Designing RS-485 Links and Networks5
Rise time is the time required for an output to switch from 10% to 90% of full
range (Figure 7-1). Fall time is the time required for an output to switch from
90% to 10% of full range. The rise and fall times are often equal or nearly
equal, so some sources use rise time to refer to transition time in general. The
rise time of a digital signal is an indication of the frequencies that make up the
signal. A faster rise time means the signal contains higher frequencies.
The data sheets for RS-485 drivers specify typical and maximum rise and fall
times. The values range from a few nanoseconds to nearly a microsecond. The
specifications assume a defined load, often 54Ω and 50 or 100 pF, with higher
capacitance resulting in a longer switching time.
A shorter rise time corresponds to a faster slew rate and faster possible bit rates.
In general, the time required to transmit a bit should be 5–10 times longer than
the rise time to ensure that the voltage has reached a valid logic level by the time
the receiver reads the bit. For example, an RS-485 driver rated for use at 2.5
MHz might have a maximum rise time of 0.06 μs, which is 15% of the bit
width.
The bit rate is also important because transmission-line effects such as ringing
and reflected voltages occur during and immediately after voltage transitions,
while receivers read logic levels near the middle of the bits. At slower bit rates,
the bits are wider, and the voltages are likely to have settled by the time the
receiver detects them.
To understand why rise and fall times are a measure of frequency, consider the
most basic digital signal, a square wave, which has alternating, equal-width high
and low voltages. Mathematically, a square wave is the sum of a sine wave of a
fundamental frequency and its odd harmonics. For example, a 100-Hz squareFigure 7-1: A circuit with short rise and fall times contains high frequencies, even if
the bit rate is slow..