Specifications

Chapter 1 - Overview RS-422 And RS-485 Applications Ebook
Page 14 of 137 Manual Documentation Number: <DocNumber>
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Some transceiver chips are manufactured with their driver and receiver
data lines connected internally, permanently configuring them as two-
wire devices.
Figure 7: RS-485 Chip with internal bridging
The EIA-485 specification labels the data lines A and B. Some
manufacturers label two-wire systems DATA A and DATA B and four-
wire systems TDA, TDB, RDA and RDB. Other manufacturers simply
label them "-" and "+". (In this document, and on B&B Electronics
equipment, two-wire systems are labeled DATA A(-) and DATA B(+)
and four-wire systems are labeled TDA(-), TDB(+), RDA(-) and
RDB(+). ) Although it is impossible to predict how other manufacturers
will label these lines, practical experience suggests that the "-" line
should be connected to the A terminal and the "+" line should be
connected to the B terminal. Reversing the polarity will not damage an
RS-485 device, but it will not communicate.
Common Mode Voltage
One of the reasons that balanced data transmission is superior to
unbalanced is that most noise picked up by the communication line is