Chapter 7
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Another concern in RS-485 links is ensuring a logic-1 input if the network
wires accidentally short together or if two drivers are enabled at the same time
and hold the differential voltage near 0V.
One solution is to use Maxim’s MAX3080. TIA-485-A says that receivers must
recognize valid logic levels when the difference between inputs A and B is at
least 200mV. The MAX3080’s receivers comply with the standard but expand
the definition for logic 1 to include the range where input A is anywhere from
50 mV more negative to 200 mV more positive than input B. The only unde-
fined range is when input A is between 50 mV and 200 mV more negative than
input B.
With these definitions, the receiver sees a logic 1 if the difference between
inputs A and B is zero, which occurs if the RS-485 wires short together. A 0V
input with up to 50 mV of noise remains a logic 1.
Figure 7-11’s circuit uses 75ALS180B or MAX491 driver/receivers with a resis-
tor network to provide fail-safe protection. The chips are full-duplex
driver/receiver pairs, similar to the SN75179B introduced in Chapter 6 but
with an enable line for each direction. Figure 7-11’s circuit is half duplex with
the 2-wire interface created by tying the driver and receiver pairs together.
Resistors R1 and R2 bias the line to a logic 1 if no driver is active, and R3 andFigure 7-11: This RS-485 interface has open-circuit and fail-safe biasing.