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// SATELLAR MANUAL // RADIO UNIT // USER GUIDE // V. 1.8
5. Interfaces
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5.1.7 Failsafe RS-485/422 termination
When there is no data on the bus (no node is transmitting), the RS-4xx signal pair floats free. In principle
both signals (‘a’ and ‘b’) should be floating at the same potential. However, due to possible outside distur-
bances, this is not always the case.
According to the RS-4xx standard, the receiver interprets signals as either logic high or low depending on
the difference in potential between a and b. A potential difference of greater than 0.4 V is required for the
receiver to decide whether the signal is low or high. In practice most receivers make the decision at greater
than 0.2 V level.
The RS-485 receiver output is typically logical ‘1’ when the inputs are floating.
When a disturbance causes, the potential difference to increase logic ‘0’ is easily detected. This is then
interpreted as a start bit by the receiver on the RS-4xx bus, resulting in bit errors or garbled extra characters.
Another method of error due to lack of failsafe termination is that once a node starts transmitting on the
line, the receiver which already senses a ‘0’, misses the transition from stop bit to start bit, needed to syn-
chronize a UART transmission. Thus the receiver in error will receive the first data byte wrong, and depend-
ing on the number of stop bits and a pause between bytes on the line, might miss also the following bytes or
even an entire packet.
This is a potential error mechanism, which can be easily overcome by pulling the ‘a’ line high and the ‘b’ line
low by connecting the wires thru a series resistor to the desired potential.
Figure 5.3 Failsafe termination examples
