Making a BS-2 Look Like a GPS

Archiver

Check out this website http://www.byonics.com. Looks at the Tiny
Trak II, this unit should work for you. It's made for APRS but it
should work fine for your apllication. I have two of these units and
they work really well. You can not beat the price, 30$.

Chris
KE6MTO

--- In basicstamps@y..., verhap@o... wrote:
> One of my new projects is creating a new controller for high
altitude
> balloons. In it, a BS-2P will capture data from a GPS into scratch
> pad memory for processing. Afterwards, the BS-2P must send a
> simplified version of that data to a MIM. The MIM is a single
board
> TNC. For those of you who aren't lucky enough to be a ham radio
> operator, a TNC is a modem for radio. I can send serial data (at
> 1200 baud) over amateur radio instead of phone lines. This really
> becomes important when a balloon reaches an altitude of 20 miles.
>
> So here's my problem. The MIM expects to be connected to a
> GPS. The NMEA standard is 4800 baud, N81. The GPS acts like
> a RS232 device, so the logic is inverted. Technically the voltage
> swings in the signal should be -10 to + 10 volts. However many
> (all?) GPS recievers operate at TTL levels (0 to 5 volts). I
program
> a BS-2P to send test data (the text is GPTEST) at 4800, N81,
> inverted. I can see the data over my o'scope and know it's getting
> to the MIM. The MIM is programmed to send telemetry at 20
> second intervals. The telemetry it's looking for is GP???, where
> ??? is a three letter wildcard.
>
> The MIM never sends the data. I know the MIM works because it
> will send beacon data 9which is programmed into the MIM). I'm
> wondering if the MIM is really looking for RS232 level signals, but
> find that hard to believe because, as I understand, most GPS
> receivers don't output true RS232 levels.
>
> If I send data inverted, the voltage level of the pin is 0 volts
until the
> signal comes in. I can see the square wave of the text string.
after
> the text is sent, the voltage level drops back to 0 volts until the
> next text is sent. If I send the data with true logic, the voltage
on
> the pin is 5 volts until the signal comes in. After the text is
sent,
> the voltage level goes to 5 volts until the next text is sent.
>
> I really true logic to sit at 0 volts when there was no signal and
> inverted logic to sit at 5 volts where was no signal. So
> I'm a little confused there.
>
> So the short and skinny of my question. Is anyone familiar with
> the MIM? Does it require RS232 level signals? Don't most GPSs
> output TTL level signals? What should serial data look like in the
> o'scope if it's inverted and when it's true?
>
> Thanks,
> Paul

Archiver

One of my new projects is creating a new controller for high altitude
balloons. In it, a BS-2P will capture data from a GPS into scratch
pad memory for processing. Afterwards, the BS-2P must send a
simplified version of that data to a MIM. The MIM is a single board
TNC. For those of you who aren't lucky enough to be a ham radio
operator, a TNC is a modem for radio. I can send serial data (at
1200 baud) over amateur radio instead of phone lines. This really
becomes important when a balloon reaches an altitude of 20 miles.

So here's my problem. The MIM expects to be connected to a
GPS. The NMEA standard is 4800 baud, N81. The GPS acts like
a RS232 device, so the logic is inverted. Technically the voltage
swings in the signal should be -10 to + 10 volts. However many
(all?) GPS recievers operate at TTL levels (0 to 5 volts). I program
a BS-2P to send test data (the text is GPTEST) at 4800, N81,
inverted. I can see the data over my o'scope and know it's getting
to the MIM. The MIM is programmed to send telemetry at 20
second intervals. The telemetry it's looking for is GP???, where
??? is a three letter wildcard.

The MIM never sends the data. I know the MIM works because it
will send beacon data 9which is programmed into the MIM). I'm
wondering if the MIM is really looking for RS232 level signals, but
find that hard to believe because, as I understand, most GPS
receivers don't output true RS232 levels.

If I send data inverted, the voltage level of the pin is 0 volts until the
signal comes in. I can see the square wave of the text string. after
the text is sent, the voltage level drops back to 0 volts until the
next text is sent. If I send the data with true logic, the voltage on
the pin is 5 volts until the signal comes in. After the text is sent,
the voltage level goes to 5 volts until the next text is sent.

I really true logic to sit at 0 volts when there was no signal and
inverted logic to sit at 5 volts where was no signal. So
I'm a little confused there.

So the short and skinny of my question. Is anyone familiar with
the MIM? Does it require RS232 level signals? Don't most GPSs
output TTL level signals? What should serial data look like in the
o'scope if it's inverted and when it's true?

Thanks,
Paul

Archiver

> > So the short and skinny of my question. Is anyone familiar with
> > the MIM? Does it require RS232 level signals? Don't most GPSs
> > output TTL level signals?

I have no experience with the MIM but I think I know what you are
trying to do, you want to capture the GPS sentences with the BSII and
inject data (telemetry) into them to be transmitted by the MIM, yes?

If so, I would get the GPS to MIM connection is working first. I
believe most consumer GPS units will output normal (inverted) RS232,
while most board-level GPS modules will output TTL (non-inverted)
signals. The MIM documentation should tell you what is needed.

Once you confirm everything is working you can break the connection,
capture the GPS signals with the BSII, modify your sentences and send
them to the MIM. The baud rate/signal type you specify in your SERIN
for receiving from the GPS should now be the same as what you will
transmit to the MIM in your SEROUT.
Good Luck.

Carl N7UZP

Archiver

> I have no experience with the MIM but I think I know what you are
> trying to do, you want to capture the GPS sentences with the BSII and
> inject data (telemetry) into them to be transmitted by the MIM, yes?

That's exactly right.

> If so, I would get the GPS to MIM connection is working first. I
> believe most consumer GPS units will output normal (inverted) RS232,
> while most board-level GPS modules will output TTL (non-inverted)
> signals. The MIM documentation should tell you what is needed.

The GPS outputs an inverted TTL level signal, so I programed the
stamp to do the same thing. An o'scope shows me the signal is
getting there, but the MIM doesn't respond.

I think I will try getting a GPS connected to he MIM. I'm beginning
to think the problem is with the MIM and this should help to prove it.

I've also wondered if there should be a pull up or pull down resistor
on the serial line.

Paul