 346:
Page 347 and 348:
334 Indexde-rating 310dielectric ab
Page 349 and 350:
336 IndexFlash ADC 209Foldback curr
Page 351 and 352:
338 IndexMultilayer PCB constructio
Page 353 and 354:
340 Indexlimiting element voltage 7
show all

The Circuit Designer's Companion - diagramas.diagram...

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?