[aprssig] GPS GT-320FW(AS)

Andrew Rich vk4tec at tech-software.net
Wed Apr 24 18:31:56 CDT 2013

I see the opentracker description says

R11 and Q2 form an inverter/buffer circuit for the RS-232 input. The RS-232 
output polarity is controlled in software. The output level swings between 0 
and 5 volts, and may not be compatible with all RS-232 devices.

R11 and Q2 turn the GPS signal upside down and make it MCU compatible.

Whats does "The RS232 output polarity is controlled in software" mean ?

- Andrew -

----- Original Message ----- 
From: "Andrew Rich" <vk4tec at tech-software.net>
To: "TAPR APRS Mailing List" <aprssig at tapr.org>
Sent: Thursday, April 25, 2013 9:08 AM
Subject: Re: [aprssig] GPS GT-320FW(AS)

> Scott,
> Thank you
> So the GPS-320 is LVTTL as opposed to TTL
> In saying TTL I have found some garmins faking RS232 by using TTL
> I have had various GPS
> 1. RS232
> 2. TTL - UART and PC compatible
> 3. LVTTL
> Maybe I can use the GPS-320 RS232 and add a FET ? like you do
> A FET is going to be less work than a MAX232 chip LOL
> Making a tracker for my quad copter
> - Andrew -
> ----- Original Message ----- 
> From: "Scott Miller" <scott at opentrac.org>
> To: "TAPR APRS Mailing List" <aprssig at tapr.org>
> Sent: Wednesday, April 24, 2013 1:51 AM
> Subject: Re: [aprssig] GPS GT-320FW(AS)
>>I haven't been following this thread closely, so forgive me if this has 
>>been covered.  The GT-320FW has both RS-232 and LVTTL outputs and inputs. 
>>To answer Andrew's original question, it'll work just fine with a UART, 
>>USART, SCI, or whatever your vendor calls their asynchronous serial 
>>interface, as long as it's OK with one of those available signals.
>> If you've got a 5v MCU, you'll need to check the threshold voltage for 
>> the I/O pins and make sure it'll work with an LVTTL input.  The 5v 
>> Freescale MCUs I use mostly don't.  Easy fix is a 2N7000 FET, with the 
>> RS-232 output from the GPS connected to the gate, and a pull-up resistor 
>> to Vcc connected to the drain.  Source is connected to ground.  The 
>> signal at the drain pin will be of the proper polarity and voltage for 
>> the MCU's USART.
>> Scott
>> N1VG
>> On 4/23/2013 7:03 AM, Jason KG4WSV wrote:
>>> On Tue, Apr 23, 2013 at 7:27 AM, Dave B <dave at g8kbv.demon.co.uk> wrote:
>>>> Come on guys, this isn't rocket scienct....
>>>> Has everyone forgotten how to think through an issue, or search online?
>>> C'mon Dave, we're just speculating in the absence of information.  One
>>> the one hand, we've got a rather inadequate datasheet from the GPS
>>> manufacturer, and on the other we don't even know what Andrew is
>>> interfacing with.
>>> As to your "google", the only really useful information will come from
>>> the datasheets of the devices in question; everything else is
>>> guessing.  And as I tell my daughter, any idiot can put up a web page,
>>> and many do.  I've even got one. :)
>>>>   You can use just one NPN transistor (and some passives) for the
>>>> incoming '232 to TTL, and similarly, one PNP device (and a -ve rail) 
>>>> for
>>>> the outgoing "driver".
>>> I personally despise this transistor trick.  I find it frequently
>>> unreliable, especially if you're connecting two devices that both use
>>> the trick.  Using a MAX232 (or equivalent) is not that hard or
>>> expensive, and it always works.  I've fabricated some little PCBs that
>>> have a DB9 on one end and TTL level tx/rx/vcc/gnd on the other end,
>>> and have them lying around for this sort of project.  They're a few $
>>> each but can save time and a ton of frustration.
>>> Besides that, transistors aren't usually rated for exposure to the
>>> outside world; I killed a couple 2N7000 transistors on OpenTrackers
>>> before Scott switched to a special 2n7000 with some ESD protection
>>> built in.  The MAX232 type devices include ESD protection since it is
>>> designed to interface to the outside world.
>>>> Next question, who remembers what U A R T actually stands for?
>>> *yawn*  universal asynchronous receiver transmitter
>>> As to the previous "USART" question, at least on some ATmega
>>> microcontrollers they have a chunk of logic for communications that
>>> may be used for UART, SPI, etc, depending on how it's configured.
>>> IIRC, one of the Xmega lines have multiple USARTs that can be used for
>>> either RS232 type or SPI (and maybe other types) communication.
>>> -Jason
>>> kg4wsv
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