| ??? 11/15/05 14:46 Modified: 11/15/05 14:48 Read: times |
#103745 - You will loose all the noise margin! Responding to: ???'s previous message |
Chris said:
This leads me to think that maybe I don't ever need to drive the line into the mark state, but I could just disable the line driver and let the fail-safe biasing take control. Additionally to what Kalpak mentioned you should also take this into consideration: The fail safe feature of modern RS485 recievers is only good for giving them a chance of detecting, that the cable is disconnected or eventually, with some luck, that the master is powered-off. To do this, the reciever input is made to be a bit asymmetric, so that the absence of signal during above situations will cause a stable, non-changing output signal. And if the output of RS485 reciever isn't toggling any more, then a failure must have occured. This is the idea of fail safe feature. But, take care, this imbalance of reciever isn't big enough to always guarantee the non-toggling of output, if, for instance, there's some relevant noise on the line. This is due to the fact, that the imbalance of reciever input must be much smaller then the regular signal amplitude, because otherwise the RS485 driver could never make the reciever output to change its state! So, when using the driver enable input to transmit data over the RS485 line you would loose all the noise margin the RS485 loop is designed for!! Kai |
