[OZAPRS] Small GPS

[Andrew Rich]

The considerations I have struck are

1) Does the device do true RS232
2) Does the device simulate RS232 at TTL levels
3) Does the device do TTL but inverted for the USART in a MICRO
4) Does the device meet the minimum requirements for volts in a logic "high"

I had to invert the TTL from the GPS 18 to make it work with the USART in 
the 16F628

I could bit bang but I am too lazy.

Some GPS modules are now coming out uP happy and are already inverted.

- Andrew -


----- Original Message ----- 
From: "James Cameron" <vk2lqz at wia.org.au>
To: "Andrew Rich" <vk4tec at tech-software.net>; "Australian APRS Users" 
<ozaprs at aprs.net.au>
Sent: Monday, January 10, 2011 10:43 AM
Subject: Re: [OZAPRS] Small GPS


> 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.
>
> -- 
> James Cameron
> http://quozl.linux.org.au/