Chapter 6
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The ideal driver-enable control requires no software control, no user configur-
ing, and no delays between receiving and transmitting data. Figure 6-14 shows
a circuit that meets these goals.
The added components add some complexity to the circuit design. However,
the components are inexpensive and the freedom of not having to determine
when it’s OK to enable and disable the driver improves network performance
and can be well worth the cost.
Chapter 7 described the two valid states of an RS-485 line. When input A is at
least 200 mV more positive than input B, the line may be idle or transmitting a
Stop bit or a logic-1 data bit. When input B is at least 200 mV more positive
than input A, the line may be transmitting a Start bit or a logic-0 data bit.
If a transmitting computer’s driver is disabled by the end of the final Stop bit,
another computer in the network can enable its driver and send a Start bit right
away. There’s no need to delay to be sure the previous driver has been disabled.Figure 6-14: This circuit controls the driver-enable line entirely in hardware. Circuit
courtesy of R.E. Smith (www.rs485.com).)