Specifications

Chapter 1 - Overview RS-422 And RS-485 Applications Ebook
Page 10 of 137 Manual Documentation Number: <DocNumber>
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When the TD line is in the idle state (not transmitting), this voltage is
negative (representing a logic 1 or mark). A logic 0 or space is
represented by a positive voltage. When transmitting data the voltage
alternates between that negative level and a positive level. Typically
data pulses are transmitted with a magnitude somewhere between ±5
and ±15 volts. Although the EIA-232 standard allows a range of ±3
volts to ±25 volts, an RS-232 receiver typically operates within the
voltage range of ±3 volts and ±12 volts.
Figure 3: RS-232 Transmission Signals
Unbalanced data transmission works well over short distances, at
limited baud rates, especially in low noise environments. Unfortunately,
unbalanced systems allow capacitively and inductively coupled noise to
be superimposed onto the signal (TD) line. If the noise attains levels
that compete with the data signal, errors will occur. In environments
where electrical noise is prevalent, RS-232 should only be used over
short distances at lower baud rates. The EIA-232 standard limits cable
length to 50 feet and bit rates to 20 kbps (although in ideal situations
these limits can be exceeded).
Note: RS-423 also uses unbalanced data transmission. For information on RS-423 refer to EIA
Standard RS-423 Data Transmission (on page 89).