Chapter 6
in most networks don’t respond immediately after the Stop bit so a slightly
longer delay causes no harm.
To reduce the delay, decrease the value of the 27k resistor. Each kilohm of resis-
tance corresponds to approximately 1 μs of delay.
The circuit has a small effect on the transmitted RS-485 data. When the data
input goes low, the driver’s output switches from disabled to logic 0. When the
data input goes high, the driver’s output is already enabled and switches from
logic 0 to logic 1. At very fast bit rates or when the bit rates of the transmitting
and receiving computers don’t closely match, differing delays in switching the
driver on and switching the output of an enabled driver can result in errors
when reading data. The transceiver’s data sheet specifies the maximum delays
for a specific load. The relevant delays are driver-enable to output and driver
input to output. At low to moderate bit rates, the asymmetry is small enough
that it causes no problems.! + $
For applications that can’t use the approach described above, Figure 6-16 shows
an alternative. The circuit uses a retriggerable 1-shot multivibrator to enable the
driver whenever the node is transmitting.Figure 6-16: This retriggerable 1-shot can automatically enable an RS-485 driver. A
Start bit or logic 0 results in a pulse of 1.5 word widths.