BlueTooth: making connections

To me one of the most exciting aspects of the new NXT is BlueTooth. The ability to have a secure means of communication, and out of line-of-sight was something that was sorely lacking in the RCX. The first step is getting two (or more!) NXTs talking to each other. To be honest, this is in the user’s manual, p34-37, but a brief run-through might be useful.

Take two NXT bricks, and turn both of them on. Now use the BT menus to make sure both have BT turned on, and make sure both are “visible”. If you’ve never paired these NXT bricks with any other BT device, you will find the “Contacts” menu item seem to do nothing – there are no previous contacts recorded, so this menu is essentially blank (if you previously paired your NXT brick with a BT-capable computer, you’ll find at least one entry under “Contacts”, that of the computer itself).

On one of the NXT bricks, under the BT menu select the “Search” option. The NXT will search the local area for any recognizable BT devices, which may take a few seconds (note: if you do this at, say, NI Week, a major technical trade conference, where literally hundreds of folks are walking around with BT-equipped cell phones, laptops, and other devices… well, let’s just say I gave up waiting after awhile). After a while, the NXT pops up a list of the BT devices it has found. Select the other NXT brick from the list, and then connect on “[1]” (connection #1; you could use any connection, but for now I’ll stick with the default). The first time you do this, a “passkey” dialog will pop up on both NXTs; both of them have to accept the offered passkey (usually “1234” unless you really want to change it). Once you’ve done all this, you’ll see that the little “half-diamond” symbol in the status bar of the NXT bricks has changed to a “full diamond”, showing a BT connection. That’s it, the two NXTs are now linked via BT. If you want to follow the directions on pg37 of the user’s guide, you can now send files (but strangely not image files; I’ve no idea why that got left out) between the two NXTs.

By the way, the next time you want to connect these two NXTs, you don’t need to go through the whole searching and passkey circus. The NXT brick has obligingly recorded the name and status of the new BT device, so all you have to do to establish a connection next time is go right to the “Contacts” submenu, and select what NXT you’d like to connect to (and what connection to use). Handy.

Why did I bother walking through this, when it’s already in the user’s guide? Besides the fact that some people (none of the readers here, I’m sure) don’t actually read the entire user’s guide, I wanted to point out some quirks that become very important later. With the two NXTs connected via BT, take a look at the “Connections” menus on both of them. The one that you started the connection from (called the “master”) lists the other as on “[1]”, while the second NXT (call it the “slave”) lists the connection to the first as over connection #0 (“[0]”). Remember that – even though these two NXTs are connected to each other, they “see” the paired device on a different “connections”. Furthermore that initial NXT that you initiated the connection from has special standing; from it you can initiate other BT connection to other devices (like still more NXTs).

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Brian Davis