Serial Port Complete - Latest Microcontroller projects

Designing RS-485 Links and Networks

data cuts power consumption dramatically. A circuit that uses spare RS-232 sig-

nals as a power source for an RS-232-to-RS-485 converter can’t use a resistive

parallel termination because it draws too much current.

In Figure 7-6, circuits C and D show how adding a termination defeats the

fail-safe circuits included in RS-485 receivers. The fail-safe circuits ensure that

the receiver sees a defined logic level when the inputs are open. Without a ter-

mination, the internal pull up and pull down in many RS-485 receivers hold

Figure 7-6: (A) and (B) show that a parallel termination increases power consumption
and decreases the noise margin. (C) and (D) show how the termination defeats a
receiver’s internal open-circuit fail-safe circuits.