[OZAPRS] Small GPS

[Tony King]

On 10/01/2011 11:43 AM, James Cameron wrote:

Hi All,

True RS232 which is based on the CCITT standard V.24 is actually a two 
rail system with each rail being required to fall within the range of 
3-15 volts
Logic 1 is negative volts (in old tech talk a Mark) and Logic 0 is 
positive volts (old tech talk is a Space).

The impedance of the source is also defined but from memory I can't 
recall for sure, but I think it is was 3k

Like a lot of things American, they just didn't want to join in with the 
long established international standards and made up there own.
Blimey next we will be talking about Baud speed instead of bps. :-)

                                                               Regards 
Tony VK3API who used to teach this stuff 25 years ago


> On Mon, Jan 10, 2011 at 10:29:52AM +1000, Andrew Rich wrote:
>> I think from memory RS232 is zero is negative volts ?
> No, a logic zero is positive on TxD and RxD.
>
> Control signals are logically inverted though, you may be confused with
> them.
>
>> And a positive in RS232 is above x volts
>> That is how GARMIN get away with the TTL
> That's inverted TTL.  Did that on http://quozl.linux.org.au/ts/ where a
> PIC output pin is connected directly to RxD on an RS-232 serial port.
> The code inverts the bits as they are transmitted, see tx_byte in
> ds1820.asm.
>
> The risk of using TTL output to drive RS-232 RxD is that the RS-232
> standard doesn't define what 0V means.  Usually works.
>