| ??? 11/16/05 01:40 Read: times |
#103776 - 3 resistors or not Responding to: ???'s previous message |
Chris said:
I was always talking about the 3 resistor method of failsafe biasing, not any particular 485 driver IC "special feature" It doesn't really matter, whether you use this well known 3 resistor fail safe biasing network or a chip which offers a built-in fail safe feature. Fact is, that the potential shift is much smaller than the regular amplitude of RS485 signal. None of these application notes mention what I was considering, which is driving the line in the SPACE condition only, so that is why I posted here. The reason that no one was mentioning this technique is, that you will get a "signal" of only about 0.25V with the fail safe biasing, calculate for yourself, assuming two resistors of 560 Ohm from each line to supply rails and 120 Ohm paralleled by 120 Ohm between the lines, which yields 0.25V. This amplitude is much much smaller than 2V amplitude of regular RS485 signal (datasheet of MAX481, figure 15...18). This is a considerable loss of noise margin, which strictly forbidds its use for professional purposes. Kai |
