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Wireless Parallel Port Low-cost design uses a 434MHz UHF data link Incorporating a UHF data link, this Wireless Parallel Port is just the shot for computer control of irrigation systems, solenoids, robots, lights or just about anything. It’s based on pre-built UHF radio modules and is low in cost. By NENAD STOJADINOVIC T HE NOVEMBER 2003 issue of SILICON CHIP included an article entitled “Smart Radio Modem for Microcontrollers”. This was basically a low-cost data link that could send up to 16 bytes of data at a time across the radio “ether” over a range of 150-200 metres. The project described here expands on that basic concept by serving up the received data in parallel format. This makes it much easier for the novice to use and opens up a world of ap- plications that were designed for the parallel port. Fig.1 shows the basic scheme for the Wireless Parallel Port. It’s based on a transmitter and receiver pair using Laipac brand 433.92MHz UHF modules, with PIC microcontrollers handling the “smarts”. As shown in Fig.1, the transmitter is exactly the same as used for the Smart Radio Modem. For those who haven’t seen it before, the modem transmitter can be driven by any microcontroller Fig.1: the basic scheme for the UHF Wireless Parallel Port. It’s based on pre-built UHF transmitter and receiver modules. 66  Silicon Chip such as a PICAXE, Stamp, etc. Alternatively, it can be attached to a computer COM port via an RS232-TTL adapter (also described in the November 2003 issue) and will accept the download of up to 16 bytes of data at a time. The downloaded data bytes are subsequently sent on their way by simply pulling the transmitter’s SEND pin low for a short period (eg, 0.25s). The modem receiver then outputs the data, giving the effect of a serial cable that can be up to 200m long. The parallel port receiver differs slightly in that it only receives and then outputs one byte of data per transmission. This data byte is latched on the output pins until the next byte comes along. Basic functions We will assume here that you have read the November 2003 article, so we won’t cover the same material again. If you don’t have a copy, it is available as a back issue for $8.80 (including postage). Basically, this unit is designed to be as easy to use as possible. For example, to transmit data to the Wireless Parallel Port using a communications program such as Hyperterminal, you simply send the number of the receiver pin that you want to turn on (ie, raise to +5V or logic “high”). You then briefly pull the SEND pin low on the transmitter module. Unfortunately, Hyperterminal (which comes standard with Windows) will not allow you to directly control the SEND pin (connected to the COM port’s RTS line), so using it requires a pushbutton switch between the SEND pin and ground. As a result, siliconchip.com.au Fig.2: this is the complete circuit for the receiving end of the UHF Wireless Parallel Port. PIC microcontroller IC1 receives data from the UHF receiver, decodes it and outputs the data (D0-D7) to pins 2-9 of a DB25F connector. LED1 flashes to indicate when data is being received. it’s more convenient to use a COM port test program which can drive the port directly. I use “Simple Term” which is available from www.ptronix.com. This program allows you to load the data, toggle the SEND (RTS) pin and watch the BUSY (DSR) pin all from the comfort of the computer screen – very convenient. Thus, to turn on pins 3, 4 & 5, you simply type in 345 (ie, no spaces) and toggle the SEND pin. In Simple Term, you type 345 in the blue bar, click Send and then change the RTS radio button to light green and back to dark. Sending 128 (for example) would then turn off the above pins and turn on pins 1, 2 & 8. Sending “0” turns all the pins off. Using a password The ability to use a password is a useful option with this unit and there are a couple of reasons why you might want to do this. If your neighbour has a similar unit, for example, the use siliconchip.com.au of a password will prevent interference between them. Alternatively, you might want to have a number of receivers, each responding to its own password. The password allowed is rudimentary. It simply consists of a single character but it does give you up to 256 different choices. To load a password, you simply type in “P” (without the quotes) and then the password; eg, P*. Note that you must use a capital P and the “learn link” must be in place on the board during this procedure – see Fig.2. Once that’s done, the receiver will only respond to commands prefaced by the password; eg, *145. To remove the password, just send the character “X” with the learn link in place. For advanced users Sending a byte to switch each of the pins is all very well but it can also be very useful to simply send a single byte that will be transferred to the port as binary data. To do this, simply send the “B” command followed by the binary data. Thus, B<01h> will set the port’s D0 pin high. Similarly, B<FFh> will set them all high. This function works in conjunction with a password if you have one in place. Circuit details The complete receiver circuit is shown in Fig 2. Apart from the UHF receiver module, it uses a PIC16F84 microcontroller (IC1) and a few other bits and pieces. The circuit essentially emulates a parallel port and outputs its data to D0-D7 (these are pins 2-9 on a standard DB25 connector). Pins 11, 12 & 13 of the DB25 connector are also connected to the microcontroller and may be used in future projects. In addition, an RX (received) data indicator is fitted, consisting of a LED connected to pin 13 via a 560Ω resistor. This LED May 2004  67 Fig.3: the PC board should only take you 30 minutes to assemble. Take care to ensure that the receiver module is correctly oriented – see photo. The PC board pattern at right is shown full size. lights and the pin goes high briefly when ever a valid data transmission has been received. The previously mentioned “learn link” is connected to pin 17 (RA0) of IC1. Fitting this link pulls pin 17 high (it’s normally pulled low via a 10kΩ resistor) and places the microcontroller in the “learn” mode. Parts List 1 PC board, code 07105041, 60 x 60mm 1 4MHz 3-pin ceramic resonator (CR1) 1 DB25 female PC-mount connector 1 18-pin IC socket 1 2-way connector 1 2-pin SIL header plus jumper shunt 1 8-pin straight SIL header 1165mm length of light-duty hookup wire Semiconductors 1 Laipac RLP-434 UHF receiver module 1 PIC16F84 programmed microcontroller (IC1) 1 1N4004 diode (D1) 1 5mm LED (LED1) 1 78L05 voltage regulator (REG1) Capacitors 2 0.1µF monolithic Resistors (0.25W, 1%) 2 10kΩ 1 560Ω 68  Silicon Chip All outputs to the port can be monitored via connector CON1, while a 4MHz ceramic resonator (CR1) provides the clock signals. The unit can be powered by an DC supply from 8-15V (eg, a plugpack). Diode D1 provides reverse polarity protection, while regulator REG1 provides a stable +5V rail to power IC1 and the UHF receiver module. Construction & testing With only a handful of components, the construction is not challenging. Fig.3 shows where all the parts go. Install the smallest components first and watch the orientation of everything except the resistors. Don’t forget the wire link and be sure to use a socket for IC1. Note that the UHF receiver module is installed with its red coil facing towards IC1 – see photo. The antenna consists simply of a 165mm length of insulated hookup wire. If you are using the receiver to control a board (eg, a relay board) that doesn’t have its own power supply, you will need to organise power to both. The DB25 connector is tied to ground, so you will probably only need to connect a positive lead from one board to the other. If you have any doubts about how it all goes together, you can leave out the PIC and the UHF receiver module until the power supply is sorted out. That way, they cannot be damaged if you make a mistake. In fact, it’s a good idea to check that there is +5V on pin 14 of IC1’s socket before switching off the power and installing the microcontroller and the receiver module. Testing simply involves sending data to the receiver and checking that the LED blinks. You can then check which pins have been set (ie, taken high) by using a multimeter to measure the voltages on CON1. Practical applications The accompanying photo shows the receiver connected to an 8-channel relay board, as sold by Oatley Electronics (see SILICON CHIP, September 2000). Basically, you can use any board which only requires eight data inputs (D0-D7) and requires no control lines. With six standard and two heavy duty relays, driving just about any load is no problem and password activation means you can selectively drive up to 256 separate relay boards. This view shows the transmitter board plugged into its companion RS232 interface board (see SC, Nov. 2003). siliconchip.com.au The UHF Wireless Parallel Port is ideal for controlling this 8-Channel Relay Board, as sold by Oatley Electronics. You can control any board which only requires eight data inputs (D0-D7) and requires no control lines. That’s a lot of relays! I’ve been using these boards in the garden to control irrigation solenoids and currently the back and front yards have one each, controlled by their own password. I had what is essentially a PIC-based alarm clock left over from another project and I simply set the start and stop times during the day for each relay. For example, in pseudo code: At 10:35 am: Send *1   ;Turn on relay 1 on card with the password “*” At 11:20 am: Send *0    ;Turn off all relays on card with the password “*” I imagine that this would be a very simple program for the PC, perhaps with a scheduling grid to show what SC is on and off at what time. siliconchip.com.au Where To Get The Parts Kits plus individual parts for this project are available from the author. Prices are as follows (kit prices include the PC board plus on-board parts only): (1). Wireless Parallel Port Receiver ................................................... $46.00 (2). Matching Transmitter Kit ............................................................. $28.00 (3). Transmitter/Receiver Kit Package Deal ....................................... $69.00 (4). RS232 Interface Kit For Transmitter ............................................ $25.00 (5). Programmed Microcontroller With Resonator ...... Rx $18.00; Tx $15.00 (6). Rubber Duck Antenna (see November 2003 article) ......... $19.00 each All prices include postage within Australia and GST. To order, write or email the author as follows: Nenad Stojadinovic, PO Box 320, Woden, ACT 2606. email: vladimir<at>u030.aone.net.au The Laipac UHF transmitter and receiver modules are also available from Commlinx Solutions at www.commlinx.com.au May 2004  69