[aprssig] GPS GT-320FW(AS)
vk4tec at tech-software.net
Tue Apr 23 13:52:57 CDT 2013
Sent from my iPhone
On 24/04/2013, at 1:51 AM, Scott Miller <scott at opentrac.org> 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.
> 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.
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