UART RS485 transciever setup

[Cristian Mitu]

Hello guys,


I am designing a board ( an adapter board for the BeagleBine black) that is un upgrade for a current system. Beacuse of this i specifically have to link a 5V TTL RS485 transceiver to one of the UARTs available. I am not sure what logic family the BeagleBone uses(though i'm guessing that it is CMOS)  and because of this i want to know if this old transceiver can be wired directly to the BeagleBone or is it unavoidable that i have to use logic level translators for the signals. I apologize for the newbie question and i thank you in advance for your response.

PS

If you could direct me to some good drivers for RS485 communication i would be overwhelmed with joy and eternally grateful :)

[John Syn]

From: Cristian Mitu <cristi...@gmail.com>
Reply-To: "beagl...@googlegroups.com" <beagl...@googlegroups.com>
Date: Wednesday, July 2, 2014 at 1:49 PM
To: "beagl...@googlegroups.com" <beagl...@googlegroups.com>
Subject: [beagleboard] UART RS485 transciever setup

Hello guys,


I am designing a board ( an adapter board for the BeagleBine black) that is un upgrade for a current system. Beacuse of this i specifically have to link a 5V TTL RS485 transceiver to one of the UARTs available. I am not sure what logic family the BeagleBone uses(though i'm guessing that it is CMOS)  and because of this i want to know if this old transceiver can be wired directly to the BeagleBone or is it unavoidable that i have to use logic level translators for the signals. I apologize for the newbie question and i thank you in advance for your response.

You will need to translate the 5V TTL signals to 3.3V before connecting to the BBB.

Regards,

John

PS

If you could direct me to some good drivers for RS485 communication i would be overwhelmed with joy and eternally grateful :)

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[Cristian Mitu]

Thank you for your response John.
 So i only need to to translate  when sending data to the BBB or for both TX and RX ?

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[Jerônimo Lopes]

Cristian,

2014-07-03 8:55 GMT-03:00 Cristian Mitu <cristi...@gmail.com>:

Thank you for your response John.
 So i only need to to translate  when sending data to the BBB or for both TX and RX ?

​

​

I'm using BBB with a 75176 transceiver (5V powered). Put a diode between BBB Rx and 75176:

75176------|<|------BBB Rx

And a pullup resistor, from BBB Rx to 3.3V.

It's working fine.

[John Syn]

From: Cristian Mitu <cristi...@gmail.com>
Reply-To: "beagl...@googlegroups.com" <beagl...@googlegroups.com>

Date: Thursday, July 3, 2014 at 4:55 AM
To: "beagl...@googlegroups.com" <beagl...@googlegroups.com>
Subject: Re: [beagleboard] UART RS485 transciever setup

Thank you for your response John.
 So i only need to to translate  when sending data to the BBB or for both TX and RX ?

I recommend using a translator in both directions. This is the type of translator that will do what you want:

http://www.ti.com/lit/ds/symlink/txs0102.pdf

If the package is difficult for you to use, use a SMD to conventional pin adapter or look for a similar device in a DIP package. 

Regards,

John

[John Syn]

From: Jerônimo Lopes <lopesj...@gmail.com>
Reply-To: "beagl...@googlegroups.com" <beagl...@googlegroups.com>
Date: Thursday, July 3, 2014 at 5:31 AM
To: "beagl...@googlegroups.com" <beagl...@googlegroups.com>
Subject: Re: [beagleboard] UART RS485 transciever setup

Cristian,

2014-07-03 8:55 GMT-03:00 Cristian Mitu <cristi...@gmail.com>:

Thank you for your response John.
 So i only need to to translate  when sending data to the BBB or for both TX and RX ?

​

​

I'm using BBB with a 75176 transceiver (5V powered). Put a diode between BBB Rx and 75176:

75176------|<|------BBB Rx

And a pullup resistor, from BBB Rx to 3.3V.

It's working fine.

Only for slow data speeds. 

[Cristian Mitu]

Thank you again Jhon you are a life saver this part looks perfect for what i need. I think i will use two, one for the  RX and TX signals seeing as it doesn't need direction input and one for the DE and RE enable signals. Thank you again 

PS

Also thank you Jerônimo for your suggestion.

[John Syn]

From: Cristian Mitu <cristi...@gmail.com>
Reply-To: "beagl...@googlegroups.com" <beagl...@googlegroups.com>

Date: Thursday, July 3, 2014 at 12:22 PM
To: "beagl...@googlegroups.com" <beagl...@googlegroups.com>

Subject: Re: [beagleboard] UART RS485 transciever setup

Thank you again Jhon you are a life saver this part looks perfect for what i need. I think i will use two, one for the  RX and TX signals seeing as it doesn't need direction input and one for the DE and RE enable signals. Thank you again 

Then use the TXS0104

Regards,

John

[Cristian Mitu]

Hello John,

Sorry to bother you but i have a question regarding the input output features of the TXS0104 that you recommended me a while back. The problem is that i am doing some calculations for the decoupling network of the power regulator. And usually in this calculation i would determine voltage noise margins by seeing the input/output min and max  voltage characteristics of the communicating ICs. Firstly i am not sure about the characteristics off the BeagleBone Black but looking at the processor datasheet i see a 2.85 min output high and a 0.45 max output low(correct me if i am wrong). Now according to the TXS0104 datasheet the min high input should be 2.9 and the max low input should be 0.15. These numbers seem  too high and do not allow for any noise margin. I just want to know if i am reading the datasheet wrong somewhere because usually there is a noise margin even if just a little. 

Sorry again for the bother but i really need to know if I am doing this right.

Regards,

Cristian

[John Syn]

From: Cristian Mitu <cristi...@gmail.com>
Reply-To: "beagl...@googlegroups.com" <beagl...@googlegroups.com>

Date: Tuesday, August 19, 2014 at 7:23 AM

To: "beagl...@googlegroups.com" <beagl...@googlegroups.com>
Subject: Re: [beagleboard] UART RS485 transciever setup

Hello John,

Sorry to bother you but i have a question regarding the input output features of the TXS0104 that you recommended me a while back. The problem is that i am doing some calculations for the decoupling network of the power regulator. And usually in this calculation i would determine voltage noise margins by seeing the input/output min and max  voltage characteristics of the communicating ICs. Firstly i am not sure about the characteristics off the BeagleBone Black but looking at the processor datasheet i see a 2.85 min output high and a 0.45 max output low(correct me if i am wrong). Now according to the TXS0104 datasheet the min high input should be 2.9 and the max low input should be 0.15. These numbers seem  too high and do not allow for any noise margin. I just want to know if i am reading the datasheet wrong somewhere because usually there is a noise margin even if just a little. 

All you need is a 0.1uF ceramic decoupling capacitor on both power rails (3v3 and 5v). Everything else will work just fine.

Regards,

John