AppendixB SerialPort Information
SerialHardware and Software for Windows B-2 ni.com
distancesof 50 ftand with data rates up to 20 kbytes/s. However, because
ofimprovements in line drivers and cabling, you can usually increase the
actual performance of the bus past the limitations on speed and distance
recommended int he specification.
RS-422
As specifiedin the EIA/RS-422-A St andard, Electrical Characteristics of
Balanced Voltage Digital Interface Circuits, RS-422 defines a serial
interfacemuch like RS-232. However, RS-422 uses balanced
(or differential)transmission lines. Balanced tran smission lines use two
transmissionlines for each signal. The state of each signal is represented,
not bya voltage level on one line referenced to ground as in RS-232, but
rather bythe relative voltage of the two lines to each other. For example,
the TX signal is carried on twowires, wire A and wire B. A logical 1 is
represented bythe voltage on line A being greater than the voltage on
line B. A logical 0 is represented by the voltageon li ne A being less than
thevoltage on line B. Differential voltage transmission creates a signal that
is more immune to noise as well as voltageloss due to tr ansmission line
effects.Thus, you can use RS-422 for longer distances (up to4,000 ft) and
greatertransmi ssion speeds (up to 10 Mbytes/s)than RS-232.
RS-485
As specifiedin the EIA-485 Standard, Standard for Electrical
Characteristicsof Generators and Receivers for Use in Balanced Digital
MultipointSystems, RS-485 expands on the RS-422 standard by increasing
the number of devicesyou can use from 10 t o 32 and byworking with
half-duplexbus architectures. Unlike the RS-42 2 standard, RS-485
addressesthe issue of using multiple transmitters on t he same line. RS-485
definesthe electrical characteristics necessary to ensure adequate signal
voltagesunder maxim um load, short-circuit protection. RS-485 can also
withstand multiple driversdriving conflicting signals at the same time.