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

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