[aprssig] GPS GT-320FW(AS)

Andrew Rich vk4tec at tech-software.net
Wed Apr 24 19:08:12 EDT 2013


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

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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