Registered: Jun 2000
Location: Lebanon, NH, USA
Posts: 40

I'm not sure which forum to post this...

I've hacked together a Hotsync serial cable for my Visor Deluxe to work with a Magellan GPS 315 handheld. The tricky part was doing it without resorting to running external power to the adaptor cable (pocketAPRS) or stealing it from one of the devices (http://get.to/engineer). As it turned out, the GPS315 doesn't have any handshake pins to steal power from.

As far as I can tell, the GPS 315 uses a low-voltage version of RS-232, while I'm fairly certain that the Visor uses inverted logic 3.3V. So I used a couple of nifty micro-power 1-Gate 3.3V logic chips from Texas Instruments, and stole power from the HS1 pin. Seems to work okay with Delorme Solus Pro 1.5. I finally have a moving map GPS! Sometimes position updates in Solus Pro takes a second or so longer to update than on the GPS display, but I don't have a Palm (which uses real RS-232 levels) to compare with. I suspect that I may be getting some line errors, as I do when manually downloading waypoints using HandTerminal (from the Palm dev kit). But I'll have to get a line sniffer to know for sure.

Other than that, I think the only things missing from my current design are protection diodes for connecting the Visor to real RS-232 levels (larger voltages swings). I'll post a how-to once I compile it. Anyone see problems with stealing 20uA from HS1?

-WaC
Wayne

Edit: HS1 correction

[This message has been edited by wac (edited 06-24-2000).]

Registered: Dec 1999
Location: Connecticut USA
Posts: 2682

The Visor RX and TX lines are TTL-level, not RS232. Also, you said you're drawing power from HS1? That's interesting because when HS1 is asserted low (held to GND, pin 4), a HotSync is initiated. What kind of voltage is it supplying?

------------------
MarkEagle - Ice is nice!

Registered: Jun 2000
Location: Lebanon, NH, USA
Posts: 40

In the strictest sense, I doubt if the 3.3V levels used by the Visor really is TTL; it's most likely "low-voltage" CMOS. I won't know for sure unless I take it apart, but I'll avoid doing that until I break something.

Anyhow, HS1 always has 3.3V on it, and supplies ~10mA when pulled low, although I can't readily verify if it's just a simple ~330 ohm pullup resistor to 3.3V. By using "micropower" logic gates, I can avoid false-triggering the Visor into HotSync mode.

I spent a few hours on the highway with this setup today, driving from NH to CT. It works adequately, despite Solus Pro 1.5's clunky interface, slightly increased Visor battery drain, and some small delays in position updates. I hope the upcoming Solus Pro 2.0 will be easier to use and will magically fix the position update delays.

I also need to take more measurements before I can post a how-to, in case there are any compatibility issues with other RS-232 equipment.

-WaC
Wayne

Registered: Apr 2000
Location: Texas
Posts: 20

wac,

I am confused as to why you need power from the Visor and hand shaking??!! I went and looked at the specs for your GPS at: http://www.magellangps.com/PDFs/gps3153.pdf and saw that all you need is send and receive from the GPS to the Visor using NMEA.

Now as for "I'm fairly certain that the Visor uses inverted logic 3.3V." Remember what was said about the logic 1 and 0 for RS-232. The Visor uses logic 1 as +3.3v and above. Logic 0 is 0v, not negative!!! So if you connect RS-232 directly to your visor and you don't have a circuit to buffer the voltage, then a �3v to -13v will be sent to the Visor on the TX line! This is NOT good! This could damage your Visor!


P.S. I just got a Eagle Explorer GPS myself, I will be connecting it to my Visor via the circuit on my website. I will post my results in a couple weeks.

Good Luck!
Pat http://get.to/engineer

[This message has been edited by EE_Fun (edited 06-27-2000).]

Registered: Jun 2000
Location: Lebanon, NH, USA
Posts: 40

Hi Pat, thanks for the feedback.

I know I don't need handshaking, and I'm quite familiar with computer communications (RS-232. RS-422/485, etc.). Sorry for coming across as a newbie. I should have said negative logic rather than inverted logic.

I finally got a scope on the signals, and found the following:

GPS 315
-------
mark: VOL = 0V (-3V to -12V in RS-232 spec)
space: VOH = 5.9V (+3V to +12V in RS-232 spec)

Visor
-----
mark: VOH = 3.3V
space: VOL = 0V

Other notes: the inverting logic gate that I used to drive the GPS 315's RXD pin pulls HS1 down as low as 2.9V. Fortunately, that's a safe margin above VIL, and the Visor has never entered HotSync mode yet. I think the KBD pin may be able to supply slightly more current than the HS1 pin, but I haven't tried that yet.

As you can see, I need the logic gates for signal inversion. And I need power to run the logic gates. Neither device provides a dedicated pin for supplying power. Palm devices (and PC's) have handshaking pins that you can steal useful power from, but not the Visor.

With the GPS 315, negative voltages won't be a problem, since it doesn't seem to care about the lack of negative transitions. In fact, most modern comm interfaces use the MAX232's Vth of about 1.5V, so you really don't even need negative voltage swings! Even then, negative RS-232 voltages (through the 300 ohm current-limiting resistor) will probably not harm the Visor, as there are most certainly protection diodes in the Visor. I think I may simply add a zener diode to my circuit to protect against overvoltage and negative voltages when I test it on other RS-232 equipment.

I presume the Eagle Explorer has handshaking pins or a power pin?

-WaC
Wayne

Registered: Apr 2000
Location: Texas
Posts: 20

wac,

I can see what your getting at with the gates , but here is the issue I am thinking of. If the Visor sends (TX) the Logic 0 the GPS may treat it as invalid because it is not a valid voltage (according to the RS-232 standards). Now when the Visor receives (RX) the logic 0 from the GPS (neg V) then I could see the Visor dealing with it, but I would recomend it. So I think you will see problems when the Visor Sends a Logic 0. Do you agree, or am I way off?

"I presume the Eagle Explorer has handshaking pins or a power pin?"

*******Changed this portion*******
Now I see what you mean about why you want handshaking, for a power source to drive the MAX chip. I forgot all about that.

How about a battery? All it would take is one 9V or two AAA batteries and they would last a long time.

I still don't know about using diodes and gate in place of the MAX chip circuit. Would the Logic 0 at 0v from the Visor be reconized by the GPS without any errors? I thought RS-232 Logic 0 had to be -3v to -13v in order it to be reconized?
**********************************

This is a great discussion, keep me posted.
Pat

[This message has been edited by EE_Fun (edited 06-27-2000).]

Registered: Jun 2000
Location: Lebanon, NH, USA
Posts: 40

Yes, I'm glad we're working together on this.

What I meant about handshaking/power from the Eagle was: where are you getting your power for the MAX203 from? Your serial cable design takes power from the DTR and RTS pins, but the Eagle Explorer doesn't have either! Are you going to supply power from an external battery or power cord?

As I was trying to say, the majority of devices these days (such as the GPS 315 and the PCs that it works with) will happily take 0V as a "mark". Negative voltages are optional. Try it out for yourself on your cable design - it should work. Large voltage swings are a relic of the past when power supplies were notoriously noisy, or for electrically noisy environments. On battery-powered equipment, we get pretty good noise margins.

I don't think I'm having any problems with the voltages I'm using (as described above). I now suspect I may be getting line errors due to the software. The HandTerminal code doesn't appear optimized, and I have no idea what kind of polling rate Solus Pro 1.5 uses for NMEA communications with the GPS.

Good luck,

-WaC
Wayne

Registered: Jun 2000
Location: Lebanon, NH, USA
Posts: 40

I have compiled a how-to, on the basis that I have verified this design in compatibility testing with a Magellan GPS 315. I have not tested the design on any other serial devices, as I don't have any to connect my Visor to.

You may find the write-up here: http://wac.addr.com/ee/visor/visor.html.

Please let me know if there are any concerns before someone finds a serial device that is incompatible with this design, or worse, will cause damage to either the serial device or the Visor.

Thanks,

-WaC
Wayne

Registered: Apr 2000
Location: Texas
Posts: 20

As an FYI, it can be done!

I finaly connected my Eagle Explorer GPS unit to the serial circuit posted on my website (http://get.to/engineer) All I added was a 9V battery to the circuit, just before the voltage regulator, and it worked great!

Pat