RS-485 driver 3.3V vs 5V

[xxtigerxx]

I want ot make a network between devices with RS-485. But some RS-485 IC will be 3.3V and other will be on 5V.

Will i have a problem?

For the 5V i use SN65HVD3082E.
For the 3.3V i would probably use the SN65HVD72D.

Thanks

 

[KlausST]

Hi,

RS485 is differential transmission.
The absolute voltage level is not that important.
The driver common mod input voltage level should be +/- 7V (if i remembering right)
More data are in the datasheets.

So everything is OK with mixed supplies.

Hope that helps.
Klaus

 

[arbj2]

in the bus side it is normally +/- 5V. I am not sure if +/- 3V will work.

 

[betwixt]

RS485 receivers will typically respond to 200mV reversal in polarity as long as it is within their common mode range so there should be no problem.

Brian.

 

[xxtigerxx]

thanks for your comments

Did anyone test it? just to be sure

 

[FvM]

If I say it works for me, are you sure that it will do for you, too? Why don't you read the RS485 driver datasheets thoroughly or try to understand the RS-485 standard and answer the question yourself?

 

[xxtigerxx]

Hi FvM,

I read the RS-485 driver and the standard. They do say anything about the supply of the IC.
The only limits are -7 +12 as i have seen.

That way i ask for your opinion and if anyone has done it.

 

[D.A.(Tony)Stewart]

Many other choices depend on datarate and supply voltage in both chips while the cable impedance, minimum signal swing & receiver hysteresis , wide operating common-mode bias range for active terminators are fairly standard.

Longer cable capacitance and faster data rates demand higher current swings.

These are designed for low power, so if it is not fast enough, next part number higher up will be faster and may consume more power.. 1~50MHz between chips typ.

 

[FvM]

I read the RS-485 driver and the standard. They do say anything about the supply of the IC.
The only limits are -7 +12 as i have seen.

Apparently you still don't understand the specification. The supply voltage doesn't matter, only the line level. It's specified by allowed common mode range and minimal required differential level. In simple words, just expect that any driver marketed as RS-485 will keep the standard in terms of line level.

That way i ask for your opinion and if anyone has done it.

I already answered the question.

 

[Gorgon]

If a driver is rated as a RS485 device, this will connect correctly to other RS485 devices/drivers, as long as you adhere to the comon mode offset of +/-7V, or +12V if you add the 5V supply to the mix. To get the proper operation you should terminate the RS485 correctly in both ends, this is normally done with a 120ohm resistor. I would advice you to check the data sheet for the drivers, if you have more than 2 drivers connected to the RS485 link.

The only difference between 3.3V and 5V supply is for the other side of the driver, the logic side. Here you need to take care of the supply voltage of the interfacing signals, as long as the supply voltage are the same you have no problem.