RS232C - Serial 2

[LearnerGP]

Hi guys

I've just been employed at a company that deals with monitoring systems. I sorta qualified in 1999, but because of a lack of experience I was unable to find employment in my field of study viz. Electronic Engineering. As a result of not working in my field over the last couple of years I seem to have forgotten most of my theory.

Please can somebody give me a brief run down of a RS232C or point me in the direction of a relevant.
- What is a RS232C?
- How does it work?
- What are its applications?
- What does it look like?

Guys, I need this info. ASAP.

Thanks
LearnerGP

 

[BrianG]

Hi Learner,
You can Google all this info of course, but I'll give you a quick summary:

1) RS232C is an interface signalling standard for serial line data transmission. It refers to the voltage levels, NOT to any particular word length, message protocol or other data information standards.

2) It's used for connections between computers and external modems, or any other pair of devices that need to communicate at moderate speeds, but only over relatively short distances. Usually there are separate lines for send and receive so that device can talk in both direction simultaneously ("full duplex") if they want.
If you don't understand serial data transmission then you will have to Google more for yourself.

3) RS232 uses an unbalanced (data + return wire)circuit and signals over this using bi-polar voltages, i.e. a logic 1 is +V and a logic 0 is -V. At any moment the line will be one or the other voltage: Zero volts is not allowed.

The minimum signal levels a receiver input must accept is +3V and -3V respectively. (I forget the exact spec maximum voltage range (plus/minus 25V?) but it can often be +12v and -12v from an unloaded driver device)

4) It does not "look like" anything! There are certain standards that have grown up from the computer/modem industry for use of standard D-Type connectors, either 9-pin or 25-pin, and these usually have agreed pin numbers for the various RS232 lines.

 

[Skogsgurra]

A small addition (and correction). The signal levels are inverted. So logic "1" is signal voltage below -3 V and logic "0" is above +3 V. The communication is asynchronuous. That means that every character sent (99.999% is using ASCII) starts with a start bit and then followed by seven (or eight) binary coded data bits starting with 2^0 and ending with 2^6 (or 2^7). Then, there is one Parity bit and one stop bit (could be two, but not very often today) and then the line is ready for the next character. Speeds could be a low 110 Baud (bits/second, as it was then) up to 115 kbits/second (and that is usually not the same as 115 kBaud).

But, as said before. Google and learn.



Gunnar Englund

http://www.gke.org

 

[MikeHalloran]

If you want to know what RS232 'looks like', program your computer to send ASCII uppercase 'U' characters continuously, and reprogram the USART for different numbers of data bits, parity bits, etc.

That particular character has a lot of 'edges', so you can easily see and identify the bits on the transmit wire with a scope.

The reason it's defined down to very low data rates is that the very first transmitters and receivers (in teletypes) were mechanical devices, that rotated in unison even if miles apart, and tripped mechanical latches in particular angular positions if the wire was in a logic 1 state, and the combination of latches selected determined which character was impressed on the paper when the hammer came down. The start and stop bits were actually required so that the motors had time to accelerate and decelerate.

Last time I checked, the current standard was EIA-232D, but you can get enough information from Google that you don't have to buy it.



Mike Halloran
Pembroke Pines, FL, USA

 

[logbook]

RS232 is quite a "difficult" interface to get working in the sense that it does not just plug in and work. Both ends of the interface have to speak in the same terms. It is typical when getting an interface set up that nothing works for a while. Then you read through all the manuals and set the baud rate, parity bits, XON/XOFF, and all the other settings so they match at both ends. Only then do you get a link!

 

[felixc]

Look at the book Serial Port Complete, from Jan Axelson. It covers just every aspect of RS232.