Designing RS-485 Links and NetworksThe amplitude of the ringing varies with the driver’s output resistance, the
wires’ inductance, the load’s capacitance, and the frequencies carried by the
wires. As with other mismatched terminations, if the ringing voltages are large
enough, the receiver may misread transmitted bits.
To reduce ringing, use a driver with a slower rise time. There’s no reason to use
a driver capable of 10 Mbps if you’re transmitting at 9600 bps. If you can’t
change the hardware, using a slow bit rate at least gives the ringing more time to
settle before the receiver reads the input.
You can also reduce ringing by reducing the circuit’s Q, which is a measurement
of the circuit’s ability to resonate. To reduce the Q, decrease the wires’ induc-
tance or increase the load’s capacitance. To decrease the inductance, use larger
diameter wires or wires that are twisted more tightly. To increase capacitance,
use an AC termination as described below.
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An AC, or active, termination can reduce power consumption of idle lines, and
may also reduce ringing voltages. However, this type of termination also
reduces the maximum possible cable length and bit rate. Figure 7-8 shows
examples.
In Figure 7-8A, a resistor and capacitor connect in series across the differential
lines. The capacitor prevents ringing by absorbing the high frequencies that
make up the ringing voltages. The capacitor also reduces power consumption
because the current on the lines is near zero when the capacitor has charged
after each transition. The added capacitance lowers the maximum bit rate and
cable length, so this termination is limited to shorter, low-speed links.
TIA-422 gives this formula to select the capacitor’s value:
CT (pF) < 2 * (1-way cable delay (ps)) / (characteristic impedance (Ω))
Assuming a propagation rate of 125 ps/in. and a characteristic impedance of
120 Ω, a 10-ft cable should use a capacitor of 250 pF or less. A 100-ft cable can
use values of up to 2500 pF.
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 Ω 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