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RS232/USB to RS485 converter

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diepvu1805

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Hi all.
I have two design:
1/ RS232 to Rs485 converter
I use Txd, Rxd from PC ==> max232 => opto => max3089. I use RTS(from PC) to control DE and /RE pin max3089
2/ USB to Rs485 convert.
I use USB=>> FT232RL => opto => max3089. I use CBUS2(pin 13 of FT232RL) to control DE and /RE pin max3089.
With 2 this design I can send/receive data to the device with speed <= 38400 baud, the data good,
When I send/receive with speed as: 57000,...,115200, the data error.
I don't understand that.
Could you help me,pls?
Thank you,
 

diepvu1805

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Thank,
But I don't think the opto is issue. I am using Si8431AB(USB to Rs485) and Si8421(Rs232 to Rs485). They are fast opto.
I've also tried removing opto and connect Txd and Rxd pin to RO and DI pin but when send/receive data with high speed. it's still error.
 

barry

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It's apparently a timing issue. I would first look at my signals with an oscilloscope and see if there's anything that looks wrong there.
 

diepvu1805

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Thank you for helping me,
The problem is Capacitor(100nF). This Capacitor is put between A and B wire of RS485, so when I send/receive the data with high speed occur the errors.
 

barry

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You DEFINITELY don't want a cap there. For RS485 you need a terminating resistor, but not a cap.
 

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