Posted by ddorling
on 3/19/2010, 12:04 pm
207.200.116.65
Setting up a chip to drive RS485 dmx512 I have come across different solutions. First is there a difference between using a 40-60ma chip or a low power 4ma chip?
Does it make sense to place a termination resistor 121ohm A-B on the board?
Is it necessary to place a pullup 1.21k on output terminal A and a pulldown 1.21k on output terminal B?
Does it help to place a 26ohm resistor in series with output A and 26ohm in series with output B. I have been using the 121ohm and 1.21k circuit without a problem for years but the 26ohm resistor is something new I've run across.
My main focus is to minimize voltage spikes generated by the chip at the transmission rate that find their way onto application signals. 
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