In addition, the product of the terminating resistance and capacitance should
be no more than 1/10 the width of a bit. For example, with 120 ?„¦ and 2500 pF,
the minimum bit width is 3 ?µs, for a maximum bit rate of 330 kHz.
Unlike a purely resistive parallel termination, this termination doesn??™t defeat the
receiver??™s internal biasing circuits. When all drivers are off, the capacitor
Chapter 7
124
Figure 7-8: These terminations can conserve power on some lines.
Designing RS-485 Links and Networks
125
remains charged and the receiver??™s internal pull-up and pull-down resistors hold
input A > B.
In Figure 7-8B, a capacitor charges to half the differential voltage. Figure 7-8C
replaces the capacitor with a resistor equal to 1/4 the line??™s characteristic impedance.
The terminations in B and C aren??™t recommended when the largest source
of noise is from magnetic fields.
* 1
When more than two computers share a communications path, how the computers
connect to the cable can also affect signal quality. Figure 7-9 shows several
network topologies, or wiring configurations. RS-485 drivers and receivers
are designed for use in a bus, or linear, topology, where the network cable
begins at one node and connects in sequence to each of the other nodes. This
topology enables using a termination at each end of the bus and brings us to
guideline #3 for RS-485:
Wire the nodes in a bus topology.
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