Specifications

Chapter 2 - System Configuration RS-422 And RS-485 Applications Ebook
Page 38 of 137 Manual Documentation Number: <DocNumber>
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applications. However, the total current requirement should still be
calculated, especially in cases such as when using port-powered
converters, which may not be able to supply enough current.
Other Biasing and Terminating Considerations
Standard RS-485 Biasing Resistors. Most B&B RS-485 equipment
includes 4.7 k pull-up and pull-down bias resistors from the factory.
This value is adequate for most systems without termination. If
termination is required it may be necessary to replace these resistors
with a different value.
Standard RS-422 Biasing Resistors. Some equipment that is used
for four-wire operation uses RS-422 chips. (E.g. B&B 422LCOR) In
these devices the included bias resistors are 1K When RS-422 is used
for an RS-485 master node the maximum number of nodes is reduced.
A repeater may be needed, especially when termination is used.
The system designer should always calculate the biasing requirements
of the network.
Biasing Resistor Locations. In an RS-485 four-wire configuration,
the bias resistors are ideally placed on the receiver lines. B&B
Electronics models that provide two- or four-wire modes of operation
(485OPDRI, 485DRCI, and FOSTCDRI) have selectable transmit and
receive line biasing resistors. Models such as 4WSD9R, 4WSD9TB, and
485BAT3 have bias resistors on their transmit lines.
Under-biasing. Symptoms of under-biasing range from decreased
noise immunity to complete data failure. Over-biasing has less effect
on a system, the primary result is increased load on the drivers.
Systems using port powered RS-232 to RS-485 converters can be
sensitive to over biasing.
Network Biasing. Bias is typically provided on the master node in
most networks.