TETRIX Using Motor Servo Controllers
|Bluetooth (BT) is an industry standard short-distance (up to 10 meters) wireless communications protocol operating at 2.4 GHz. It can optionally use a higher power transmission to achieve distances up to 100 meters. The NXT utilizes the 10 meter option.
BT includes not only the low-level radio transmission (at 2.5 GHz) but also several higher layer message protocols (or profiles) designed for different applications. There are over 25 different BT profiles currently defined. Some of the more popular profiles include:
In order to connect two devices via BT, the devices must not only support BT but also support the type of profile that will be used for the connection. The NXT only supports the SPP so that it cannot, for example, directly connect to a Nintendo or Sony game controller.
It is very easy to set up Bluetooth communications between two NXTs.
ROBOTC measured performance to send a BT message, receive it at far end, process it, generate a reply and receive it at original device is about 36 such transactions per second. Other implementations (e.g. NXT-G) typically support about 13 transactions per second.
ROBOTC allows full duplex operation over a single BT stream. Measured performance indicates that each end of the stream can autonomously send 250 messages per second. The half-duplex implementation is limited to 13.
This functionality is similar to that found on the LEGO Mindstorms RCX with a few notable exceptions:
Connecting Two NXT Bricks
Disconnecting Two NXT Bricks
PC/NXT Pairing with ROBOTC
|To pair your NXT with your Bluetooth adapter to use ROBOTC, follow these steps.
Note: You cannot download firmware to an NXT over Bluetooth. Please download ROBOTC's firmware before trying to connect via Bluetooth.
|1. Go to the "Robot" menu, and under the "NXT Brick" submenu, select "Link Setup."|
|File:1 link setup.gif|
|2. In the NXT Brick Link Selection screen, we want to enable Bluetooth seaching. Click the checkbox labeled "Include Bluetooth in Brick Search" and then click the "Refresh Lists" button.|
|3. ROBOTC will use the bluetooth adapter to search for NXT bricks. This may take 30 seconds.|
|4. When ROBOTC is competed searching for Bricks, you can select which brick you would like to connect to. Click the name of the brick under the "NXT Bricks Reachable via Bluetooth Wireless" menu and then click the "Select" button on the right.|
|5. ROBOTC will remind you to press the orange button on the NXT when the NXT make a sound. Click "OK" to continue.
This is the NXT requiring input for the Bluetooth passcode. The default passcode will automatically be entered, so just press the orange button.
|6. ROBOTC will start to pair with the NXT. This may take 15-30 seconds and you will have to press the orange button once during this time.|
|7. Once the brick has connected via Bluetooth, you will see the brick's name appear under the "NXT Brick Connection History". Your NXT is now connected via Bluetooth! You can continue to use ROBOTC as if your brick was connected via USB... everything will work, including the debugger and program downloading. Click "Close" to exit this screen.|
Connecting via Bluetooth - Pairing
|Each BT device is identified by a unique 12 hexadecimal digit address. It's a little awkward to refer to devices with this address so the BT protocols include a "friendly name" that is a more conventional 15-character string. The default friendly name for a NXT brick is "NXT" but you can modify this using the NXT's on-brick user interface, or from the ROBOTC IDE, or directly from within a ROBOTC program.
Before you can connect two devices via BT they must be "paired". The pairing process exchanges messages between the two devices where they share their BT address, their friendly names and their supported profiles. The devices confirm that they support the same profiles and that the (optional) password matches. Once two devices have been paired, you can make subsequent connections using just the friendly names. The devices remember the passwords so that they don’t have to be re-entered each time
There's a lot of low-level messages used in establishing a paired connection. Fortunately, this is hidden from the user on the NXT with a simple user interface via the user interface on the NXT.
From the BT menu on the NXT, a search is performed to build a internal list in the NXT of all the devices that are currently available via BT. The NXT simply broadcasts a "who's available" message over BT and collects a list of all the devices that replied.
To learn how to connect two NXTs together, please see the "Connecting NXTs" page here.
Once connected, the "slave" device will be added to the NXT's internal table of paired devices, i.e. the "My Contacts" list on the NXT. Next time you want to make a connection you can select the device from the "My Contacts" and avoid the 30-second search.
NOTE: Sometimes you'll find that you cannot get two previously paired devices to connect. One cause of this is that one device is no longer powered on. Another cause is that somehow the devices have got out of sync on the pairing status where one device has "lost" the paired status. If this happens, try removing both devices from the "My Contacts" list on each NXT and restart the connection using the "Search" function.
Sending Messages via Bluetooth
|Once you have two NXTs connected via BT, it is very easy to send messages between them. There are only three functions that are needed:
After calling each of the above functions you should check the returned value to determine the success/failure of the function. Use the function cCmdMessageWriteToBluetooth(nQueueID, nXmitBuffer, nSizeOfMessage) to send a BT message to the far end NXT. Check the error code to make sure message was transmitted successfully. This sends the message (up to 58 bytes in length) in the variable nXmitBuffer to queue or mailbox number nQueueID on the far end NXT. nSizeOfMessage is the length of the message.
When messages are received over BT by a NXT they are automatically added to the end of one of the 10 message or mailbox queues. Use the function cCmdMessageGetSize(nQueueID) to determine whether any messages have been received. A positive return value indicates that a message was received and is the number of bytes in the message.
Use the function cCmdMessageRead(nQueueID, nRcvBuffer, nSizeOfMessage) to retrieve the first message from the specified mailbox and copy it to a user’s buffer at nRcvBuffer. Only the first nSizeOfMessage bytes of the message are copied. nQueueID is the mailbox number to obtain the message from.
The sample program "NXT BT Messaging No Error Checking.c" is a simple program to show how to use all three of the functions.
|There are two functions for easily sending messages. One sends a single 16-bit value and the other sends three 16-bit values. Either of the two NXTs can send messages at either time. It is the responsibility of the user program to not send messages too frequently as they may cause congestion on either the Bluetooth link or overflow of the NXT’s transmit and receive queues.
Sends a single 16-bit word message. nMessageID should range in value form -32767 to +32767. Message value 0 is invalid and should not be used. It is a special value to indicate "no message" received when using the message variable.
This function is identical to the sendMessage function except that the message contains three 16-bit values. This is useful in easily sending separate items of information in a single message. Do not use a value of zero for nMessageID.
|The NXT firmware automatically receives messages and adds them to a queue of incoming messages. The application program takes the messages from this queue and processes them one at a time. The variables message and messageParm contain the contents of the current message being processed. The function ClearMessage discards the current message and sets up to process the next message.
This variable contains the 16-bit value of message received over the Bluetooth channel. It has a range of -32767 to 32767. A value of zero is special and indicates that there is “no message”. Whenever the value is zero and the message variable is accessed, the firmware will check to see if it has any received messages in its queue; if so, it will take the first message and transfer its contents to the message and messageParms variables. These two variables will continue to contain the message contents until the user’s program indicates it has finished processing the message by calling the ClearMessage() function.
Array containing optional message parameters (up to 3 16-bit words) for messages received over the RCX infrared channel. messageParm is the same as message. messageParm and messageParm are additional 16-bit values.
Boolean function that indicates whether a unprocessed message is available in the NXT's received message queue. This is useful when multiple messages have been queued and your program wants to skip to the last message received. Your program can simply read and discard messages as long as a message is available in the queue.
Clears the current message. The next time the message variable is accessed, the firmware will attempt to obtain the first message from the queue of messages received by the NXT. Do not send messages faster than about one message per 30 milliseconds or it is possible for some messages to be lost.
Skipping Queued Messages
|A typical application might have one NXT send a message to the NXT on a periodic basis. For example, it might send a message containing the current values of sensors S1 and S2 every 100 milliseconds. Due to processing delays in the receiving NXT, several messages may have been queued and your program may want to rapidly skip to the last message received. The following are two code snippets that show how this might be accomplished.|
|The code in the NXT sending the sensor values:
|The code in the receiving NXT:
Master vs Slave Device
|One end of a BT connection is the master device and the other end is the slave device. The master device generates the clocking signal used for the BT connection and the slave device derives its clock from the received radio signal. This results in the following restrictions:
Bluecore Bluetooth Information
|BT implementation on the NXT uses a "Bluecore" chip from CSR. Bluecore is a self-contained implementation of BT that manages the BT hardware and protocols. It has a few limitations that are common to many other BT hardware implementations:
NXT-G Bluetooth Messaging Compatibility
|The NXT-G firmware has built a message passing system on top of the NXT Bluecore implementation. It works as follows: