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Digital circuits 201terminating equipment”, also abbreviated to DCE. It does not clarify exactly what is aDTE and what is a DCE, and since many applications are simple DTE (computer) toDTE (terminal or printer) connections, it is often open to debate as to what is at whichend of the interface. Although a point-to-point connection provides the correct pinterminations for DTE-to-DCE, a useful extra gadget is a cable known as a “nullmodem” (Figure 6.19) which creates a DTE-to-DTE connection. The common sight ofan installation technician crouched over a 9-way connector swapping pins 2 and 3, tomake one end’s receiver listen to the other end’s driver, has yet to disappear.Figure 6.19 The null modemDCD 1RD 2TD 3DTR 4GND 5DSR 6RTS 7CTS 8RI 9DB9S connectors1 DCD2 RD3 TD4 DTR5 GND6 DSR7 RTS8 CTS9 RIEIA-232F’s transmission distance is limited by its unbalanced design and restrictedTable 6.2 Major electrical characteristics of EIA-232F, EIA-422 and EIA-485Interface EIA-232F EIA-422 EIA-485Line type Unbalanced, point to point Balanced, differential,multidrop (one driver per bus)Balanced, differential,multiple drivers per bus (halfduplex)Line impedance Not applicable 100Ω 120ΩMax. line lengthLoad dependent, typically15m depending oncapacitanceL ≈ 10 5 /B metresB = bit rate, kB/sMax. recommended 1200m,depending on attenuationMax data rate 20kB/s 10MB/s 10MB/sDriverOutput voltage ±5 to ±15V loaded with 3–7kΩ+V = logic 0, –V = logic 1±10V max differentialunloaded, ±2V min loadedwith 100Ω±6V max differentialunloaded, ±1.5V min loadedwith 54ΩShort circuit current 500mA max 150mA max 150mA to gnd, 250mA to -7or +12VRise time4% of unit interval (1ms max)30V/µs max. slew rate10% of unit interval (min20ns)30% of unit intervalOutput with power off > 300Ω output resistance ±100µA max leakage > 12kΩ output resistanceReceiverSensitivity ±3V max thresholds ±200mV ±200mVInput impedance 3kΩ–7kΩ, < 2500pF 4kΩ min 12kΩCommon mode range Not applicable ±7V +12 to -7V
202 The Circuit Designer’s Companiondrive current. The unbalanced design is very susceptible to external noise pick-up andto ground shifts between the driver and receiver. The limited drive current means thatthe slew rate must be kept slow enough to prevent the cable becoming a transmissionline, and this puts a limit on the fastest data rate that can be accommodated. Maximumcable length, originally fixed at 50 feet, is now restricted by a requirement for maximumload capacitance (including receiver input) for each circuit of 2500pF. As the linelength increases so does its capacitance, requiring more current to maintain the sametransition time. The graph of Figure 6.20 shows the drive current versus loadcapacitance required to maintain the 4% transition time relationship at different datarates. In practice, the line length is limited to 3 metres or less for data rates more than20kb/s. Most drivers can handle the higher transmission rates over such a short lengthwithout drawing excessive supply current.4030Driver output current mAfor rise time = 4% of unit interval20116kb/s50kb/sFigure 6.20 EIA-232F transmit driver outputcurrent versus C L 0 500 1000 1500 2000 250010Load capacitance pFNote that there are several common “enhancements” that are not permitted by strictadherence to the standard. EIA-232F makes no provision for tri-stating the driveroutput, so multiple driver access to one line is not possible. Similarly, parallelingreceivers is not allowed unless the combined input impedance is held between 3kΩ and7kΩ. It does not consider electrically isolated interfaces: no specification is offered forisolation requirements, despite their desirability. It does not specify the communicationdata format. The usual “one start bit, eight data bits, two stop bits” format is not part ofthe standard, just its most common application. It is not directly compatible withanother common single-ended standard, EIA-423, although such connections willusually work. Also, you cannot legitimately run EIA-232F off a ±5V supply rail − theminimum driver output voltage is specified as ±5V, loaded with 3−7kΩ and with anoutput impedance of 300Ω.The standard calls for slew-rate limiting to 30V/µs maximum. Although you can dothis with an output capacitor, which operates in conjunction with the output transistor’scurrent limit while it is slewing, this will increase the dissipation, and reduces themaximum possible cable length. It is preferable to use a driver which has on-chip slewrate limiting, requiring no external capacitors and making the slew rate independent ofcable length.EIA-422Many data communications applications now require data rates in the megabaudregion, for which EIA-232F is inadequate. This need is fulfilled by the EIA-422standard, which is an electrical specification for drivers and receivers for use in abalanced or differential, point-to-point or multi-drop high speed interface using twistedpair cable. Table 6.2 summarises the EIA-422 specification in comparison with EIA-232F. One driver and up to ten receivers are allowed. The maximum data rate is10kb/s20kb/s
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The Circuit Designer’s Companion
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NewnesAn imprint of ElsevierLinacre
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vi Contents2.2 Design rules 452.2.1
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viii ContentsChapter 5Analogue inte
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x Contents7.5.3 Secondary cells 256
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xii Contents
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2 The Circuit Designer’s Companio
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10 The Circuit Designer’s Compani
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Grounding and wiring 25Cable type R
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Grounding and wiring 27Shielding an
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Grounding and wiring 29successive h
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Grounding and wiring 31(a) Two audi
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Grounding and wiring 331. Side-by-s
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Grounding and wiring 35ABV pZ out =
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Grounding and wiring 37times will d
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Grounding and wiring 39of Z o . Thu
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Printed circuits 41material. The co
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Printed circuits 43board cost you s
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Printed circuits 45Board ABoard ABo
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Printed circuits 47which they can w
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Table 3.3 Survey of capacitor types
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334 Indexde-rating 310dielectric ab
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336 IndexFlash ADC 209Foldback curr
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338 IndexMultilayer PCB constructio
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340 Indexlimiting element voltage 7
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