Negative Voltage on RS232

This topic has 2 replies, 2 voices, and was last updated 16 years, 3 months ago by Col.

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March 24, 2008 at 11:24 am #1391

Col
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Just trying to get my head round the theory of a ‘negative’ voltage on RS232/485, could someone clarify it for me?

My understanding is that the GND is actually just a reference of zero, and would actually have a voltage when measured to a true 0V. The negative voltage part of the signal would actually just be a lower voltage with respect to this ‘virtual zero.’ Is this correct, as it is what I understood it to be but the other tech on my shift disagrees…

The sups more of a clanky, he gave us this look 😯

Cheers for any clarification!

March 24, 2008 at 12:11 pm #16789

Martin Wareham
Participant

Hi Col,

Communication GND is normally at the same potential (voltage) as power 0V; unless you have an isolated communication link and your measuring from the input side of the DC/DC converter.

RS232 normally alternates between about +10VDC and -10VDC depending upon the RS232 driver chip. So if you measure between communication GND and RX or TX you should see a square wave on an oscilloscope that jumps from +10V to -10V and vice versa.

A negative voltage is created when the current in a circuit flows in the opposite direction to the norm (V=IR).

RS485 is a bit different as it’s a differential voltage whereas RS232 is single ended. But the same theory applies.

Hope this helps 🙂

Martin

March 25, 2008 at 9:07 pm #16790

Col
Participant

Cheers Martin, that makes sense. 🙂
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