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Network
Topologies
Network configuration isn't defined in the RS-422 or RS-485
specification. In most cases the designer can use a configuration
that best fits the physical requirements of the system.
Two Wire
or Four Wire Systems
RS-422 systems require a dedicated pair of wires for each
signal, a transmit pair, a receive pair and an additional
pair for each handshake/control signal used (if required).
The tristate capabilities of RS-485 allow a single pair of
wires to share transmit and receive signals for half-duplex
communications. This "two wire" configuration (note that an
additional ground conductor should be used) reduces cabling
cost. RS-485 devices may be internally or externally configured
for two wire systems. Internally configured RS-485 devices
simply provide A and B connections (sometimes labeled "-"
and "+").
Devices configured for four wire communications
bring out A and B connections for both the transmit and the
receive pairs. The user can connect the transmit lines to
the receive lines to create a two wire configuration. The
latter type device provides the system designer with the most
configuration flexibility. Note that the signal ground line
should also be connected in the system. This connection is
necessary to keep the Vcm common mode voltage at the receiver
within a safe range. The interface circuit may operate without
the signal ground connection, but may sacrifice reliability
and noise immunity. Figures 2.1 and 2.2 illustrate connections
of two and four wire systems.
Figure 2.1 Typical RS-485 Four Wire Multidrop Configuration
Figure 2.2 Typical RS-485 Two Wire Multidrop Configuration
Termination
Termination is used to match impedance of a node to the impedance
of the transmission line being used. When impedance are mismatched,
the transmitted signal is not completely absorbed by the load
and a portion is reflected back into the transmission line.
If the source, transmission line and load impedance are equal
these reflections are eliminated. There are disadvantages
of termination as well. Termination increases load on the
drivers, increases installation complexity, changes biasing
requirements and makes system modification more difficult.
The decision whether or not to use termination
should be based on the cable length and data rate used by
the system. A good rule of thumb is if the propagation delay
of the data line is much less than one bit width, termination
is not needed. This rule makes the assumption that reflections
will damp out in several trips up and down the data line.
Since the receiving UART will sample the data in the middle
of the bit, it is important that the signal level be solid
at that point. For example, in a system with 2000 feet of
data line the propagation delay can be calculated by multiplying
the cable length by the propagation velocity of the cable.
This value, typically 66 to 75% of the speed of light (c),
is specified by the cable manufacturer.
For our example, a round trip covers
4000 feet of cable. Using a propagation velocity of 0.66 ×
c, one round trip is completed in approximately 6.2 µs. If
we assume the reflections will damp out in three "round trips"
up and down the cable length, the signal will stabilize 18.6 µs after the leading edge of a bit. At 9600 baud one bit is
104 µs wide. Since the reflections are damped out much before
the center of the bit, termination is not required.
There are several methods of terminating
data lines. The method recommended by B&B is parallel termination.
A resistor is added in parallel with the receiver's "A" and
"B" lines in order to match the data line characteristic impedance
specified by the cable manufacturer (120 ohms. is a common
value). This value describes the intrinsic impedance of the
transmission line and is not a function of the line length.
A terminating resistor of less than 90 ohms should not be
used. Termination resistors should be placed only at the extreme
ends of the data line, and no more than two terminations should
be placed in any system that does not use repeaters. This
type of termination clearly adds heavy DC loading to a system
and may overload port powered RS-232 to RS-485 converters.
Another type of termination, AC coupled termination, adds
a small capacitor in series with the termination resistor
to eliminate the DC loading effect. Although this method eliminates
DC loading, capacitor selection is highly dependent on the
system properties. System designers interested in AC termination
are encouraged to read National Semiconductors Application
Note 903 (note 2) for further
information. Figure 2.3 illustrates both parallel and AC termination
on an RS-485 two-wire node. In four-wire systems, the termination
is placed across the receiver of the node.
Note 2: Refer to Chapter 7 for Information
on National Semiconductors Application Notes
Figure 2.3 Parallel and AC Termination
Biasing
an RS-485 Network
When an RS-485 network is in an idle state, all nodes are
in listen (receive) mode. Under this condition there are no
active drivers on the network. All drivers are tristated.
Without anything driving the network, the state of the line
is unknown. If the voltage level at the receiver's A and B
inputs is less than ±200mV the logic level at the output
of the receivers will be the value of the last bit received.
In order to maintain the proper idle voltage state, bias resistors
must be applied to force the data lines to the idle condition.
Bias resistors are nothing more than a pullup resistor on
the data B line (typically to 5 volts) and a pulldown resistor
(to ground) on the data A line. Figure 2.4 illustrates the
placement of bias resistors on a transceiver in a two-wire
configuration. Note that in an RS-485 four-wire configuration,
the bias resistors should be placed on the receiver lines.
The value of the bias resistors is dependent on termination
and number of nodes in the system. The goal is to generate
enough DC bias current in the network to maintain a minimum
of 200mV between the B and A data lines. Consider the following
two examples of bias resistor calculation.
Figure 2.4 - Transceiver with Bias Resistors
Example 1. 10 node, RS-485 network
with two 120 ohm termination resistors
Each RS-485 node has a load impedance of 12K. 10 nodes in
parallel give a load of 1200 ohms. Additionally, the two 120
ohm termination resistors result in another 60 ohm load, for
a total load of 57 ohms. Clearly the termination resistors
are responsible for a majority of the loading. In order to
maintain at least 200mV between the B and A line, we need
a bias current of 3.5 mA to flow through the load. To create
this bias from a 5V supply a total series resistance of 1428
ohms or less is required. Subtract the 57 ohms that is already
a part of the load, and we are left with 1371 ohms. Placing
half of this value as a pullup to 5V and half as a pulldown
to ground gives a maximum bias resistor value of 685 for each
of the two biasing resistors.
Example 2. 32 node, RS-485 network
without termination
Each RS-485 node has a load impedance of 12Kohms 32 nodes
in parallel give a total load of 375 ohms. In order to maintain
at least 200 mV across 375 ohms we need a current of 0.53
mA. To generate this current from a 5V supply requires a total
resistance of 9375 maximum. Since 375 ohms of this total is
in the receiver load, our bias resistors must add to 9Kohm
or less. Notice that very little bias current is required
in systems without termination.
Bias resistors can be placed anywhere
in the network or can be split among multiple nodes. The parallel
combination of all bias resistors in a system must be equal
to or less than the calculated biasing requirements. B&B Electronics
uses 4.7Kohm bias resistors in all RS-485 products. This value
is adequate for most systems without termination. The system
designer should always calculate the biasing requirements
of the network. 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.
Extending
the Specification
Some systems require longer distances or higher numbers of
nodes than supported by RS-422 or RS-485. Repeaters are commonly
used to overcome these barriers. An RS-485 repeater such as
B&B Electronics' 485OP can be placed in a system to divide
the load into multiple segments. Each "refreshed" signal is
capable of driving another 4000 feet of cable and an additional
31 RS-485 loads.
Another method of increasing the number
of RS-485 nodes is to use low load type RS-485 receivers.
These receivers use a higher input impedance to reduce the
load on the RS-485 drivers to increase the total number of
nodes. There are currently half and quarter load integrated
circuit receivers available, extending the total allowable
number of nodes to 64 and 128.
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