Robot Communications

I need to talk to the robot to operate it remotely. This raises all sorts of questions of what sort of bandwidth do you need, etc.

Nominal Requirement:

Moderately high rate >9600 bps range 1 km

Fall back low rate >300 bps?


TNC on both ends - 1200 bps or 9600 bps - 2m or 440 MHz, or FRS radios - Using the FRS radios for data is probably illegal, even though hooking them up to the TNC is trivial. The real problem is that TNCs seem to be made for a somewhat archaic market. They're big, clunky, stupid, and the modulation is designed to be compatible with wireline modems from the 1960's. Interoperability with other folks is a fine reason for using these in ham radio (like APRS, for instance), but where I control both ends of the link, they are pretty non-optimal.

Linx Technologies ( Inexpensive modules designed for the Part 15 market. Of particular interest is the HP-II series in the 900 MHz band (actually 903.37 to 921.37). I've got the evaluation boards (which work pretty well), although they have weird reverse polarity SMA connectors (needed for Part 15). Since it's legal for me, I can replace those with normal connectors, dump the attenuator pad on the transmitter, and put a much better antenna on (and, potentially, put an amplifier on the TX). The ham band is 902 to 928, so the Linx parts fall within. More on the Linx mods and usage, as well as antennas.

Wireless LAN (802.11b) - I've been fooling with a wireless LAN at home, and while they claim ranges of 10's of meters, that's clearly in a big open room. A realistic range at full speed is more like 5 or 6 meters, and intervening walls are apparently pretty good attenuators at 2.5 GHz. Maybe with a gain antenna (these have those funky Part15 connectors too!) and a suitable diplexer/amplifier, this might work. The cool thing would be that you could make the robot an internet accessible device. Of course, a decent serial link, and an appropriate TCP/IP stack would do the same thing. Rabbit makes an internet peripheral, which might be a better approach.

Cellular telephone - take advantage of those "free cellphone to cellphone calls" deals and use a cell phone on each end. A very clever scheme, and it would work over long distances too. Data rate is pretty low (no more than 8 kbps, really), so you're not sending video at full speed. This is an intriguing way to control a large, free ranging robot.


Using Amateur radio has some implications, as does using Part 15 (unmodified). Some discussion is here.


revised 19 November 2001, Jim Lux
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