[aprssig] GPS GT-320FW(AS)

Scott Miller scott at opentrac.org
Tue Apr 23 11:53:22 EDT 2013

And yes, the gate on a standard 2N7000 is really ESD sensitive.  I've 
only had unprotected FETs die on the older OpenTrackers when they had 
their gate exposed on an external pin, though, and not when the signals 
were only internal to the board.


On 4/23/2013 8:51 AM, Scott Miller wrote:
> 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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