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Penguini with Mint: the Linux Mint desktop, from a distribution supplied by P. J. Radcliffe of RMIT. The root versions of the file browser and terminal provide you with unlimited power but no handrails or safety net. The tool bar pops up when the cursor hits the bottom of the screen. Control Your World Using Linux By NENAD NENAD STOJADINOVIC Microcontrollers are becoming more powerful and sophisticated, to the point where they are challenging the supremacy of the PC. On the other hand, PCs running Linux are quite open and accessible to the experimenter. Yes, your computer can control your air-conditioning and lights and this article will show you how. T HIS ARTICLE is basically the first step (actually there are only two steps) to embedded computing using single-board computers (SBCs). SBCs are generally just 5-8cm square and cram a complete PC onto a single circuit board. They are extensively used in applications requiring small size and/or resistance to vibration, such as automated teller machines, motor vehicles and portable machinery etc. These days, many run a version of Linux as the operating system due to its low cost, flexibility and range of development tools (some instead use one of the BSD-based Unix clones or Microsoft Windows). 22  Silicon Chip They do tend to be rather basic, so it’s much easier to do all development work in the luxury of your desktop PC, then stuff the finished software into the SBC when it’s all debugged and running. Technology directions In the last year or two, SILICON CHIP has presented several spectacular examples of just what can be done with the newest generation of powerful microcontrollers. There has been the Web Server In A Box (WIB), the Data Logger and just recently, the Maximite computer. It gives an indication of the direction that technology has taken and it’s amazing to think that a tiny piece of plastic and metal with a silicon “brain” can present you with seemingly endless pages of full-colour web content. PCs have mirrored this upward trend but in so doing, have made themselves more and more inaccessible to the casual experimenter. It’s harder to interface with USB than the older serial and parallel ports; even if you still have one of the latter, the latest Microsoft operating systems make writing software to access it a difficult task. But then there’s Linux. Linux is most certainly not such a black box. siliconchip.com.au In fact, everything is out in the open and it’s really quite easy to access the various PC communication ports. By using Linux as the operating system (OS), you once again have a PC that you can experiment with. Want to log data, surf the web, flash LEDs or switch relays in response to varying light levels? No problem at all. You may never have found much need to use to Linux and I certainly hadn’t. But during the course of this work, I found that Linux is the number one choice for an embedded operating system, with “Android” (now commonly used in mobile phones) being perhaps the most famous derivative. This and many other embedded operating systems are based on the Linux kernel, which is the core component that provides all the basic functions. The kernel is now up to version 2.6, and has a well-deserved reputation for functionality, stability and security. CONTROL SMART I/O MODULE SENSOR DATA SMART I/O MODULE SMART I/O MODULE CONTROL & DATA CONTROLLING COMPUTER Fig.1: a simple supervisory control scenario. Smart modules run a process while a central supervisory computer provides broad operating parameters and responds to any alarms. SMART I/O MODULE SENSOR DATA CONTROL Linux control paths There are three clear paths to Linux control and it’s worth spending some time to explore them. The first is the simple, old-fashioned use of the serial and parallel ports. Yes, I know that they have been largely phased out but rumours of their deaths have been greatly exaggerated. Suitable interface cards are readily and cheaply available (including USBto-serial or USB-to-parallel adaptors) and the average desktop PC has lots of space to fit them. Likewise, many embedded applications still use them and a glance through the Ocean Controls catalog will quickly illustrate the point. The second path involves the use of USB I/O (input/output) devices. They come in every imaginable configuration, from simple USB converter cables to boxes full of relays, ADCs and digital outputs – all driven from the USB port. For a (very) good example, take a look at the Arduino-compatible I/O controller featured in the April 2010 issue of SILICON CHIP. However, while the above approach is very useful, it has two failings: (1) the devices are generally “brainless”; and (2) it is difficult to use a PC in real time for such tasks. Your controlled system can be going haywire and overrunning its limit switches while the computer is blithely servicing some trivial interrupt. A better approach is “supervisory siliconchip.com.au control” (see Fig 1). It can be used with a bewildering variety of busses and protocols but let’s assume for the moment that it’s all USB. In that case, the PC and one or more USB I/O devices are linked together in a network, except now the USB I/Os are intelligent. An an analogy, imagine a ship’s captain and engine room crew. The captain (supervisory computer) drives the boat according to a plan involving high order functions such as navigation, sea state, schedule etc. The captain issues orders to the engine room for a certain speed and the engine room crew (USB I/O modules) takes care of monitoring and adjusting for steam pressure, engine operating parameters, lubrication and all the myriad functions that the captain does not want (or need) to worry about. The point is that each control module can monitor and adjust for its own feedbacks in real-time so the PC can then interact with them on its own schedule. The battle plan Let’s start with a simple example of PC hardware control, a basic serial/ parallel control system. While there are many more complex systems, this has the potential to occupy the ardent experimenter for some time. As mentioned above, using Linux is essential to making this easy. The Linux interface is not hugely difficult to master but it has some important differences compared to Windows (and many similarities). An article in the March 2009 issue of SILICON CHIP (“Reviving Old Laptops With Puppy Linux”) covered the basics quite well and is recommended for the new chum (thanks Warrick). In my case, I chose to purchase a Linux disk from P. J. Radcliffe of RMIT, who presented an intelligent USB I/O Interface in the October 2009 SILICON CHIP. As part of the development, he produced a live DVD that is set up specifically for this sort of work and is furthermore stuffed with all sorts of tools and data (see http://interestingbytes.wordpress.com/hello/openusb-io-interface-board/). Selling for a paltry $8.80, it is one of the world’s few remaining bargains. For this project, I obtained a PC built in 2005 with both parallel and serial ports and set it up near my usual computer so that I could hop onto the net at any time to check up on some arcane Linux command. It would also let me experiment without the fear of “killing” my good computer. The total cost of the set-up was just the price of the Linux DVD plus the computer, which still came to a total of $8.80, ie, the computer was scrounged for free (I did have to duck the hordes of people that also tried to “donate” their excess computers). Now Linux is available in many July 2011  23 less familiar beyond this point (but if you’re old enough to remember DOS, this will be nostalgic). Open up a root terminal using the desktop icon and enter the following commands: user ~ # cd /media user media # cd work user work # A relay board with a parallel port interface. These are available from many sources, eg, Ocean Controls. Setting an output bit high on the parallel port closes the corresponding relay. different “flavours” (called distributions). These contain the same basic components (kernel, graphical user interface etc) but they are pre-configured in various different ways. Just about any distribution will do for this task but I rather like Ubuntu. Also many distributions (Ubuntu included) provide a “live CD” or “live DVD” mode where the operating system is booted off the installation disk. The advantage of working this way is that if you make a mistake and cause it to lock up or you mess up some critical system file, you simply reboot and all will be pristine again. And if you use P. J. Radcliffe’s DVD in this manner, you will find that the desktop and the directories are prearranged for ease of use in this mode. Into the breach The first step is to make sure you can boot from the DVD. Reboot your computer and watch for the message that tells you how to get into BIOS – generally you must press DEL, ESC or F1 early on in the boot process. Once in the BIOS menu, wade through the options until you find the setting for “Boot Order”, then follow the onscreen directions to move the DVDROM to the first line (if it isn’t there already). Next, go to the peripherals set-up menu. There you will find several options for the parallel port and you will need to set it to “SPP” mode (or bi-directional or compatible, depending on the BIOS). Having done that, put the Linux DVD in the drive, then save and exit the BIOS settings, which should reboot 24  Silicon Chip the computer. Booting from DVD is a bit slow but you will eventually be rewarded with the Linux Mint desktop (assuming that you are using the recommended distribution; see lead photo). Note the terminal and Dolphin icons – they will become your friends. Booting the machine from a DVD means that you will have to store your working files elsewhere. If you have an old 1GB USB stick laying around, it will be more than sufficient. Plug it into your favourite computer and rename it something simple like “work” because you’ll be typing it a lot (it’s easier if there are no spaces in the name). Watch out – unlike Windows, Linux is also case sensitive, ie, “work” and “Work” are treated differently. Next, download the files you will need from the SILICON CHIP website (see the panel for a list of files and sources) and save them to the USB drive. That done, plug the USB stick into the Linux machine and open up the Dolphin (or other) file browser. Note that Linux does NOT identify disk drives with letters like Windows (C:, D: etc), though it does show you your disk as an icon for convenience. The topmost directory level is root, denoted by a single forward slash (/). Everything else is then under root as in /bin or /usr etc and you will find your drive mounted as /media/work (depending on what exact name you assigned to it). Rummage around and make sure it is there, as it’s harder for beginners to do this when working in the terminal. Things now terminal For Windows users, it all gets much The hash (#) in the prompt shows that you are working as root, which means you have full access to the computer. Otherwise, you would see a $ (dollar) sign (a bit ironic given that Linux is free) and would have more limited access. You will need to learn the following commands: ls -1 (list the files and do it neatly) pwd (print working directory, ie, current location) cd .. (go up one directory level) ./my_program (run my_program from this directory) Note that you don’t normally use the root terminal, as it does not have any safety constraints. Instead, it is more usual to prefix a command with sudo, which has the same effect but only for that command. Linux has various levels of permissions for reading and writing files and executing programs but the root (or “super user”/administrator) rules them all and can do anything and everything (including trashing important files!). That’s why we’re operating from a DVD. Also, some programs are not usable without root access, so it’s easier in this case to simply use the root account. The terminal will now have access to the directory that holds your USB drive. Type ls -1 to see if your files are all present and accounted for. The provided programs are all written in the “C” language, so they will need to be “compiled” into executable program files before they can be run. Linux generally has a C compiler ready to go and you can invoke it by typing “gcc” (which stands for Gnu Compiler Collection) at the command prompt: user work # gcc -O -o lp_tty_start lp_tty_start.c This compiles the C program lp_tty_ start.c into an executable binary file called lp_tty_start. The “-O” flag tells the compiler to perform an optimisation pass, producing a faster program. This is not to be confused with “-o” which tells it that the output file name follows. siliconchip.com.au Sources For Information, Hardware & Software (1). http://linuxgazette.net/118/chang.html (interface ADC with parallel port) (2). http://linuxgazette.net/112/radcliffe.html (general introduction to interfacing serial and parallel ports) (3). www.interestingbytes.wordpress.com (Linux Mint development system on DVD-ROM and intelligent USB IO) (4). www.oceancontrols.com.au (network and control hardware, for industry and hobby users) (5). www.siliconchip.com.au (download site for the software relating to this article in zip format) Do the same for port_write_then_ read.c, port_read.c and tx_rx_serial.c, remembering to change both the C source file and executable file names for each; ie, Now type in the following (mind the spaces and underscores): user work # ./pp_serial_check run user work # gcc -O -o port_read port_read.c Note that the serial check part only checks for the presence of a functioning serial port. It does not test the data transfer (see below). If that’s all successful, then try outputting a value via the parallel port: user work # gcc -O -o tx_rx_serial tx_rx_serial.c user work # ./lp_tty_start ./port_ write_then_read 888 85 Finally, create an executable called pp_serial_check, which combines no less than three separate C source files. To compile it type: That outputs a value of 85 (decimal) or 55 (hex) or 01010101 (binary) onto LPT1, which is located at port 888 (often written as 378 hexadecimal). You can then connect the ground lead of your multimeter to any of pins 18-25 (all ground) and measure the port’s output. Pin 2 is the bottom bit (D0), which should be 1 and the remaining bits run through to pin 9 (D7). The voltages should alternate as you scan the pins. For this type of programming, it helps to become familiar with converting between decimal (base 10), hexadecimal (base 16) and binary (base two) numbers. Sending the number 1 (decimal) to the port will result in 00000001 (binary) going to the output pins (D0 high). Sending 128 (decimal) gives 1000000 (binary), ie, D7 high. If your calculator can’t handle the maths, there are lots of web sites that will do the job. You can read the state of the port pins by invoking ./port_read; eg: user work # gcc -O -o port_write_ then_read port_write_then_read.c user work # gcc -O -o pp_serial_check main.c pp_access.c serial_access.c It is worth noting that the source code listings of these serial programs are useful items in themselves and are neatly commented to make them as easy as possible to understand and use. Limited hair, proceed carefully If, like me, you don’t have much hair left, you can’t afford much in the way of frustration and need to consider each step with care. First, check the operation of the ports with a loopback test. To do this, connect serial and parallel cables to the PC and look closely at the free ends of these cables (or peer into the ports at the back of the computer if you’re a masochist). You will see little numbers and you need to connect pin 14 to pin 15 on the parallel cable and pin 6 to pin 7 on a 9-pin serial cable (or pin 6 to pin 4 on a 25-pin serial cable). In my case, I used a paper clip to short the relevant pins for the parallel port and I got my daughter to hold a screwdriver to short out the relevant serial pins. siliconchip.com.au user work # ./lp_tty_start ./port_ read 889 This will read the input of LPT1. Why 889? Parallel ports were originally designed for printers and the input port is at a different address to the output port. There are only five Helping to put you in Control Control Equipment Digital Spirit Level Has the normal liquid bubble vials and a digital inclinometer to give you an accurate 0-360degree readout. Also fitted with a laser to assist with alignment. SRS-105 $119.00+GST Dual Axis Inclinometer using the latest MEMS technology the sensor mounts flat and provides two orthogonal 0 to 5 V outputs for X and Y tilt from -45º to +45º. SRS-038 $159+GST Infra-Red Temperature Sensor Non contact sensor measures temperature over 0 to 400degC. 4-20mA output STW-080 $179.00+GST CNC Controller This is a 4 Axis stand alone CNC Controller which can execute GCode from a USB memory stick or its internal memory, eliminating the need for a separate PC running Mach3 or EMC. CNC-703 $995+GST MP3 Player Shield Fitted with a SD micro card your arduino controller can become a fully functional MP3 player SFA-405 $39+GST JPEG Trigger interfaces with the LinkSprite JPEG Color Camera to simplify picture taking. Activate one of six I/O lines to take a picture and save it to a onboard SD micro card SFC-060 $29.00+GST Industrial Grade Pushbuttons with screw terminals and NO+NC contact HER-201 $9.95+GST Contact Ocean Controls Ph: 03 9782 5882 www.oceancontrols.com.au July 2011  25 #409 N 6 CO TRO L 6 DATA #409 SENSOR MODULE #4096 MASTER SERIAL MODULE SENSOR DATA SENSOR MODULE #4097 CONTROLLING COMPUTER SENSOR DATA PARALLEL PORT RELAY BLOCK SENSOR MODULE #4098 CONTROLLED HARDWARE Fig:2: a more advanced control system. In this case, a radio serial transceiver module (eg, from Ocean Controls or Parallax) sends commands to remote stations which return data to be processed by the PC. A parallel port relay interface then switches pumps, valves etc. input bits for a parallel port, located at pin 15 (D3), pin 13 (D4), pin 12 (D5), pin 10 (D6) and pin 11 (D7). Yes they are scrambled and the data for bit D7 is inverted! There were plans afoot to make port 888 bidirectional but my computer certainly doesn’t have that option. It’s worth a try, though – just read port 888 while using a resistor to pull the pins high or low. Serial comes to us Serial data is much harder to deal with at first, because the data goes past in a blink and so you must capture it to debug it. The solution is to use another computer to send and receive test messages. I scrounged an old laptop that runs Windows 3.1 (guess how much it cost!), with the Hyperterminal program for serial port I/O. I also splashed out and bought a null-modem interface cable (which connects one serial port’s transmit pin to the other port’s receive pin, and vice versa) from Jaycar for about $12 (Cat. WC 7513). Serial ports are composite in that the serial data goes in and out of pins 2 & 3 while the rest of the pins are for handshaking signals and so are in fact quite like a little parallel port! However, it’s complicated by the fact that the application must decide which is 26  Silicon Chip the master and which is the slave; the master cues up data and signals when it is ready to transmit, while the slave is generally sending data and must signal when it’s ready to receive. This can make life a bit complicated, especially when sending from peer to peer, but it’s usually possible to simply ignore the physical handshaking functions and arrange for control signals to be embedded in the data if it becomes really necessary. Linux serial ports are prefixed with “tty”, which is from its early days as Unix, where the ports were used to drive teletypes. What is referred to as COM1 in Windows is ttyS0 in Linux (S is for serial). For USB, it is ttyUSB. Linux also differs in that the serial ports are accessible as files, eg, the first port is /dev/ttyS0. There is a huge variety of serial software available for Linux, most from amateur programmers who needed a particular function or had a good idea that they pursued. I found that GTKterm (an improved version of Hyperterminal) is very useful for testing, as it will display any data that passes by. It is included in many Linux distributions (or “distros”) and Mint makes it available from the toolbar which is at the bottom of the screen (under “Utilities”). Fig.2 shows a sample system that I built, with a number of microcontroller modules sending serial data to my control program which then switches pumps, heaters or valves as appropriate. I used radio modules for the serial link, because the remote sensors were in awkward positions. For the serial communications, I started by running the program tx_rx_serial with port pins 2 & 3 linked together so that the data sent was immediately looped around to the data input. Once I knew that the port was running correctly, I simplified the program and set it up to send the appropriate commands to each sensor. This version is called serialstim.c and it will need to be compiled in the usual way. Run the compiled program and immediately the number 4096 should appear on the screen (why 4096? In binary it is 100000000000). To test this, I then connected the laptop via the null modem cable and fired up Hyperterminal. With the appropriate port speed selected, I made up a text file with a number in it (actually it was the number 11) and cued it up with “send a file”. It was then a simple matter of starting serialstim at the same time as I pressed the <enter> key on the laptop. Success was getting the number 4096 to appear on the laptop and the number 11 on the Linux machine. siliconchip.com.au LISTING 1: BASIC SCRIPT FILE (PASSWORDNDATA) #!/bin/bash set_val=11 #Preset value return=$( ./serialstim ) #Send command, get data in response echo $return #Print received data to screen if [ $return -ge $set_val ] # Is reading above preset value? then (./lp_tty_start ./port_write_then_read 888 1) #Yes it is, set D0 else (./lp_tty_start ./port_write_then_read 888 128) #No it’s not, set D7 fi This basic script file (passwordndata.sh) sends a command to a remote terminal, receives data in return and switches a relay if a condition is met. The command went one way and data came back the other. Cool. Close the loop with a script Passing data back and forth is all very well but nobody wants to sit around and operate stuff manually. How about controlling those pumps and valves? To achieve that, I tied it all together with script files. When you type commands into the terminal, they are actually interpreted by a program called BASH (which stands for Bourne Again SHell). BASH is basically the command line interface between you and Linux and it was the only way of doing things in this operating system before its graphical user interfaces were developed. Some clever person(s) realised that to save effort, it would be useful to be able to run common sets of commands without having to type them in every time. This is achieved using files called “shell scripts” which are a bit like DOS/Windows batch (*.BAT) or command (*.CMD) files. Additional commands were added to BASH to allow these scripts to be smarter; commands like “if”, “while”, “do” etc. Suddenly the command line had a form of programming called “BASH shell programming”, based on script files. With a shell script, you can do most of the things you expect from any program, with the added advantage of being able to easily utilise other programs, written in other languages such as “C” or Python (as well as other shell scripts). Furthermore, you can “pipe” the output from one program (say, your serial reception program) to the input of another program, perhaps a data analysis and control program. Simple script file The subject of BASH programming is much too large for an article such as this. The Linux community has obviously spent a lot of time developing the software and there are boundless references on the internet, though it can take a bit of hunting to find stuff that’s written by a native of Earth. However, to demonstrate, I’ve written a short script that uses all of the aspects covered so far – see Listing 1. It does not need to be compiled and it is run by typing: user work # sh ./passwordndata.sh The script calls serialstim to send a command and then receives data transmitted from the remote sensor, which is then echoed (printed) to the screen. It compares the returned data to a set value. If it is equal to or above this value (ge = Greater than or Equal to), it activates the parallel port enabling program and the control program to send binary 00000001 to the port, thus activating the relay connected to D0. Conversely, if it’s below the set value, it sends 1000000 to activate the relay on D7 and make sure that D0 is switched off. It’s a simple script and it demonstrates what can be done by unskilled labour. Remember though that you are only pottering around in one corner of a powerful computer. This simple script could easily be extended to, say, write the data to a file with a time stamp and then FTP the files to an address of your choosing. Then it would be easy to install web server software like Xampp so that you can access the system from all over the world (remote control is also possible using a program called ssh or Secure SHell). In short, the applications are limitless, the software and support is all out SC there, and the price is right! Are Your Issues Getting Dog-Eared? Are your SILICON CHIP copies getting damaged or dog-eared just lying around in a cupboard or on a shelf? Can you quickly find a particular issue that you need to refer to? REAL VALUE AT $14.95 PLUS P & P Keep your copies of SILICON CHIP safe, secure and always available with these handy binders Available Aust, only. Price: $A14.95 plus $10 p&p per order (includes GST). Just fill in and mail the handy order form in this issue; or fax (02) 9939 2648; or call (02) 9939 3295 and quote your credit card number. siliconchip.com.au July 2011  27