usa1169:
I will test this on a breadboard and try to get the XBee to pass through the TTL data as is, if that fails then I will try inverted, and if that fails I will put an RS-232 shifter in. From the XBee Pro 900 documentation it appeared that it might operate more like a modem and require the RS-232 protocol. But you have encouraged me to give the simple route a try first. Always good to keep it as simple as possible to reduce power consumption and size.
I am virtually certain that you will not need an RS-232 circuit. Look at the Serial Communications section of the manual, beginning on page 10. You’ll see this:
XBee product manual:
The XBee module interfaces to a host device through a CMOS-level asynchronous serial port.
Through its serial port, the module can communicate with any UART voltage compatible device or
through a level translator to any RS-232/485/422 device.
That’s how the 2.4GHz modules work as well - I’ve been feeding them TTL-level data happily for some time now.
I forgot to answer your earlier question about the RF Data Rate vs. the Serial Data Rate. RF Data Rate is the rate at which two modules communicate with each other, using RF. That rate is fixed at 9600 Baud, and I would assume that if signal conditions are less than ideal, the actual rate will be lower than that (but you won’t need to worry about it). For example, the module supports automatic retries (read the section on the RR setting), and obviously if the module has to send the same data several times your overall throughput will be lower than it otherwise would be.
The Serial Data Rate (or “Interface Data Rate”) is the rate at which your microprocessor (or terminal program) communicates with the XBee. This will default to 9600, and the manual points out that increasing the interface data rate won’t necessarily get you more overall throughput. It’s at least as likely that you’ll just overwhelm the (fixed) RF data rate, and lose data. However, you do need to make certain that the interface data rate matches the output rate of the GPS. It looks like the GPS defaults to 9600 Baud, which is convenient, as that’s the default interface rate of the XBee. It looks to me like you can leave things at the default speed, and expect 9600 Baud throughout, except when the signal deteriorates.
usa1169:
On the remote battery powered unit I had planned to put a diode in for reverse polarity protection. But now I will also add a voltage regulator with the appropriate capacitors like I would for a source that started out AC.
The XBee manual also clearly specifies “3.0-3.6 VDC regulated” as the power supply. I think you’ll find that the voltage of a Lipo cell is all over the place, depending on the cell and its charge state, and may easily exceed 3.6V. You’ll want to use a good low dropout voltage regulator capable of providing a comfortable overhead over the 265 mA transmit current required by the XBee. Be careful in choosing the regulator. I see that many low dropout 3.3V regulators require more than 4V to supply 3.3V, and you’re not going to get that from a single Lipo. Read the dropout voltage part of the datasheet. I see that none of the regulators I’ve been using would do what you’re asking for, and a quick look at Digikey suggests that using a Lipo cell to power a 3.3V device requiring over 250 mA might be a little difficult. There are some regulators that say they can do that, but you have to look carefully for them. You’d make life a lot easier if you went to a somewhat higher voltage supply (5V+).
usa1169:
An thank you for the real-world data on range of the 2.4 GHZ PRO units. Was that a 50nW or a 60mW unit?
60mW. This unit:
http://www.sparkfun.com/commerce/produc … ts_id=8742
usa1169:
My remote sensor will be attached to an athlete on the ground so I am going to be dealing with some Fresnel zone interference that you do not get. But it will be clear line of sight. The 900MHZ should deal with the fresnel issues better than 2.4GHz… but since I have a ton of 2.4GHz equipment I may try a 2.4GHz XBee like you used for the remote sensor and then use a high gain parabolic on the base station. I know from past experience that a good directional antenna will allow you to link to a laptop wireless over two miles. That just requires that the antenna track the target. If you are getting a few thousand feet with those tiny omni directional wire antennas I would not be surprised if I can get the range I need with 2.4GHz and a really good directional on one end. Thanks again for the help.
Now you’re out of my range of expertise (grin), but if you can use a good directional antenna I’m sure that your odds will improve a lot.
Remember again that in my situation, the transmitter is located free and clear in the air over my head, which also makes a HUGE difference. If you already have the 2.4GHz XBees, though, yeah, trying them is a good idea. I’ve been pleasantly surprised so far: I believe that I just might get the full 1 mile line-of-sight range they advertise in the manual. I’m already getting around 4000 feet of range, and that’s with that little wire antenna, and a transmitter that’s mounted inside a phenolic tube. And I haven’t lost data even right up to landing (though again, if it’s behind a hill, that’s another story), so it’s not entirely the altitude that’s doing it for me.
