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Mahmood Elnasser
12/27/12 13:35
Modified:
  12/27/12 13:42

Read: 982 times
Sacramento
USA


 
#189049 - Found it
Responding to: Erik Malund's previous message
As Kai pointed out above:
In ten ways to bulletproof RS485 http://www.ti.com/lit/an/snla049a/snla049a.pdf it says:
6 Termination and Stubs
A common mistake is to connect a terminating resistor at each node a practice that causes trouble on buses that have four or more nodes. The active driver sees the four termination resistors in parallel, a condition that excessively loads the driver. If each of the four nodes connects a 100 Ohm termination resistor across the bus, the active driver sees a load of 25 ohm instead of the intended 50 ohm. The problem becomes substantially worse with 32 nodes. If each node includes a 100 Ohm termination resistor, the load becomes 3.12 Ohm.

Jeez this was my mistake, I added a termination resistor on each node, should have read this app note again and not relied on memory!.
Mahmood

List of 13 messages in thread
TopicAuthorDate
RS485 supply voltage      Mahmood Elnasser      12/26/12 12:00      
   Could the data rate be a problem?       Richard Erlacher      12/26/12 14:12      
   5V      Erik Malund      12/26/12 17:17      
   No!      Kai Klaas      12/27/12 06:24      
      Strange!      Mahmood Elnasser      12/27/12 10:56      
         stubs?      Erik Malund      12/27/12 11:38      
            Found it      Mahmood Elnasser      12/27/12 13:35      
               I made $$ on that one      Erik Malund      12/28/12 10:22      
   How to Protect ?      Satish S Munot      01/09/13 23:32      
      Use MAX3430      Oliver Sedlacek      01/10/13 01:49      
         Or the LT1785...      Kai Klaas      01/10/13 05:10      
      MAX485      Satish S Munot      01/10/13 07:43      
         low      Erik Malund      01/11/13 13:41      

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