| ??? 11/16/05 15:35 Read: times |
#103803 - Yes, but it can fail from time to time Responding to: ???'s previous message |
Erik said:
It's the best we have and it works when you add CRC/checksum. I have done some statistics on 485 in very noisy environments and found that a 3 resistor 485 bus took about one hit per hour. In a company where we started to provide outdoor sensors with RS485 loops, we had certain problems with the fail safe biasing methode from time to time, as we experienced relevant common mode interference at certain plants. Because we couldn't improve the grounding management of every plant we had to deal with, we decided to make the RS485 loop more robust. Since this time we only use RS485 loops with drivers which are permanently turned-on. If we need handshaking, then we use a full-duplex RS485, again with drivers, which are permanently turned-on. The according RS485 loop does only address this single sensor. If more than one sensor is mounted, then we use individual RS485 loops. It's interesting to note, that PROFIBUS, which also uses half-duplex RS485 being driven by many individual drivers, does not recommend the direct connection to outdoor sensors either. And this allthough they use a floating RS485 loop, which should be highly immune against common mode interference! Happily, up to now we never needed this optical link for our applications, due to the use of drivers which are permanently turned-on, and of course due to a redundant signal protocol. Kai |
