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This 2-line Telephone Exchange Simulator can be used to test telephone handsets, fax machines, modems, answering machines and other telephone equipment such as diallers on burglar alarms. It contains the all the circuitry necessary to accept decadic (pulse) or DTMF (tone) dialling. Telephone Exchange Simulator For Testing Have you ever wanted to test the modem section on a piece of electronic equipment but were unable to afford the luxury of a small PABX? Or are you in the production side of electronics and need to simulate a telephone exchange to test the finished product? Well, this Telephone Exchange Simulator can overcome these problems. By MIKE ZENERE Testing faulty or new pieces of tele­ phone equipment over the switched network is illegal and can incur large fines if you are detected. Best not to do it. What you really need is a test box which can automatically detect decadic or tone dialling and can display the progress of a call via LEDs on the front panel. The unit to be described can also be an interface between your fax machine and PC, enabling you to scan in documents or pic­tures. Modems and faxes present a real problem if you want to test them. Say you have a fully approved and working modem or fax and you want to test it out. Sure, you can legally test them over the phone lines but you need two phone lines to do it and that’s not always easy. It might be easy enough if you have two lines coming into your residence but if yours is a commercial organisation, getting access to telephone lines which are already connected to your PABX is not convenient or legal either. So this Telephone Exchange Simulator fills a real need. The Telephone Exchange Simulator is housed in a plastic instrument case and has a telephone socket on each side. You can connect two tele­phone handsets and place a call between them, in either direction. The phones can use either decadic (ie, pulse) or tone (DTMF) dialling and the unit will automatically detect either mode. In the following example, a tele­ phone will be used to illus­trate the call procedure but it could be any sort of appliance that might use the public switched telephone network (PSTN). February 1998  25 26  Silicon Chip Fig.1 (left): the heart of the circuit is the 68705P3 processor which controls all the phone functions apart from DTMF decoding which is done by IC3. A call is made in this way: lift the handset of one tele­phone and listen for dial tone. At this point both the LOOP LED and the DIAL TONE LED should be on, signifying that a call is in progress. Also an audible sound should be heard from the internal speaker. Start dialling, noticing that the dial tone disappears and either the DTMF LED or the LOOP LED are flashing, in accor­dance with the digits dialled. If the exchange receives a correct number, ring tone will be heard in both the speaker and the ear piece as well as an audible ringing of the phone. If the called phone is answered, the second LOOP LED and the CONNECT LED will light, showing that the call is connected. A speech path is now formed from one telephone to the other. This simple test procedure will not only enable you to test typical tele­phone handsets but it is also very useful for testing cordless phones. And as already noted, it will let you test fax machines and modems and answering machines too. Some useful terminology Listed below are some terms that may be useful: On hook: the telephone receiver is on the phone and the phone is disconnected from the line. Off hook: the telephone receiver is off the phone and the phone is connected to the line. Dial tone: the sound you hear when you first pick up the receiv­er before you start dialling. Ring tone: the sound you hear when the exchange is calling the other end. Busy tone: the sound you hear when you have called the other end but their phone is in use. No progress tone: the sound you hear when the wrong number has been dialled. How it works Fig.1 shows the complete circuit of the Telephone Exchange Simulator. At the heart of the circuit is IC1, a February 1998  27 Where To Buy A Kit A complete kit of parts for the Telephone Exchange Simula­tor is available from the author who owns the design copyright. This kit includes all components, including the programmed micro­processor, transformers and case. The price is $190.00 plus $8.50 for postage and packing. If the documented source code is re­quired on disk, please add a further $20.00. Please make payments (Postal Orders only) payable to M. Zenere, 1/83 Headingley Road, Mt. Waverley, Victoria 3149. Telephone (03) 9806 0110. Also available is a kit for the Magnetic Card Reader featured in the January 1996 issue of SILICON CHIP. The Card Reader can store up to eight magnetic cards in memory and can be used as a door lock. The kit price is $68.00 plus $7 for postage and packing. 68705P3 single chip microcontroller. This device is a complete computer on a chip and controls the entire exchange simulator. This device is somewhat old now but as they are in plentiful supply and fulfil the requirements of this project, they were used. A review of the functions of the 68705P3 was featured in the September 1992 issue of SILICON CHIP. Another key feature of the circuit is the two Line Loop Detectors, comprising zener diode ZD1, diode D9 and transistor Q8 for the first detector and ZD2, D13 and Q9 for the second detec­tor. Line loop detectors Line loop detectors are the curse of the telephone exchange designer and at first glance these two line loop detectors may appear to be quite simple but the amount of design time and testing that went into this part of the circuit was enormous. In fact, more time was spent getting this part of the circuit to work properly than was spent on the rest of the project, includ­ing writing the article. The line loop detectors are used to sense a low resistance loop in the line; eg, someone has lifted a handset. It was decided that a loop current of 20-25mA minimum would be required to cause the Simulator to accept that a call was being made. Looking at the line one circuit, we can see that the basic telephone circuit is made up of +50V, resistor R15, RLY2 contacts, the telephone handset itself, RLY2 contacts, resistor R17 and ground. With the telephone on-hook, the line appears as an open circuit to the exchange and as such, no vol­tage 28  Silicon Chip is developed across R17. When the telephone handset is lifted, a low resistance loop is placed across the TIP and RING of socket J1 and as current flows through the loop, a DC voltage is developed across resistor R17. Just how much voltage depends on the type of telephone, modem or whatever is making the call. But in any case, we need to produce around 12V across R17 to get our 2025mA flowing through the circuit. When the voltage across R17 reaches or rises above this level, the loop detector comes into play. Zener diode ZD2 con­ducts via diode D13 and feeds current into the base of transistor Q9 to turn it on. This pulls pin 23 of IC1 low, which signals to the processor that a call is under way. “So what’s so hard about loop detection?” you may ask. Well not much at this point but let’s go to the other end where after the correct number has been dialled by the calling end, bursts of 50Hz ring current are fed out to the called telephone. The exchange is now in calling mode and is sending bursts of 50Hz at 200V peak-to-peak imposed on 50V DC at one instant and then in the next, is sending 50V DC to line. This means that at any time the called end answers the call, the telephone may be seeing anything between +150V to -50V in the ring cycle or straight 50V DC. In any case we want the exchange to answer the call within a short time and to turn off the ring current. To help with the explanation, let’s divide this up a bit. Case 1: Relay RLY4 is not operated as we are between ring bursts, thus we are sending 50V DC to line. The circuit path is now +50V, R16, RLY4 contacts, the telephone, RLY4 contacts, resistor R21 and ground (ie, 0V). No current flows in the loop until the telephone is an­swered at which point more than 12V appears across resistor R21. This causes zener diode ZD1 to conduct via diode D9, causing base current to flow into transistor Q8 which now turns on. This pulls pin 22 of IC1 low; the processor is now signalled. Case 2: Relay RLY4 is operated as we are sending ring cur­rent to the line. The circuit path is now +50V, ring transformer T1, RLY4 contacts, the telephone, RLY4 contacts, resistor R21 and ground. Remember, at this point the tele­ phone is unanswered but a capacitor in the phone passes the AC to the bells or ringer and causes voltage fluctuations across resistor R21. These may well be enough to turn on the line loop detector if the voltage rises above +12V, causing the exchange to think the phone has been answered. This is where the problem lies, as how can the exchange tell if the call is being answered or it is being tricked by the ring current? The answer lies in the software. Let’s assume that the capacitor in the phone is quite large and is causing a 50Hz AC signal to appear at the line loop detec­tor. This in turn is causing a signal to be sent to the proces­sor. Anything above 12V will cause the line loop detector to be on and anything below 12V will cause it to be off. As the 50Hz AC ring signal is symmetrical, the line loop detector will be on for less time than it is off. How can this be? Well, a complete cycle takes 20ms so each peak is active for 10ms. This would normally send a square wave to the processor but as we need to reach +12V before the loop detector operates, the signal to the processor now has a longer on time than off time. When the call is answered, the line is biased positive by the +50V rail on one side of transformer T1. This has the effect of lifting the line DC potential and causing the line loop detectors to be more on than off. The signal to the processor now has a longer off time than on time. During the calling cycle the processor is doing what we will call a data acquisition on its associated line loop detector port pin. In this case, line two’s line loop detector is being read by the software at 800 times a second and a record is kept of its on and off times. This information is sent through a subroutine in software and if the conditions are right the call is deemed to be answered. Power supplies The Telephone Exchange Simulator requires five different supply rails to work properly and these are derived mainly from a 12V AC transformer. The different sections are described below. The logic side of the board draws around 150mA and its 5V rail is derived from the 12V secondary winding using a half-wave rectifier D1 and a 2000µF filter capacitor C26. This feeds 3-terminal 5V regulator REG1. There are two 12V supplies one of which powers the audio section of the circuit involving dual op amp IC2 while the other 12V rail powers the relays. Separating the relay circuitry from the op amp section helps reduce noise and distortion. The first 12V source is derived via diode D2 and capacitor C30, while the second 12V rail source is derived from diode D1 and capacitors C25 and C14. +50V supply Three diodes, D6, D7 & D8 and three capacitors C22, C23 & C24 make up a voltage Fig.2: the component layout of the PC board. The LEDs are bent at rightangles to tripler from the 12VAC and protrude through the front panel. this produces around 50VDC. This voltage is used to drive the telephone hand­sets and provide our speech path to the other will stop sending ring current in a When the processor is running end. very short period. The two line re- properly, it toggles its EXCHANGE lays RLY2, and RLY4 were needed to OK port pin every second or so which 200V supply totally isolate the high voltage from tempo­rarily turns on transistor Q5 and The voltage to ring a standard issue the rest of the circuit. discharges C15. Telstra phone is quite high and conWhile C15 is unable to charge via Watchdog circuitry sidering a customer could be over 4km R18 and R26, the output of the 555 from the ex­change a voltage of 200V timer stays high, allowing the proThe watchdog circuitry is used to peak-to-peak (70V RMS) is required. prevent the processor from “locking cessor to continue normal operation. The simplest way to provide this is up” and thereby causing the unit to If the program were to lock up, Q5 to use a step-up transformer fed from become inopera­tive. The circuit em- would remain off and allow C15 to 6VAC. ploys a 555 timer IC4 which is used in charge thus switching pin 3 of the Notice that one side of the output an astable mode to reset the processor. 555 low. The reset line of the prowinding is tied to +50V DC so that If allowed, IC4 would os­cillate at a cessor would now be pulled low via if the called end is answered in the frequency of about 0.25Hz, as set by diode D11 and is held low until the middle of a ring burst, the simulator 555 changes state. At this point the the values of R18, R26 and C15. February 1998  29 processor starts again and continues its pulsing of its port pin. Audio monitoring When testing equipment, it is useful to hear what is being sent from the calling end or even from one caller to another. With DTMF dialling, tones are sent from the telephone to the exchange and are decoded by a special chip. If you suspect your telephone or modem is not sending DTMF you will be able to pick it up. Capacitor C18 is used to provide DC isolation between op amp IC2b and the external telephone circuit. When an AC signal appears (due to DTMF, tones or voice) across C18 they are ampli­fied by IC2b. This op amp drives a complementary output stage consisting of transistors Q6 & Q7 and these drive the loudspeaker via coupling capacitor C28. SILICON CHIP SOFTWARE Now available: the complete index to all SILICON CHIP articles since the first issue in November 1987. The Floppy Index comes with a handy file viewer that lets you look at the index line by line or page by page for quick browsing, or you can use the search function. All commands are listed on the screen, so you’ll always know what to do next. Notes & Errata also now available: this file lets you quickly check out the Notes & Errata (if any) for all articles published in SILICON CHIP. Not an index but a complete copy of all Notes & Errata text (diagrams not included). The file viewer is included in the price, so that you can quickly locate the item of interest. The Floppy Index and Notes & Errata files are supplied in ASCII format on a 3.5-inch or 5.25-inch floppy disc to suit PC-compatible computers. Note: the File Viewer requires MSDOS 3.3 or above. Relay driver ❏ Floppy Index (incl. file viewer): $A7 ❏ Notes & Errata (incl. file viewer): $A7 ❏ Alphanumeric LCD Demo Board Software (May 1993): $A7 ❏ Stepper Motor Controller Software (January 1994): $A7 ❏ Gamesbvm.bas /obj /exe (Nicad Battery Monitor, June 1994): $A7 Under normal conditions the processor’s port pins are low, thereby leaving the relay driver transistors in the off state. When the processor wishes to enable a relay its associated port pin goes high and causes base current to flow to the transistor which turns on to operate the relay. The diode across each relay coil prevents any spikes from damaging the associated transistor when it turns off. ❏ Diskinfo.exe (Identifies IDE Hard Disc Parameters, August 1995): $A7 Tone injector ❏ Computer Controlled Power Supply Software (Jan/Feb. 1997): $A7 ❏ Spacewri.exe & Spacewri.bas (for Spacewriter, May 1997): $A7 ❏ I/O Card (July 1997) + Stepper Motor Software (1997 series): $A7 ORDER FORM PRICE When making a call, certain tones are sent to the calling end to inform the user as to what’s happening; eg, ring tone, busy tone or no progress tone (wrong number). The tones are injected in the following way. One port pin is used to try and reproduce all of the tones required. This process comes pretty close to doing what we want. IC2a is configured as an amplifier with its gain set by trimpot VR1 and resistor R10. The signal waveform from the pro­cessors is rounded off by R32 and C6 and it is then coupled by C29 to the op amp which amplifies it and sends it out to line via R13 and C7. POSTAGE & PACKING: Aust. & NZ add $A3 per order; elsewhere $A5 Disc size required:    ❏ 3.5-inch disc   ❏ 5.25-inch disc TOTAL $A Enclosed is my cheque/money order for $­A__________ or please debit my Bankcard   ❏ Visa Card   ❏ MasterCard ❏ Card No. Signature­­­­­­­­­­­­_______________________________ Card expiry date______/______ Name ___________________________________________________________ PLEASE PRINT DTMF detection Suburb/town ________________________________ Postcode______________ Send your order to: SILICON CHIP, PO Box 139, Collaroy, NSW 2097; or fax your order to (02) 9979 6503; or ring (02) 9979 5644 and quote your credit card number (Bankcard, Visa Card or MasterCard). 30  Silicon Chip ✂ Street ___________________________________________________________ DTMF (dual tone multi frequency) detection is done using IC3, a Motorola MC145436 tone decoder which receives the incoming tones via a filter network comprising resistors R12 & R14 and capacitor C11. When Inside the Telephone Exchange Simulator. Note that the PC board and wiring layout of the prototype pictured here has been fairly significantly modified in the final PC board depicted in Fig.2. a valid tone is detected the DV line (pin 12) of IC3 goes high, signalling to the processor that a digit is being pushed. At this point the processor enables the decoder’s output pins by taking the EN line high (pin 3) and reads in the data. Assembly procedure Most of the circuitry of the Tele­ phone Exchange Simulator is accommodated on a PC board measuring 161 x 128mm. The compon­ents off the board are the power transformer and speaker. By the way, the prototype shown in the photos has undergone a number of fairly substantial changes so the assembly notes apply only to the circuit of Fig.1 and the PC component layout of Fig.2. Note also that the prototype photos show two power trans­formers inside the rear panel but the final ver- sion uses just one power transformer. You can begin the PC board assembly by mounting the four standoffs, one on each corner of the board. Next, all of the resistors, links relays, diodes and capacitors can be soldered in. Screw the 7805 regulator to the heatsink with the screw, washer and nut provided and solder this into place. The remainder of the components, with the exception of the ICs can then be mounted. This done, mount the two telephone sockets and transformer and glue the speaker onto the side of the case with some silas­tic. You will need to drill holes for the mains fuse and cordgrip grommet for the mains power cord. The mains wiring can be run, taking care to insulate with heatshrink any exposed termi­ nals. Don’t forget to attach the earth wire to a solder lug separately bolted to the case rear panel. An earth wire should also be run from this point to a solder lug securely bolted to the front panel (not shown on photo of prototype). Temporarily connect up the sockets, speaker and transformer with longer pieces of wire to enable you to test the board out of the case. Testing Before proceeding, it is well to note that although the ring transformer (T1) looks fairly insignificant, it puts out quite a bite if you get caught across its output. I found this out the hard way! Without any ICs plugged in, turn on the power and check voltages around the board, especially the supply rails to the processor. If all is OK, turn off the power and plug in the ICs. Turn on the power again and use a small screwdriver to short out the TIP and RING connectors of each telephone line in turn. Each time you do so, the LOOP LED for that line should come on. February 1998  31 Use cable ties to neatly secure the wiring and insulate the terminals of the fuseholder with heatshrink tubing, to prevent accidental contact with the mains. Be sure to earth both the front and rear panels of the case (see text). Plug a phone in at each end and lift one of the receivers. Listen for dial tone and use trimpot VR1 to set the tone to the desired level. If the tone level is too high, you may swamp the DTMF from the phone, causing the Exchange to miss any dialled digits. Also at this time use the volume control (VR2) on the front panel to set the volume coming out of the speaker. With the receiver off hook, hit some of the keys on the telephone and listen for tones through the speaker. If all seems well, you can shorten the wires and solder them to the posts. If you have connectors that are spaced at 0.1 inch you can use these instead of hard wiring. Storing a telephone number As this is a two-line telephone exchange simulator we need a telephone number for each end. These are stored in the serially fed EEPROM, IC5. Pick up one end and wait for dial tone. Hit *6805 and wait for two beeps before 32  Silicon Chip dialling in your telephone number of up to 20 digits in length. When this is done, hit the # button to terminate and wait for two beeps. You have now programmed that extension with its own number. Do the same for the other end and yes, you are allowed to have the same number at both ends. Detailed talk-through For this procedure we’ll assume a phone is plugged in at each end. Lift the handset for line one. This causes a voltage of more than 12V to appear across the line loop detectors, thus signalling the processor. The exchange now realises that you want to make a call so it switches RLY1 over and starts injecting Dial tone out through its port pin, through op amp IC2a where it is amplified, through RLY1, through C1 and out to the line. At the same time, the tone is also fed to op amp IC2b via C18 and R27 where it is amplified and buffered by transistors Q6 & Q7. This audio is now heard through the speaker. The user starts dialling and the tones are passed by C1 back through RLY1, through C10 and the filter network to the DTMF decoder, IC3. Once a tone pair has been recognised, DV (pin 12) on the MC145436 goes high, signalling to the processor to get the data in. The digit is retrieved and stored until the whole number is complete or until it gets a wrong digit, at which time the “No progress” tone is sent back to the caller. Once the correct number has been loaded, the exchange starts toggling RLY4, causing bursts of ring current to be fed out to line. Also ring tone is sent back to the user to indicate what is happening. If the second phone is answered, the line loop detector signals to the processor to stop sending ring current and RLY4 remains in its normal state. The ring tone is stopped and RLY3 operates, causing a speech path to be established. The call is now complete. During the progress of the call the LEDS on the front panel will be operSC ating to indicate the progress.