Hypothetical

say you have an rs422 driver on a piece of hardware (A) trying to talk to another piece of hardware (B)

Driver A is powered by 10V and Receiver (B) is powered by 3.3V.
What happens when the 3.3V part receives a signal that has a magnitude of 10V?

Is this common? Does it cause damage? The receiver is in theory infinite input impedance so there should be no current flow besides leakage right?
Thanks

 

Hypothetical

say you have an rs422 driver on a piece of hardware (A) trying to talk to another piece of hardware (B)

Driver A is powered by 10V and Receiver (B) is powered by 3.3V.
What happens when the 3.3V part receives a signal that has a magnitude of 10V?

Is this common? Does it cause damage? The receiver is in theory infinite input impedance so there should be no current flow besides leakage right?
Thanks

You look at the input common mode range of the transceiver. This is not usually a problem, but attention to details is essential. A link to the respective datasheets would be most helpful.

 

You look at the input common mode range of the transceiver. This is not usually a problem, but attention to details is essential. A link to the respective datasheets would be most helpful.

Thanks , no data sheet, im strictly just thinking outloud.

What does the common mode have to do with part to part voltage? I thought the common mode was more just a diff signal A to diff signal B thing

 

Thanks , no data sheet, im strictly just thinking outloud.

What does the common mode have to do with part to part voltage? I thought the common mode was more just a diff signal A to diff signal B thing

Thinking out loud will not be much help in practice, and your view of device specifications is somewhat limited. FYI RS-422 is a differential signaling method as the following excerpt from the Wikipedia article demonstrates.

RS-422 specifies differential signaling, with every data line paired with a dedicated return line. It is the voltage difference between these two lines that define the mark and space, rather than, as in RS-232, the difference in voltage between a data line and a local ground. ​

It is an "interface" part, and as such may be used to connect to other devices with different internal voltage supplies and signal grounds. The RS-422 specification defines voltage levels and drive capability for the transmitter and the receiver connected to the "bus". The datasheet will define what supply voltages and drive capabilities are acceptable on the "non-interface" side.

Because RS-422 is NOT referenced to a common ground, the common mode voltage range is precisely the parameter of interest. Doing this exercise WITHOUT datasheets is a useless fool's errand. The lack of objective evidence about the parts being used will prevent you from drawing any meaningful conclusions about what is and what is not possible.

 

Thinking out loud will not be much help in practice, and your view of device specifications is somewhat limited. FYI RS-422 is a differential signaling method as the following excerpt from the Wikipedia article demonstrates.

RS-422 specifies differential signaling, with every data line paired with a dedicated return line. It is the voltage difference between these two lines that define the mark and space, rather than, as in RS-232, the difference in voltage between a data line and a local ground. ​

It is an "interface" part, and as such may be used to connect to other devices with different internal voltage supplies and signal grounds. The RS-422 specification defines voltage levels and drive capability for the transmitter and the receiver connected to the "bus". The datasheet will define what supply voltages and drive capabilities are acceptable on the "non-interface" side.

Because RS-422 is NOT referenced to a common ground, the common mode voltage range is precisely the parameter of interest. Doing this exercise WITHOUT datasheets is a useless fool's errand. The lack of objective evidence about the parts being used will prevent you from drawing any meaningful conclusions about what is and what is not possible.

as usual, i will agree to disagree with you. For years i have been reading your replies that are just untrue.
For someone with so much understanding, its unfortunate you are so close minded to teaching in other ways then just how you see fit.

 

as usual, i will agree to disagree with you. For years i have been reading your replies that are just untrue.
For someone with so much understanding, its unfortunate you are so close minded to teaching in other ways then just how you see fit.

Yup, I'm obviously too "close minded[sic]" to teach you anything, so I'll pass it over to any of my learned colleagues, that wish to take a stab at it.

 

What do you find to be untrue with @Papabravo #4 comment above?
The common mode voltage IS important. In fact, I've just helped another poster with an RS485 problem that was exactly that.
It seems to me you are the close minded party here.

 

as usual, i will agree to disagree with you. For years i have been reading your replies that are just untrue.
For someone with so much understanding, its unfortunate you are so close minded to teaching in other ways then just how you see fit.

Wow!
https://forum.allaboutcircuits.com/threads/rs485-to-usb-communications.194522/post-1830636

 

Wow!
https://forum.allaboutcircuits.com/threads/rs485-to-usb-communications.194522/post-1830636

Thanks for the concrete example.

 

What do you find to be untrue with @Papabravo #4 comment above?
The common mode voltage IS important. In fact, I've just helped another poster with an RS485 problem that was exactly that.
It seems to me you are the close minded party here.

He must be talking about all of my other posts....perhaps?

 

Hi,
According to EIA422 standard your drive voltages must not exceed 6V. So driver A (10V) does not comply with the standard.
Thats the theoretical part. What happens in the real world depends on the receiver and driver you will use. But i think it will be hard do find a EIA422 compliant driver that ouputs 10V (because than it would not comply with the standard)

 

What do you find to be untrue with @Papabravo #4 comment above?
The common mode voltage IS important. In fact, I've just helped another poster with an RS485 problem that was exactly that.
It seems to me you are the close minded party here.

i wont get baited into a pissing contest with you. However, i find it funny how you two constantly come to each others rescue when anyone has a difference of opinion. That is EXACTLY the definition of close minded. It does this excellent forum a dis-service that you claim to cheerish. Just because someone does not agree with you does not give you the right to lash out at people like you always do around here. All it does is show your insecurities and hamper the point of this forum which is to learn.

You saying discussing hypothetical situations is a waste of time? Where did you go to school then because in my text books it was 5 years of hypothetical situations with no data sheets. hmmmm.... a clue sherlock!
And i never said the answer was not about common mode. I stated MY misunderstanding of when common mode is important and was hoping one of you would adjust my understanding of common mode so i could LEARN.... what a novel concept!

 

i wont get baited into a pissing contest with you. However, i find it funny how you two constantly come to each others rescue when anyone has a difference of opinion. That is EXACTLY the definition of close minded. It does this excellent forum a dis-service that you claim to cheerish. Just because someone does not agree with you does not give you the right to lash out at people like you always do around here. All it does is show your insecurities and hamper the point of this forum which is to learn.

You saying discussing hypothetical situations is a waste of time? Where did you go to school then because in my text books it was 5 years of hypothetical situations with no data sheets. hmmmm.... a clue sherlock!
And i never said the answer was not about common mode. I stated MY misunderstanding of when common mode is important and was hoping one of you would adjust my understanding of common mode so i could LEARN.... what a novel concept!

What a projection of a person's own actions onto others. Totally amazing example.

 

In my opinion AAC is a superb site for technical information on circuits, if I am allowed to say so openly.
Someone comes for advice and expert advice is provided.

Who is seeking advice here and who is giving it?

RS-422 is a physical interface standard with voltage specifications. 10V signalling exceeds the voltage specifications.
In order to assess compatibility, one has to refer to the datasheets of the RS-422 sending device and that of the RS-422 receiving device.

The bottom line is, one has to refer to the datasheets as well as how the RS-422 interface is implemented on both devices.

 

In my opinion AAC is a superb site for technical information on circuits, if I am allowed to say so openly.
Someone comes for advice and expert advice is provided.

Who is seeking advice here and who is giving it?

RS-422 is a physical interface standard with voltage specifications. 10V signalling exceeds the voltage specifications.
In order to assess compatibility, one has to refer to the datasheets of the RS-422 sending device and that of the RS-422 receiving device.

The bottom line is, one has to refer to the datasheets as well as how the RS-422 interface is implemented on both devices.

This is absolutely untrue.
You dont need a data sheet to discuss a standard.

 

True, if you're willing to just make stuff up in a Hypothetical.

 

This is absolutely untrue.
You dont need a data sheet to discuss a standard.

Here we go again.

You come here for advice.
You receive expert advice.
You choose to ignore and contradict the advice given.
What else do you expect from AAC?

 

I wrote what the standard says. You can connect a driver with a higher voltage output to a receiver with a lower operating voltage and it works in the other direction to. BUT a +-10V driving voltage is outside of the standard

 

This is absolutely untrue.
You dont need a data sheet to discuss a standard.

Then, by all means, discuss the standard. As long as the discussion remains within the scope of the standard, then the discussion is applicable to any device that is compliant with the standard. But as soon as you ask about something that is outside the scope of the standard, then you have wandered off into undefined behavior and anything is fair game. If you want to know what is going to happen in that case, you have to go to the datasheet for the behavior of that particular part because there is no requirement that any part is going to behave similarly to any other part under those condition.

So, what does the standard say about what the defined range of the input signals is for RS-422?