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

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