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RS232 voltage

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I read that RS232 voltages are 3 to 15v for '0' and -3v to -15v for '1' while some sites say that it is 3 to 25v for '0' and -3v to -25v for '1'. So which is correct?
 
Spectacular Butter said:
I read that RS232 voltages are 3 to 15v for '0' and -3v to -15v for '1' while some sites say that it is 3 to 25v for '0' and -3v to -25v for '1'. So which is correct?

Why does it matter?, either will work just as well as the other.

Generally RS232 is perceived as +/-12V, and that's the value you should be aiming to transmit - on the receive side it should make no difference.
 
There are some RS-232 standards, but there are so many variants that about the only guarantee is that two properly-working RS-232 devices that are plugged together will not destroy each other. Otherwise, those voltages, various extra lines and the actual code sent across as data can vary so much that it can provide for some mighty fine headaches when trying to get two devices to "talk" to each other. Hence, such books as "The RS-232 Cookbook" and other titles have been useful.

Dean
 
Spectacular Butter said:
I read that RS232 voltages are 3 to 15v for '0' and -3v to -15v for '1' while some sites say that it is 3 to 25v for '0' and -3v to -25v for '1'. So which is correct?
From the book 'Data Transmission' by Tugal and Tugal:

Driver max levels (unloaded): +/-25v
Driver loaded: +/-5v to +/-15v
Driver output resistance (power off): 300ohms min.
Driver o/p s/c current: +/-500mA max.

Max cable capacitance: 2500pF

Receiver input resistance: 3K to 7Kohms
Receiver input threholds: -3v to +3v max
Receiver input voltage: -25v to +25v max

Hope that helps.
 
A freind asked for some help with his TTL to RS-232 converter circuit. I didn't understand the real meaning of RS-232 so I did some reading. I learned that orginally RS-232 was developed for one specific purpose and has been borrowed, modified, etc, and used for all sorts of things for which it wasn't intended.

Armed with a little knowledge I inspected my freind's circuit and determined that it can't possibly output anything resembling RS-232 - with only 5 volts to work with (no charge pump,etc). I also read where circuits just like that were said to function acceptably. Eventually I discovered some coherent explanations that said some computers can handle the 0 volts and 5 volts just as well as proper RS-232. It also said that some can't. A electronics professional shared an explanation about an 'asserted state' - and if the signal is not the 'asserted state' it's the other. Since the RS-232 port is looking for one state or the other this makes sense - though I thought the spirit of RS-232 was to not let any low level of signal mean anything.

So from what I've read I'd say that you'd have to go back to a specific standard if it is referenced but also expect that the difference between the 'standard' and what will actually work could be quite different.
 
stevez said:
A freind asked for some help with his TTL to RS-232 converter circuit. I didn't understand the real meaning of RS-232 so I did some reading. I learned that orginally RS-232 was developed for one specific purpose and has been borrowed, modified, etc, and used for all sorts of things for which it wasn't intended.

Armed with a little knowledge I inspected my freind's circuit and determined that it can't possibly output anything resembling RS-232 - with only 5 volts to work with (no charge pump,etc). I also read where circuits just like that were said to function acceptably. Eventually I discovered some coherent explanations that said some computers can handle the 0 volts and 5 volts just as well as proper RS-232. It also said that some can't. A electronics professional shared an explanation about an 'asserted state' - and if the signal is not the 'asserted state' it's the other. Since the RS-232 port is looking for one state or the other this makes sense - though I thought the spirit of RS-232 was to not let any low level of signal mean anything.

So from what I've read I'd say that you'd have to go back to a specific standard if it is referenced but also expect that the difference between the 'standard' and what will actually work could be quite different.
Yes, I have seen circuits published in magazines where an RS232 '1' is given a value of 0v. The authors should have known better.

True, many modern serial chips in computers will work quite happily with that figure, but many more will not. Furthermore, when you consider the inherent cable resistance, it is unlikely that the receiver input will ever get down to 0v.

To use 0v as the norm for a '1' is inviting problems. Levels for RS232 are specified in the standard - and standards should be adhered to.
 
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