Garmin GPS to TinyTrak3 problems
 

Hi, Everybody,

I am trying to set up an APRS station consisting of a Garmin GPSmap 60 and
one of the TinyTrak3 TNCs. However, the green 'GPS Valid' LED never lights.

I can connect the GPS receiver directly to my computer's serial port and
read good data, and I can capture some of that data and play it back from
computer to TinyTrak and the green LED lights up, but a direct connection
between GPS and TT just does not work.

I see a waveform coming out of the GPS serial (J2 pin 2 of the TT) using a
scope. Quiescent voltage is -5V, and when the data burst comes it is -5 V to
+5 V.

Could this be a voltage level problem on the serial? Is there a converter
that can be used? Or is it another problem, possibly protocol-related?

Any ideas?

Thank you,

Al W6LX

On Mon, 30 May 2005 17:06:59 -0700, "Anonymous"
wrote:

Hi, Everybody,

I am trying to set up an APRS station consisting of a Garmin GPSmap 60 and
one of the TinyTrak3 TNCs. However, the green 'GPS Valid' LED never lights.

I can connect the GPS receiver directly to my computer's serial port and
read good data, and I can capture some of that data and play it back from
computer to TinyTrak and the green LED lights up, but a direct connection
between GPS and TT just does not work.

I see a waveform coming out of the GPS serial (J2 pin 2 of the TT) using a
scope. Quiescent voltage is -5V, and when the data burst comes it is -5 V to
+5 V.

Could this be a voltage level problem on the serial? Is there a converter
that can be used? Or is it another problem, possibly protocol-related?

Any ideas?

Thank you,

Al W6LX


OK Al, I'm going to go out on a limb here. The GPS doesn't work on
the TNC. It does work on the PC. The PC can send GPS data it
recorded from the GPS out to the TNC and the TNC recognizes the dya as
GPS data and interprets the protocol correctly (the green LED goes
on). You measured the waveform from the GPS and it shows +- 5V.

I. The GPS is good, it works on the PC.
II. The PC is good, it reads and writes GPS data.
III. The TNC is good, it reacts correctly to the PC and the GPS data
from the PC.
IV. The GPS waveform is +- 5V.

My conclusion: The voltage is too small for the TNC to recognize.
Did you 'scope the PS serial data? If you do, you will see that it is
+- 12V. The PC's serial port can read and write both +-5V and +-12V
levels. You need a level converter to change the GPS data to the
level that the TNC needs to see.

Russ

In article , "Anonymous"
wrote:

I am trying to set up an APRS station consisting of a Garmin GPSmap 60 and
one of the TinyTrak3 TNCs. However, the green 'GPS Valid' LED never lights.

Al-

Russ may be right about needing level conversion. Another possibility is
that you need a null-modem adapter in the line between the GPS and the
TNC.

And what about baud rate? What if the PC has automatic baud-rate
adjustment and the GPS & TNC don't?

73, Fred, K4DII

Thanks to all who replied. In the end, I never got it working. It appears to
be a basic incompatibility between the Garmin GPSmap 60 and the TinyTrak3. I
should have done better research into exactly which GPS models are
compatible with the TinyTrak3.

An exhaustive search of the Internet turns up absolutely no information on
incompatibilities between the TinyTrak3 and the vast number of GPS units on
the market today. But neither could I find any complaints of people running
into incompatibility problems. Strange.

Incidentally, the latest generation of Garmin units have only USB ports, no
more serial ports. By definition, all of these are incompatible with the
TinyTrak3.

I have a feeling that the TinyTrak3 hasn't kept up with the times and if it
expects to survive it has to be updated to allow operation with the latest
NMEA protocol version and the new USB units.

The TinyTrak is a good idea. I hope it gets updated, and I hope to see a
section on its web site with information about which GPS's are okay and
which ones to avoid.

Regards,

Al W6LX







"Anonymous" wrote in message
...
Hi, Everybody,

I am trying to set up an APRS station consisting of a Garmin GPSmap 60 and
one of the TinyTrak3 TNCs. However, the green 'GPS Valid' LED never
lights.

I can connect the GPS receiver directly to my computer's serial port and
read good data, and I can capture some of that data and play it back from
computer to TinyTrak and the green LED lights up, but a direct connection
between GPS and TT just does not work.

I see a waveform coming out of the GPS serial (J2 pin 2 of the TT) using a
scope. Quiescent voltage is -5V, and when the data burst comes it is -5 V
to
+5 V.

Could this be a voltage level problem on the serial? Is there a converter
that can be used? Or is it another problem, possibly protocol-related?

Any ideas?

Thank you,

Al W6LX

Al,

"Anonymous" wrote in message
...
I have a feeling that the TinyTrak3 hasn't kept up with the times and if it
expects to survive it has to be updated to allow operation with the latest
NMEA protocol version and the new USB units.

Unfortunately, whereas "RS-232 and NMEA" are very much a standard, the newer
GPS receivers with USB ports neither all show up as a "standard serial-type
device" nor do they encode NMEA the same way. :-( Updating the TinyTrak to
support USB receivers is a _major_ undertaking... whereas you can talk RS-232
with any old microcontroller (especially at the 4800bps that NMEA typically
uses), I believe that the commercial USB receivers are running full speed
(12Mbps) and there is simply no way to build a full speed USB master (what you
need to talk to one of these receivers) in software alone. I.e., the TinyTrak
would have to change to incorporate a microcontroller containing a USB _host_
(and these are few and far between -- Cypress has one), or it would have to
add a separate chip to perform this function.

OEM GPS modules that spit out RS-232 won't be going away any time soon; even
in 2010 you'll still have no problem getting them.

---Joel Kolstad

"Joel Kolstad" wrote in message
...
Al,

"Anonymous" wrote in message
...
I have a feeling that the TinyTrak3 hasn't kept up with the times and if
it
expects to survive it has to be updated to allow operation with the
latest
NMEA protocol version and the new USB units.

Unfortunately, whereas "RS-232 and NMEA" are very much a standard, the
newer
GPS receivers with USB ports neither all show up as a "standard
serial-type
device" nor do they encode NMEA the same way. :-( Updating the TinyTrak
to
support USB receivers is a _major_ undertaking... whereas you can talk
RS-232
with any old microcontroller (especially at the 4800bps that NMEA
typically
uses), I believe that the commercial USB receivers are running full speed
(12Mbps) and there is simply no way to build a full speed USB master (what
you
need to talk to one of these receivers) in software alone. I.e., the
TinyTrak
would have to change to incorporate a microcontroller containing a USB
_host_
(and these are few and far between -- Cypress has one), or it would have
to
add a separate chip to perform this function.

OEM GPS modules that spit out RS-232 won't be going away any time soon;
even
in 2010 you'll still have no problem getting them.

---Joel Kolstad

A lot of GPSs these days have Bluetooth and so practically all PDAs and
almost all notebooks - now all we need is for the radio manufacturers to
include it.
73
Roger ZR3RC

A lot of GPSs these days have Bluetooth and so practically all PDAs and
almost all notebooks - now all we need is for the radio manufacturers to
include it.
===========================
Perhaps there are Bluetooth to serial port RS232 /USB1 /USB2 converters ?

Frank GM0CSZ / KN6WH

"Highland Ham" wrote in message
...
A lot of GPSs these days have Bluetooth and so practically all PDAs and
almost all notebooks - now all we need is for the radio manufacturers to
include it.
===========================
Perhaps there are Bluetooth to serial port RS232 /USB1 /USB2 converters
?

Frank GM0CSZ / KN6WH



USB --- Bluetooth are very common and quite cheap - getting to RS232 might
take a bit more research.

Hi Roger,

"Roger Conroy" wrote in message
...
A lot of GPSs these days have Bluetooth and so practically all PDAs and
almost all notebooks - now all we need is for the radio manufacturers to
include it.

AFAIK, there aren't any simple "microcontroller to Bluetooth" interfaces
available either. I forget the exact chip, but I'm fairly certain the
TinyTrak 3 uses some little 8 bit microcontroller with probably no more than
"many kilobytes" of flash ROM and "a handful of kilobytes" of RAM. That's
nowhere near enough to support Bluetooth.

Bluetooth is also a lot more power hungry than a wired connection would be,
and of course is prone to interference since it's operating in the 2.4GHz ISM
bands.

---Joel

Unfortunately, whereas "RS-232 and NMEA" are very much a standard,

Hi, Joel,

Actually, RS-232 has always baffled me, and from some measurements I made it
doesn't seem to be a very strict standard. For example, the waveform coming
out of the GPS receiver was +/- 5 V, whereas I have seen the waveform out of
the RS-232 on the computer at +/- 12 V. Is the voltage level arbitrary?

Al

Hi Al,

"Anonymous" wrote in message
...
Actually, RS-232 has always baffled me, and from some measurements I made it
doesn't seem to be a very strict standard.

There really is an RS-232 standard, it's just commonly abused. Here's what I
have from a "cheat sheet" I have at hand:

-- Transition levels are between +/-3V (this is what you have to detect)
-- Maximum open circuit voltage is 25V (!)
-- Maximum load capacitance is 2.5nF
-- Load resistance is 3-7kohm (this is ignored all the times -- often the load
is much, much higher than this)
-- Maximum slew rate is 30V/us (probably violated occasionally but no one
checks :-) )
-- Maximum transition time is 4% of bit time
-- Maximum data rate is 20,000 bits/second (obviously ignored with good
reason...)

The above is supposedly the official EIA-232 standard, "revision D," approved
in November, 1986.

As you can see, both +/-5V (open circuit) and +/-12V drivers can readily meet
the spec so long as they still swing +/-3V into 3k loads.

Often times the receivers are designed so that the "logic 1" input is anything
below about a voltage or two and anything above that is a "logic 0". This is
done (rather than having the threshold at 0V) so that you can get away with
hooking a TTL or CMOS logic output directly to the RS-232 input and still
successfully receive characters. You'll occasionally see people ask whether
or not some given serial card or USB to serial adapter will support such
shenanigans. :-)

---Joel

Thanks, Joel! This is all very good information. It does bring up another
point, though, related to my original question about interfacing the GPS to
the TinyTrak3.

The GPS puts out +/- 5 V serial, but the Tiny Trak's serial input is a
TTL-level input to the PIC microprocessor. The PIC runs off of a single +5V
supply. I guess it can withstand the negative voltage swing without failing?
And... if I looked at the serial output from the GPS into an open circuit,
there was no signal. It was as if the GPS shut off its serial if it detected
an open circuit. Is this normal? Even after connecting it to the TinyTrak,
many times it wouldn't 'wake up' and start sending serial data. This was
very confusing.

Thanks

Al W6LX





There really is an RS-232 standard, it's just commonly abused. Here's
what I
have from a "cheat sheet" I have at hand:

-- Transition levels are between +/-3V (this is what you have to detect)
-- Maximum open circuit voltage is 25V (!)
-- Maximum load capacitance is 2.5nF
-- Load resistance is 3-7kohm (this is ignored all the times -- often the
load
is much, much higher than this)
-- Maximum slew rate is 30V/us (probably violated occasionally but no one
checks :-) )
-- Maximum transition time is 4% of bit time
-- Maximum data rate is 20,000 bits/second (obviously ignored with good
reason...)

The above is supposedly the official EIA-232 standard, "revision D,"
approved
in November, 1986.

As you can see, both +/-5V (open circuit) and +/-12V drivers can readily
meet
the spec so long as they still swing +/-3V into 3k loads.

Often times the receivers are designed so that the "logic 1" input is
anything
below about a voltage or two and anything above that is a "logic 0". This
is
done (rather than having the threshold at 0V) so that you can get away
with
hooking a TTL or CMOS logic output directly to the RS-232 input and still
successfully receive characters. You'll occasionally see people ask
whether
or not some given serial card or USB to serial adapter will support such
shenanigans. :-)

---Joel

Hi Al,

"Al Lorona" wrote in message
...
The GPS puts out +/- 5 V serial, but the Tiny Trak's serial input is a
TTL-level input to the PIC microprocessor. The PIC runs off of a single +5V
supply. I guess it can withstand the negative voltage swing without failing?

You'd have to look at the PIC's data sheet, but it's very common for digital
ICs to have appropriately connected diodes to Vcc and Ground for the sake of
ESD protection. It's not the most elegant design, but it's not uncommon to
use these built-in diodes to limit (clamp) input voltages to a diode drop
above Vcc or below ground, which -- assuming the current through the diodes is
limited, hence the power dissipated by them is -- is usually harmless.

And... if I looked at the serial output from the GPS into an open circuit,
there was no signal. It was as if the GPS shut off its serial if it detected
an open circuit. Is this normal?

I am a little surprised, to tell you the truth, but given that the Garmin is
typically battery powered, it would make sense for them to shut down the
serial interface circuitry if they don't detect anyone "listening."

---Joel

On Tue, 7 Jun 2005 19:47:54 -0700, "Joel Kolstad"
wrote:

Hi Al,

"Anonymous" wrote in message
.. .
Actually, RS-232 has always baffled me, and from some measurements I made it
doesn't seem to be a very strict standard.

There really is an RS-232 standard, it's just commonly abused. Here's what I
have from a "cheat sheet" I have at hand:

-- Transition levels are between +/-3V (this is what you have to detect)
-- Maximum open circuit voltage is 25V (!)
-- Maximum load capacitance is 2.5nF
-- Load resistance is 3-7kohm (this is ignored all the times -- often the load
is much, much higher than this)
-- Maximum slew rate is 30V/us (probably violated occasionally but no one
checks :-) )
-- Maximum transition time is 4% of bit time
-- Maximum data rate is 20,000 bits/second (obviously ignored with good
reason...)
-- The maximum distance is 15 m (or was it 50 ft originally). This is
often ignored and usually the problems are related to ground potential
differences.

The 20 kbit/s makes sense, when you look at the other requirements. At
this speed, the bit time is 50 us and a 4 % transition is 2 us. At 30
V/us, that would be a 60 V transition from -30 V to +30 V. Compare
this to the 25 V open circuit voltage.

To charge a 2.5 nF capacitor with the 30 V/us slew rate, a 75 mA
current is required. IIRC, the standard also contained a short circuit
current limit about that magnitude. The power dissipation can be quite
high in the transmitter.

These calculations clearly indicate the fundamental problems with
RS-232.

The RS-422/485 and later LVDS solved the problem by radically reducing
the driver voltage swing to a few volts or even below 1 V. Even with a
modest 30 V/us slew rate, the transition from one state to an other
does not take a long time. With the termination resistance of the
order of 100 ohms, the connection can also be analyzed as a bipolar
current loop. Since the characteristic impedance of a twisted pair
cable is typically around 100 ohms, this is also a well matched (low
SWR :-) transmission line system, so there are not much reflections
that would mess with the bit transitions. Also the differential
construction avoids most of the ground potential difference problems.
RS-422/485 is usually implemented with transceivers running from a
single +5V supply

It is sad that RS-422/485 is not used in common PCs, but instead all
kinds of hacks violating the RS-232 are used (some even claim TTL
signal levels RS-232 "compatible").

Paul OH3LWR

In article ,
Anonymous wrote:

Unfortunately, whereas "RS-232 and NMEA" are very much a standard,

Hi, Joel,

Actually, RS-232 has always baffled me, and from some measurements I made it
doesn't seem to be a very strict standard. For example, the waveform coming
out of the GPS receiver was +/- 5 V, whereas I have seen the waveform out of
the RS-232 on the computer at +/- 12 V. Is the voltage level arbitrary?

The RS-232 specification formally requires that the voltage swing both
positive and negative, with respect to ground, by at least 5 volts.
Swings for a loaded circuit can go up to +/- 15 volts, and the swing
must not exceed +/- 25 for an unloaded circuit.

RS-232 receivers are required to be sensitive enough to detect
reliably at +/- 3 volt levels. Voltages between +3 and -3 are
somewhat of a "no-man's land" and as far as I know the RS-232 standard
is silent on how they should be interpreted.

The slightly newer RS-423 spec also requires positive and negative
swings, but they only need to be +/- 3.6 volts at a minimum, +/- 6
volts at a maximum, and the receivers are required to have +/- 200
millivolt sensitivity. This standard allows for simpler operation
from a 5-volt-only circuit (a simpler charge pump can generate a -5
supply), and the increased receiver sensitivity allows the system to
work with longer lines (up to 4000 feet, where RS-232 is formally
limited to 50 feet).

A lot of devices these days don't produce properly-conforming RS-232
*or* RS-423 signal levels on their serial ports. It's fairly common
to see devices which use TTL/CMOS voltage levels - swinging up no
higher than 5 volts, and down to ground.

To cope with such nonstandard signals, it has become fairly common to
use serial-port receivers which are deliberately asymmetric... they
have a single detection threshold, somewhere in the 1-volt range I
think. A receiver of this sort will work properly (with a slight
reduction in noise margin) with a real RS-232 or RS-423 transmitter
which swings to +5/-5 or above, and will also work with a "pseudo-RS-
423" transmitter which swings +5/0.

--
Dave Platt AE6EO
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