November 2007

You are currently browsing the monthly archive for November 2007.

One the main problem with my bots (or the like…) is the way communications occur. I mean it can only use RS232 with a cable, and bots must stay closed to the PC from which they are actually controlled and monitored, and where data is stored. Basically, I need them to be wireless.

A first approach would use a PDA. The bot is connected to, with a RS232 cable (iPaq PDA have a serial connector). PDA then talks to the server using its builtin wifi or bluetooth and sends collected data. There’re even Linux distros which can be used on an iPaq, such as the familiar (works nice!). The problem is each time I need a bot to be wireless, I need an iPaq. And an iPaq is expensive (~ 120€ @ ebay). And big. And the one I have can’t have wifi or bluetooth.
Wifi ? Why can’t I use a Wifi board ? It’s way too expensive (~ 200€)… UART to wifi boards cost a lot, because from UART to Wifi, there’s a lot. Another problem is the battery life. If I want my bots to be wireless, they also need batteries. And wifi eats a lot of energy.
Recently, I’ve found some nice bluetooth OEM boards @ Lextronic. Cheap (~ 30€) and easy to use (on paper). Battery life is ok using bluetooth. Yeah, bluetooth seems to be the solution. When I’ve come to see those boards, I’ve realized how small they are. Way too small… That would be an advantage. But not for me, as I can’t even solder it (no pins, connectors are under the board). There’s another version: the OEM board is soldered on another board, with ready-to-use connectors. Even those connectors are too small, and this time this is twice the price (~ 65€).
So, what now ? Should I give up wireless robots ? When I’ve seen those bluetooth boards, the seller said: “Why don’t you use those tiny XBee boards ? Problem is you’ll have to have another boards connected to your PC, since none handles the ZigBee protocol“. And I said :”No way, I want a standard stuff”. Back to home, desperately giving up bluetooth, I’ve looked to those Xbee boards. ZigBee protocol seems to be a standard, but not well-known (Master Fenyo said: “It’s very usual, it’s also called the professional bluetooth”. Quite nice ! ZigBee is also known to be more reliable and uses less energy. Users report on forum those are very easy to use. And price is ok (~ 25€). Wikipedia shows an interesting comparison, showing why ZigBee is a good choice for embedded systems:

Comparison: Wifi, bluetooth, ZigBee
Protocol ZigBee Bluetooth Wifi
IEEE 802.15.4 802.15.1 802.11a/b/g
Memory needs 4-32 Kb 250 Kb + 1 Mb +
Autonomy (with battery) Years Days Hours
Number of nodes 65 000 + 7 32
Speed 250 Kb/s 1 Mb/s 11-54-108 Mb/s
Range 100m 10-100 m 300 m

ZigBee is my better choice. For now… The only problem is I need one on my PC. Next step is to check how this could be (very) easily done).