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You don’t need to be an expert to build a reliable 2-way radio link – just a pair of these new boards from Revolution Education! Being PICAXE driven, they’re dead easy to build and program and have a myriad of serious applications. PICAXE Goes Pt.1: By CLIVE SEAGER* Wireless Get your PICAXE projects talking over the airwaves using the latest high-tech 2.4GHz XBee modules! I N THIS, the first of a 2-part series, we describe how to build and test a pair of wireless data communications “nodes”. Each node is based around “XBee” radio modules from US company MaxStream, Inc (www. maxstream.net). All parts, including the XBee module, are carried on a small PC board that’s designed specifically for experimenters. With only a minor change, the XBee module can be connected to either an on-board PICAXE microcontroller or an external computer. This means that 2-way PICAXE-to-PICAXE or PICAXEto-PC communications are possible, *Clive Seager is the Technical Director of Revolution Education Ltd, the developers of the PICAXE system. 88  Silicon Chip opening up a vast array of remote control and sensing applications. Construction is very easy, as the XBee modules are pre-built and simply plug into header sockets on the PC board. These radio modules utilise the “ZigBee” standard for ultra-lowpower, high-reliability, short-range wireless communications (see the ZigBee feature in February 2006 SILICON CHIP for more information). Why XBee? When considering a wireless PIC­ AXE application for the first time, some constructors will undoubtedly compare the 2.4GHz XBee modules with the lower-cost 433MHz RF modules that are now available from hobbyist outlets (see Stan Swan’s article in SILICON CHIP, January 2006 to find out how to interface these units to a PICAXE micro). However, while the low-cost 433MHz modules are suitable for some very simple PICAXE applications, the XBee modules offer considerable advantages. For a start, a typical budget 433MHz system would offer only 1-way communication (one transmitter and one receiver module), whereas each XBee module supports 2-way data communication. And although 1-way communication may seem sufficient for some applications, it can also be unreliable, as the transmitter has no idea whether the receiver is actually receiving the data! Another big advantage is that of unique addressing. Each XBee unit siliconchip.com.au Fig.1: here’s the complete circuit diagram (minus power supply) of the AXE210 Connect Board when a PICAXE chip is plugged in. Note the 3 x 10kW resistor strings from the PICAXE outputs to ground. These reduce the signal levels by 1/3 before they are applied to the lower voltage XBee inputs. The simplicity of this circuit belies the true power of these incredibly versatile communications devices. has a unique serial number, so two (or more) units can be set up to exclusively “talk” to each other, ignoring signals from other modules. This is not easily achieved with the budget 433MHz modules as, unlike the XBee, they don’t contain any networking “intelligence”. This intelligence leads us to several other important features such as the XBee’s selectable communications channels and its in-built data packet building and error checking. These features ensure reliable data exchange under less than optimal conditions. So what applications would suit a PICAXE-based wireless node? Here are just a few examples (we know you’ll think of many more): • Remote control of robots and equipment (PC to remote PICAXE). • Data collection from a greenhouse (remote PICAXE to PC). • An advanced security system (multiple remote PICAXEs to PC). • Sensor modules in a weather station (remote PICAXE to remote PICAXE). Node hardware The so-called “nodes” described here are officially known as “AXE210 siliconchip.com.au Connect Boards”. This rather nondescript name was chosen because the boards can be built in several different configurations. For the moment, it’s only important to note that when an XBee module is plugged in, the boards can be operated in one of two distinct modes, as follows: (1). As a PICAXE to XBee wireless modem interface; or (2). As a computer (PC) to XBee wireless modem interface. As an example, this allows a sensor connected to a PICAXE chip (on one AXE210 board) to transmit data to a second AXE210 board that’s directly connected to a PC – allowing the remote data to be recorded and displayed on the PC’s screen! Let’s examine these two modes of operation and the hardware configurations needed to make them work in a little more detail. while another bit (input7) receives serial data. So the BASIC command necessary to send data from the PICAXE to the XBee module would look something like this: serout 7,T2400,(“Data”) Likewise, data received by the The XBee is a hybrid radio modem built on a tiny PC board measuring just 24 x 27mm. All of the parts are hidden beneath a metal shield, with external connections made via two rows of 2mm-pitch header pins spaced 22mm apart. PICAXE to XBee interface (mode #1) Fig.1 shows the connections when a PICAXE-18X is inserted into its 18-pin socket. One port bit (output7) of the PICAXE is used to send serial data to the XBee for transmission, April 2006  89 Fig.2: an external computer can communicate directly with the XBee module by plugging in a MAX3232. This IC acts as an RS232 level converter, ensuring reliable 2-way communications between the XBee and a PC serial port or USB-to-serial converter. XBee module can be processed by the PICAXE with an equally simple serial command, for example: serin 7,T2400, b1 A third port bit (output6) of the PICAXE is connected to the SLEEP input of the XBee module, allowing it to be placed in a low-power mode under software control, if desired. Note that as the PICAXE operates at 5V, the signal level on its outputs must be reduced before connection to the XBee’s 3.3V inputs. This is achieved using simple voltage dividers composed of three 10kW resistors. No conversion is necessary in the opposite direction, as the PICAXE readily interprets a 3.3V signal level at its digital inputs as a valid logic high. PC to XBee interface (mode #2) When a MAX3232 chip is inserted in its on-board 16-pin socket, the XBee module is connected (via the download cable) directly to the PC’s serial port. This can be seen in the circuit diagram of Fig.2. Obviously, this mode allows data to Table 1: Jumper Summary Jumper Position Function J1 Open Top Bottom (not used) Normal Ping test (XBee DOUT connected to DIN) J2 Open Fitted XBee VREF pin not connected XBee VREF pin connected to 3.3V J3 Open Top Bottom IND LED not connected IND LED connected to XBee RF_TX (transmit) pin IND LED connected to XBee ASSOCIATE pin J4 Open Fitted PICAXE output6 not connected PICAXE output6 connected to XBee SLEEP pin Note: the default position of each jumper is shown in bold. Orientate the PC board as shown in Fig.4 when referring to this table. 90  Silicon Chip be transmitted from and received by the computer. In addition, it allows the XBee’s internal configuration to be altered via the Programming Editor’s XBee Wizard or by user-entered commands (see the XBee manual for more details on configuration). Important: the PC board has provision for both a PICAXE-18X micro (IC1) and a MAX3232 (IC2), the latter being for computer (PC) support. When a PICAXE is to be used on the board (mode #1 above), it is inserted in its 18-pin socket and the MAX3232 is removed from its socket. Conversely, when used with a computer (mode #2 above), the PICAXE micro must be removed from its socket (if installed) and the MAX3232 chip inserted in its 16-pin socket. Never have both the PICAXE chip and MAX3232 chip installed at the same time! Power supply For ease of use, the AXE210 is designed to operate from either a regulated 5V DC supply or an unregulated 9V DC supply. It can also be used with a 4.5V battery pack (typically three AA cells). Fig.3 shows the circuit details. Note that two voltage regulators are included on the PC board – the ZSR500C (RG1) siliconchip.com.au Par t s Lis t Fig.3: two regulators provide +5V and +3.3V for the PICAXE micro and XBee module. Advanced experimenters looking for maximum battery life in remote nodes may wish to design their own power supply and bypass these regulators entirely. provides +5V for the PICAXE micro, whereas the ZSR330C (RG2) provides +3.3V for the XBee module. Power input to the board is via a 3-way terminal block (CONN2). If a 9V DC supply is used, its positive (red) lead is connected to the top terminal (see Fig.4). Alternatively, a regulated 5V supply (or 4.5V battery pack) can be connected to the bottom terminal. Note that the bottom terminal bypasses the on-board 5V regulator. The ground, or 0V (black) lead is connected to the centre terminal of the connector in both cases. Important: take great care with the power supply connections to the board. Accidental reversal of the power leads (or batteries) or the application of more than 5V to the 5V input will destroy many components, including the PICAXE micro or MAX3232 and the XBee module. Construction Assembly of the AXE210 Connect Board is very straightforward, with all parts mounting on a small, doublesided PC board. Fig.4 shows the assembly details. Install the resistors first, followed by the IC sockets, voltage regulators, LEDs and then all remaining parts. Take particular care with the two regulators, as they look identical. Check the package markings with a magnifying glass if necessary; the 5V regulator (marked “500”) goes into the RG1 position and the 3.3V regulator (marked “330”) goes into the RG2 position. Be sure to orientate the flat side of each device as indicated on the overlay diagram. The orientation of several other siliconchip.com.au components is also critical. The positive leads of the two electrolytic capacitors must go in as indicated by the “+” markings on Fig.4, while the anode (longer) leads of the three LEDs must go into the holes marked “A” or “+”. Finally, be sure to match the notched end of the IC sockets with that shown. Initially, both boards must be configured for mode #2 (computer to XBee) use, so insert the MAX3232 (IC2) into its 16-pin socket, making sure that its notched (pin 1) end matches the notched end of the socket. Do not install the PICAXE chip (IC1); its socket must remain empty whenever IC2 is installed! The jumpers (J1-J4) are made from the longer 10-way section supplied in the kit by cutting it into two 3-way and two 2-way sections. A sharp knife or side cutters can be used here. The XBee module is carefully inserted into the two header sockets (H1 & H2), while making sure that the chamfered corners of the module match the outline marked on the PC board. The correct orientation is also evident in the photos. Note that header socket H3 is not used with this project and therefore the position is left empty. This header is used for connecting a GPS (rather than XBee) module to the board and this will be covered in a future article. You board should now be complete. Before moving on, repeat the above steps to assemble the second board in preparation for the testing phase. XBee reset Initially, you have to connect each board to a PC to initialise the XBee Two complete kits containing the following parts are required for this project: 1 AXE210 PC board 1 3-way screw terminal block 2 6mm miniature tactile switches (SW1,SW2) 1 10-pin 2.54mm pitch header (cut down for J1-J4) 2 10-way 2.0mm pitch header sockets (H1, H2) 1 3.5mm stereo socket 4 jumper shunts (links) 1 16-pin IC socket 1 18-pin IC socket 1 3 x AA battery holder 1 battery clip Semiconductors 1 PICAXE-18X micro (IC1) 1 MAX3232CPE RS232 transceiver (IC2) 1 ZSR500C +5V 200mA regulator (RG1) (Farnell 384-8541) 1 ZSR330C +3.3V 200mA regulator (RG2) (Farnell 384-7780) 3 3mm red LEDs Capacitors 1 100mF 16V PC electrolytic 1 4.7mF 16V PC electrolytic 7 100nF MKT polyester Resistors (0.25W 5%) 1 22kW 1 4.7kW 7 10kW 4 180W Also required (not in kits): 2 MaxStream XBee modules 1 AXE026 PICAXE download cable 6 AA alkaline cells Note 1: if your PC does not have a 9-pin serial port, you also require a USB-to-serial adapter, part no. USB010. Note 2: the PICAXE Programming Editor software (v4.1.15 or later) can be downloaded free of charge from www.picaxe.co.uk or ordered on CD, part no. BAS805. Note 3: the design copyright for this project is owned by Revolution Education Ltd. Complete kits (part No. AXE210) and the XBee modules are available from authorised PICAXE distributors. Australian readers can contact MicroZed Computers on 1300 735 420 or go to www.microzed.com.au. April 2006  91 Fig.4: follow this diagram very closely when assembling the PC boards. Note in particular the orientation of the two regulators, the electrolytic capacitors, LEDs, ICs and the XBee module! For ease of experimentation, all PICAXE and XBee I/O lines have been made available on rows of pads, shown here with their designated signal names (refer to the AXE210 & XBee manuals for detailed information). modules and ready them for the communications tests. To begin, select one of your completed boards and install links (shunts) on jumpers J1-J4 as shown in the configuration panel below. XBee Reset Board Configuration Board #1 Board #2 MAX3232 fitted MAX3232 fitted J1 at “top” J1 at “top” J2 fitted J2 fitted J3 at “bottom” J3 at “bottom” J4 not fitted J4 not fitted That done, connect the board to your chosen power supply, as described earlier in the “Power supply” section. If using the supplied 3-cell battery pack, the red wire goes in the bottom (5V) terminal and the black wire goes in the centre (0V) terminal. Next, connect the board to your PC via the PICAXE download cable and launch the Programming Editor software. From the main toolbar, select View -> Options and click the “Serial Port” tab. Make sure that the selected serial port matches the physical port that you’re plugged the cable into. Next, open the “XBee Setup” window by selecting PICAXE -> Wizards -> AXE210 PICAXE Connect from the toolbar. Click the “Factory Reset” button, which you’ll note also changes the baud rate to “9600”. Now change the XBee baud rate to “2400” and click on 92  Silicon Chip the “Write” button. This operation also automatically changes the PC baud rate to “2400”. To confirm that the module is operating correctly, click on the “Read Version” button. The results should appear in the “Buffer” window, as shown in Fig.5. Of course, you may get different version numbers to that shown in our screen shot, which is perfectly OK! That completes the preliminary setup for the first board, so disconnect power and unplug the serial cable. You must now repeat all of the above steps to initialise your second board. Once done, both XBee modules have default settings, including a baud rate of 2400 bits/s. In this condition, they will respond to any other XBee module – ie, they are address independent (we will look at how to use unique addresses in Pt.2 next month). The boards are now ready for their first communications test! Ping test To check communication between the modules we’ll first perform a “ping” test. In this test, one board is connected to the PC (board #1) and the other is stand-alone (board #2). To prevent any confusion, mark one of your boards as “#1” and the second as “#2” before proceeding. The PC first sends data to board #1, where the XBee module transmits it over the airwaves. Board #2 receives the data and immediately re-transmits it (ie, echoes or loops it back). Board #1 receives the data back from board #2 and sends it on to the PC via the serial port cable, where the software compares the sent data against the received data for discrepancies. As noted, one of the boards is connected to the PC (board #1). This board operates in mode #2, so all its jumpers must be set to their default positions and the MAX3232 must be installed. The “Ping Test Board Configuration” panel below shows what you have to do. Ping Test Board Configuration Board #1 Board #2 MAX3232 fitted PICAXE fitted J1 at “top” J1 at “bottom” J2 fitted J2 fitted J3 at “bottom” J3 at “bottom” J4 not fitted J4 not fitted As you can see, changes are required to the second board (board #2). It will be operated stand-alone, so remove the MAX3232 chip and install the PICAXE-18X in its 18-pin socket. Also, move J1 from its default (top) position to the bottom position, which connects the XBee’s DOUT & DIN pins together for the ping test. As before, connect board #1 to your PC via the PICAXE download cable. That done, position board #2 about 1m away from board #1 and apply power to both units. Next, launch the Programming Edisiliconchip.com.au If both boards are correctly configured and operating normally, a text string will be transmitted by board #1 and echoed back by board #2. The result is displayed in the “XBee Setup” window. In addition, the RSSI LED on both boards should flash while they are communicating. PICAXE to PC test Fig.5: the XBee module can be reset and tested using the Programming Editor’s XBee Setup wizard. This screen shot shows the results after clicking on the wizard’s “Read Version” button (you may get different version numbers). tor software. From the main toolbar, select View -> Options and click on the “Serial Port” tab. Make sure that the selected serial port matches the physical port that you’re using with the download cable. Next, open the “XBee Setup” window by selecting PICAXE -> Wizards -> AXE210 PICAXE Connect from the toolbar. Make sure the PC baud rate matches the current XBee baud rate setting, which should be “2400”. Now click on the “Ping Test” button. siliconchip.com.au In this test, data is transmitted from the PICAXE microcontroller on board #2 to board #1. It is then is sent to the PC via the download cable and subsequently displayed in a terminal window. If you have just performed the ping test, then move J1’s jumper from the bottom position to the top position on board #2, as shown below. PICAXE To PC Test Configuration Board #1 Board #2 MAX3232 fitted PICAXE fitted J1 at “top” J1 at “top” J2 fitted J2 fitted J3 at “bottom” J3 at “bottom” J4 not fitted J4 not fitted Next, we need to program the PIC­AXE chip with a simple BASIC program. To do this, disconnect the download cable from board #1 and plug it into board #2. Using the Pro- Table 2: LED Indicators LED Indication RSSI XBee received signal strength STAT XBee module is active or sleeping IND XBee module has associated with another module or is transmitting (depends on position of J3). gramming Editor, enter the following program and download it into the PICAXE chip: init: high 7 pause 100 main: serout 7, T2400, (“Value =”, #b1,CR,LF) let b1 = b1 + 1 pause 500 goto main When that’s complete, disconnect the download cable from board #2 and plug it back into board #1. If all is well, the data being transmitted by the PICAXE over the wireless link should now be displayed in the Programming Editor’s terminal window. Summary And that’s it! Next month, we’ll see how to get two PICAXE chips talking to each other. We’ll also show you how to use computer software to save any data that’s received from a remote SC wireless node! April 2006  93