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???
11/16/05 14:02
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 		 #103798 - It's the best we have and it works when
Responding to: ???'s previous message
I would never trust a noise margin of only 50mV, when routing a RS485 loop over hundreds of feets! Think only about the imbalance of cables, which convert common mode noise into differential noise...
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.

3 resistor 485 works very well keeping the "hit" frequency very low and when you detect and recover from them it is one of the best buses.

Now, Kai, I agree with you that to "trust" the noise margin i.e. no CRC/checksumming would be foolhardly.

Erik


List of 34 messages in thread
TopicAuthorDate
RS485 Question            01/01/70 00:00      
   drive capacity            01/01/70 00:00      
      3 resistor termination            01/01/70 00:00      
         Only if you have power to burn            01/01/70 00:00      
            15 milliwatts. A lot of power?            01/01/70 00:00      
               Depends on your application            01/01/70 00:00      
                  how are you going to terminate at that            01/01/70 00:00      
                  RS485 loops are hungry...            01/01/70 00:00      
      You will loose all the noise margin!            01/01/70 00:00      
         not if you use 3 resistor termination            01/01/70 00:00      
            Better, but...            01/01/70 00:00      
               works for me, what do you have that is b            01/01/70 00:00      
                  No disagree...look what weird is planned            01/01/70 00:00      
                     Kai, I did not think you did            01/01/70 00:00      
                        Clarification            01/01/70 00:00      
                           3 resistors or not            01/01/70 00:00      
                              Then is all fail-safe bad?            01/01/70 00:00      
                                 Can you see the difference?            01/01/70 00:00      
                                    Kai, you keep ignoring the fact that RS4            01/01/70 00:00      
                                       I ignore nothing            01/01/70 00:00      
                                          It's the best we have and it works when            01/01/70 00:00      
                                             Yes, but it can fail from time to time            01/01/70 00:00      
                                                I have handled that with isolated RS485            01/01/70 00:00      
                                                   We do already            01/01/70 00:00      
                                                      485-422            01/01/70 00:00      
                                                         I challenge you to find a RS422 driver            01/01/70 00:00      
                                                            In the past.            01/01/70 00:00      
                                 All fail safe is good and 0.25V is plent            01/01/70 00:00      
                           That is not RS485, that is J1708            01/01/70 00:00      
                              trick to get around pseudo non random            01/01/70 00:00      
                                 This won't really help.            01/01/70 00:00      
                              Helpful            01/01/70 00:00      
                                 I am doing that using the Dallas DS2401            01/01/70 00:00      
                                    Yes that is what I am planning            01/01/70 00:00      

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