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Old August 4th, 2008   #2
Task Group Member
Join Date: Aug 2008
Posts: 369

The R1 bias resistor is recommended to be 562 ohm. If the 485 line is driven to 5v, you're looking at (5v-3.3v)/562ohm = 3mA. As for pulling your power supply out of spec, 3mA is no worse than a pullup resistor. It's hard to see this causing the problems you're describing unless you're building an extremely low power device or you have a very high ESR power supply.

Could you have some other path between your 5v and 3.3 volt supplies? Are you using a true RS485 driver? A 3.3v line driver with data lines driven to 5v is no different than a 5v line driver driven to 12v (at max 485 common mode voltage)

Are you using ESD protection diodes on your 485 data lines? Are those installed backwards? That seems like the most likely suspect.

Last edited by ericthegeek; August 4th, 2008 at 11:40 PM.
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