[aprssig] GPS GT-320FW(AS)

Scott Miller scott at opentrac.org
Wed Apr 24 19:35:06 EDT 2013

The HC08 MCUs have a TX invert option.  The tracker just sets the output 
to be inverted and it's compatible with RS-232, but only 0 to 5v swing.


On 4/24/2013 4:31 PM, Andrew Rich wrote:
> 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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