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???
11/15/05 20:38
Modified:
  11/15/05 20:40
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 		 #103771 - 15 milliwatts. A lot of power?
Responding to: ???'s previous message
The drawback of 3 resistor termination to drive the bus is that it burns a lot of power
750 + 130 +750 ohms across 5 volts is approx 3mA at 5 volts = 15 milliwatts. A lot of power?

If you, for whatever reason, can not terminate your bus properly, you have chosen the wrong bus.

and the 485 drivers have to drive harder too,
The drivers drive exatly as hard as into a standard termination. If there was a difference there would be termination resistance mismatch.

Anyhow RS485 is not for the powersavers a fully loaded bus will make the transmitter suck current. I recall a case where someone had a problem "it works great till we add more nodes" it was the drivers sucking the power supply dry.

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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