Chassis Plans ATXP-875P, Техническое описание

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Appendix C: Communications Devices ATXP-875P – Technical Reference
74 Chassis Plans
RS-422 is Differential and may be either Point-to-Point or Multi-Drop Connected
Differential, Point-to-Point
Differential, Multi-Drop
Differential Transmission
For balanced or differential transmission, a pair of signal lines is necessary for each channel. On one line, a true
signal is transmitted, while on the second one, the inverted signal is transmitted. The receiver detects voltage
difference between the inputs and switches the output depending on which input line is more positive. As shown
below, there is additionally a ground return path.
Balanced interface circuits consist of a generator with differential outputs and a receiver with differential inputs.
Better noise performance stems from the fact that noise is coupled into both wires of the signal pair in much the
same way and is common to both signals. Due to the common mode rejection capability of a differential amplifier,
this noise will be rejected. Additionally, since the signal line emits the opposite signal like the adjacent signal return
line, the emissions cancel each other. This is true in any case for crosstalk from and to neighboring signal lines. It is
also true for noise from other sources as long as the common mode voltage does not go beyond the common mode
range of the receiver. Since ground noise is also common to both signals, the receiver rejects this noise as well. The
twisted pair cable used in these interfaces in combination with a correct line termination—to avoid line reflections—
allows very high data rates and a cable length of up to 1200 m.
Advantages of Differential Transmission
Differential data transmission schemes are less susceptible to common-mode noise than single-ended schemes.
Because this kind of transmission uses two wires with opposite current and voltage swings compared to only one
wire for single-ended, any external noise is coupled onto the two wires as a common mode voltage and is rejected by
the receivers. This two-wire approach with opposite current and voltage swings also radiates less electro-magnetic
interference (EMI) noise than single-ended signals due to the canceling of magnetic fields.