Designing RS-485 Links and Networks/ #$#
A typical half-duplex RS-485 network has one pair of signal wires that connect
to every node. Some networks can increase their capabilities or efficiency by
using multiple RS-485 buses. Examples includes networks that have large num-
bers of nodes, span very long distances, or have a physical layout that benefits
from a star topology.
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TIA-485-A says that a bus can have up to 32 unit loads. As Chapter 6
explained, if you need more than 32 transceivers, you can use transceivers that
are less than a unit load each. Every node adds capacitance to the bus, however,
and interface chips that are a fraction of a unit load often don’t support fast bit
rates.
Another option is to add a repeater that regenerates the RS-485 signals, allow-
ing up to 32 more unit loads. You can also use a repeater to extend the length of
a network or to add a spoke to a bus.
Figure 7-16 shows an optoisolated repeater that contains two half-duplex
RS-485 transceivers. The TTL/CMOS input of each transceiver loops through
a pair of optocouplers to the TTL/CMOS output of the other transceiver.
RS-485 data received on one transceiver transmits out the other transceiver’s
RS-485 line. Each of the two RS-485 lines can support up to 32 unit loads. To
eliminate having to provide a separate driver-enable signal for the repeater, both
transceivers derive their driver-enable signals from the data as described in
Chapter 6.$#
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Another use for multiple buses is to configure a network in a star topology. Fig-
ure 7-17 shows a network with a primary node and multiple secondary nodes.
Each secondary node has its own physical bus and can branch from the primary
node in any direction without having to double back to connect to other nodes.
Each bus can also have multiple nodes.
Each secondary node receives everything sent by the primary node but receives
nothing sent by the other secondary nodes. The resulting reduced traffic at the
secondary nodes lightens their processing load.
The primary node has a full-duplex TTL/CMOS serial port, which can be a
microcontroller port or a TTL/CMOS interface on an RS-232 interface chip,