Request for Clarification on SCI Application Note : AN1753

[mailtoarup]

Posted on December 27, 2006 at 15:11

Request for Clarification on SCI Application Note : AN1753

[mailtoarup]

Posted on December 09, 2006 at 04:17

Dear Sir,

This is with reference to the Application Note AN1753 : Software UART using ST7 12 bit AutoReload Timer.

On Page No 5 under 2.1 MAIN FEATURES .... it is written as under

Furthermore, the polarities are different. A ‘1‘ bit coming from the UART corresponds to a ‘0‘bit

 

in RS232, and a ‘0‘ bit to a ‘1‘ bit. It is true for all bits including the START and STOP bits.

********************* what i believe the statement is not true(little confusing). UART binary '1' bit is always binary '1' bit in RS-232.Yes in UART '1' bit level is 5 volt whereas in RS 232 '1'(MARK) bit is typically -7V .Are the above statememts mean the same.

Please clarify.

Thanks

Arup

[wolfgang2399]

Posted on December 11, 2006 at 06:44

Hi,

it's quite a little bit complicated. There are to different groups of signals:

- the data signal lines: TXD and RXD where +3 to +12 volts indicates an 0-state (SPACE inclusive the STOP-bits) while -3 to -12 volts indicates an 1-state (MARK inclusive the START-bit)

- the control signal lines: RTS, CTS, DSR, DTR etc. where +3 to +12 volts indicates an ON condition while -3 to -12 volts will indicate an OFF condition

Thus you have to ''invert'' the data signals on the way from your mcu to the RS232 interface and vice versa, and ''not to invert'' the control signals.

For further study you'll find a lot of good pages in the internet. I took the information above from

http://pinouts.ru/SerialPorts/RS232_pinout.shtml

Or simply go to

http://www.wikipedia.org/

search for RS232 and follow the links.

Hope it helps. Regards

WoRo

[mailtoarup]

Posted on December 11, 2006 at 08:20

hai,

yes i agree with you !! there is no doubt that on data signals +3 to +12 V will represent 'binary 0' and -3 to -12V will represent 'binary 1'

But the confusion was the statement on page no 5 of Application note 1753.

Only the level is changing. Not the binary 1 in UART to binary 0 in Rs-232 and vice versa.

Thanks,

Arup

[sisto]

Posted on December 27, 2006 at 15:11

there is a logic inverter internal of all EIA RS232 drivers, (MAX232 etc) so a negative RS232 RXD (in) voltage correspond to a UART ''1'' bit and viceversa.

:)