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Build this fax/modem for your computer Have you been holding off on buying or building a modem for your computer? You have? It's just as well because you can now build this combination fax and modem for less than the cost of a modem alone. It's a freestanding unit that's fully software driven. By LEO SIMPSON Th ese days , everyone who has a computer probably has a yen for a modem - so that yo u can access bulletin boards and exchange programs and correspondence with fri ends, and so on. In parallel with this , more and more people are getting their own fax machin e - they're so convenient for leaving messages at any time, ordering goods from suppliers , etc. Some time ago, people recogn ised that there was a problem with fax machin es and that was junk faxes . Thermal fax pap er is not cheap to buy and if yo ur fax machine automat58 SIUCCJ .'c C II II' ically prints out every silly message that comes through, including those wasteful cover sheets that tell you that you are getting a fax message , you quickly go through a lot of paper. To get around this problem, faxcards were developed. We reviewed one of these units in the June 1990 issue of SILICON CHIP. With a faxcard installed in your computer, you can receive all the faxes and then review them on screen before deciding to print them out on your printer, which can be just about any old dot matrix machine printing on ordinary paper. So potentially, faxcards can give big savings on fax paper and they have the advantage that you don't have to print out a message before you fax to someone else. But technology moves on. Since a fax is really just a glorified modem linked to a thermal printer (with a few bells and whistles thrown in), it was inevitable that there would be a melding of the two technologies hence a fax/modem. We are very pleased to present one of the first examples of this new combined technology, by courtesy of Comma Corporation and PC Marketplace. And the good news is that this technology comes to you quite cheaply by virtue of being supplied as a complete kit. What you get Alright , what do you get with this box of tricks? The Fax/Mo dem is housed in a compact extruded aluminium case measuring 150mm wide, 38mm high and 173mm deep. There are no controls on the front of the unit , just a bunch of LEDs in a row. On the back, there are US modular phone sockets, a 25-pin D socket for RS-232 connection, a miniature on/ off switch and a male socket for connection of an AC plugpack supply. The reason there are no controls on the Fax/Modem is that it is completely driven by software, which is also supplied as part of the deal. You just boot this software up on your machine (it practically does this itself - we're not kidding) and away you go, guided by menus. This Fax/Modem has been designed as a free-standing unit rather than as a card to go inside a PC. This means that it can be used with any PC or Macintosh which has a spare RS-232 port. Computer requirements If you are using a PC , you need to have at least 640Kb of RAM, a free serial RS232 port, preferably a mouse (this means you probably need two ports), and a hard disc drive (although strictly speaking you can install it on a floppy only machine). The display can be CGA, EGA or VGA. A monochrome display can be used but you won't be able to display any graphics from incoming or outgoing faxes. So let's go through what you can do with the Fax/Modem. First, it allows your PC (or Macintosh) to work as a fax machine which can receive and transmit faxes at up to 9600 baud. When receiving faxes it works in the background, without interrupting the programs you are using. Then, when The Fax/Modem is a standalone unit so that it can be easily transferred from one computer to another & doesn't tie up an expansion slot. A double-sided PC board accommodates all the components, including the RS-232 input socket. you are ready, you can have a look at any received fax on the screen and, if you want to, print .it out on any printer which can be a dot matrix, laser printer or inkjet. The quality of these fax printouts will be every bit as good if not better than that from a conventional fax machine. And you will have the advantage of a permanent record , rather than one which will fade with Specifications V.29/ V.27ter/ V.21 Ch2 fax modes; V.22bis/ V.22/ Bell 212A/ Bell 103 data modes; V.23/ V.21 optional. 9600/ 7200/ 4800/ 2400/ 300 baud half-duplex fax operation; 2400/ 1200/ 300 baud full duplex data operation. Group Ill fax compatible send & receive . TR29 class 2 fax command set compatible (EIA PN2388). "AT" series V modem command set compatible . Automatic answer; tone & decadic dialling; analog, remote & local digital test facilities; automatic adaptive equalisation ; guard tone generators; call progress monitoring . Note: please check your serial 1/0 card. It must have a 16C450 UART, not an 8250 or 82C50. If it has either of the latter, it must be replaced. If it is not socketed, you may have to replace the 1/0 card . The 16C450 is available from PC Marketplace for $14.50. time, as thermal fax printouts do. Second, the Fax/Modem will work as a conventional modern at rates up to 2400 baud. When set up by the software (it uses the Hayes command set), it does auto dialling, auto answer and disconnect. It is compatible with most communications software programs and most bulletin boards. What's in the kit Basically, you get everything bar a soldering iron and solder. The kit comes with a screen printed, solder masked, double-sided PC board; all the ICs and IC sockets ; and all the other parts. Plus, you get the software and documentation , as already men- H.g.1 (following page): the key devices in the circuit are U10 & Ull - a fax/modem chipset, made by Exar (XR2901 and XR-2902). Microcontroller U4 & its associated EPROMs provide the control signals. The line isolation circuitry is in the top lefthand corner, while the circuitry at top right provides interfacing to the RS232 socket. S1•:l'TE 1\IHE/l ·1991 59 Vo□ l R18 22k C40I.L 01 R28l:~ 01 "'1 ~ ' m E L1 INDUCT .I I R22 10k 20--- • r JM7 3PLINK Voo 0 - XR-2900 C49I,l;-· 10 - Vooo ' , I 47 RCVIN1~!.!.~f--!}' - - - - - - - - - - 7 - 3 f -;;r--7---;~rTT7-;i'7n TXOUTl!'45~~*1_ _ _ _ _ _ _---;l~l~(t---;r-,r-7-t-" 1 2 C1 TXCLK!f.--':)-➔----------r--r--,t---, 1 I R22 2.2k R23 100k 30 Voo 41 ..__] MCl*MP _ _ _ _ ___, 7 X1 Y1 20 .2752MHz --j □~_J! C14 •• 20pFI cso.i4 RXCLK iNf 35 PAO AO/PAOll-'2' - - - - - - - 9 1 A1 /PA111-' - - - - - - - " "3I ' PA1 A2/PA21"4"-0- - - - - - "37, PA2 U11 TO DO 26 25 TOO XR-2901 C13• 20pFI GNO I '----=r---------------:r-------;j--j~-+--+--;--- TX01~ I WR14 U10 XR-2902 RO 12 3--~------------------,-;r--7""T7""77-Rxo,µ21ALE 13 I HOO~ 26 TOl H01~ T2 27 T02 H02 ~ 03 23 T3 28 T03 H03 04 22 T4 29 T04 HD4~ 05 21 T5 30 T05 HOS~ 06 20 T6 31 T06 H06~ H07~ T7 32 T07 T8 33 TOB 09 12 T9 RESET I ~± ---.----...----v □ o T1 07 19 I I 01 25 08 11 I I 7 31 4 WENl-'"-- - - - - - 0 WEN 2 - - - - - - - =39t DENl-"3"'DEN 6 38 CLKIN CLKOUTl'' - - - - - - - 9 X2/CIN I lR 5 ~--------........-v,, I. ~ GF124 AGNO 6 34 T09 VssA Vss□ RESET ;-.-~r.1:c'o=1r.18__, 42 4 XR-2 1 0 0 r - i B 60 SILICON C HIP Voo ALEL ,)lM--1 RXO.!l.._ v1181--? ROl'B--~ 7 ~ WR ~ A 0 3 XRUi321 TXOll-'1.:.9_ _ _ _._. 11 5 TXC:1-' .:.-------:r---, ~ Rei cs:rg'-----:t----' 47T C46""' a 13 Rxc:F----1-----~ ..--J-!l RCO 7 .l,, 0C433.I.,. FAX/MODEM 20 JM40_ __ _ _ _1--18 FREQ 3PLINK 0 ADO XR-2321 ADl ' EL J c 02 VIEWED FROM BELOW I 1 EXTXC:l-"6'--➔'~+-----------r --r--r--- i1~~f _ _43 C2 0. 2400 I Vao ..EsPKO ~ C23.L 1 OxR- ( 5 MSCLKt-- - - - , Vss G GND µ □ 114. r J - . 1 2 ' 0.1 -I Vss ( 14 U14 J3 RS 232 ll S 12 11 C27 C48 0.1 01! C43 0.1 I 74 04 U19 14 DSR SEIZE 16 Vee ooie2- - - - -~ --'l>-4---l----'l..--+--l----..:i,-, 011-5- - - -021-6- - - -- A14 A15 2 1A 3 18 U17 74HC139 CD +----;>----+--+---;>----+--+---;>--~ +---;1----+--+---;>----+--; 9 U7 0 3l-'- - - - -+---ll---+-+---ll---+---' 74HC374 1-1~2_ _ _ _~ 04 05 15 5 1v11-- + - + - - - ~ TX CLK .,. 3 06 16 --+-+--· OA7 Q1 __J 18 07 1 EXTCLK 024 XR-1489 3~N~K u-=---+-~1-'11 CLK 1Y3...7 GNO 15 a 6 RXC LK 17 0 7 19 GNO U14c 10 OE TXO RXO RTS RI 22 f<' 7 ADDRESS BU Voo C29 01+ + '!1 9 40 Vee RESET EA/UP 31 MASK UP JM5 3PLINK 28 0 Vpp 20 :i: C12 01 1 2 R21 8.2k.,. NON-MASr UP 12 INTO 13 INT1 1 0 RXO 16 4 5 6 7 8 DA4 15 T1 3 Voo iNR 17 RO U4 P07 32 DA7 1 11 TXO XR-2903/8031 20 3t~~K 2 GNO JO ALE/P .,. U9 272 56/ 27128 9 10 A11 12 13 14 OE 22 PSEN 29 GND 14 A8 P16 P2 2 23 p ~--l>--,t----"IP14 23 24 A10 A11 A12 ----;>---+--"P1 3 XL-9~i46 0011'4'--t-----'l----"1 3 P1 2 - - - ~ 1 - - + - - 2- P11 CS•.-1-------~l---'I-P10 14 01 3 X1 19 ADDRES S BU S X2 18 A14 Y2 11 .0592MH z a~ 20pf:i: JM3 2~ 3PLINK ~ ~15 1 SEPTEM BER 1991 61 IC sockets are supplied for all the chips except for the 48-pin chip (U10) for which machine pin IC socket strips will be supplied. Note how the miniature loudspeaker is mounted above the PC board on plastic pillars. tioned, and a plugpack power supply. Most important, there is a repair service available. If you do assemble the kit and, horror of horrors, it doesn't work, you can get it fixed for a very reasonable fee. More about that later. Before going any further though, let's make a number of important points. First, this is no project for a beginner. If you have not already built a number of reasonably complex projects in the past, don't decide to have your initiation on this one. Second, it uses an expensive double sided board with tracks and pads that are very close together. Even though the PC board does have a green solder mask, you can still short out tracks and do other damage if you are not proficient with a soldering iron. Third, and this is most important, you will need a temperature controlled soldering iron with a small tip. It should be cleaned regularly - keep 62 SILICON CHIP that foam pad clean and moist, and use if often! Finally, if you buy the kit and then decide that it really is too complex for you to attempt, then you can return it for a refund (less shipping charges), provided certain conditions are met. These are spelt out in instructions which come with the kit. OK, happy with all that? Let's proceed to describe the circuit. It really consists of just three main chips which are the fax/modem chipset (two) and the microcontroller. All the rest is the interfacing, RAM, ROM and other incidentals to make it all operate. Circuit diagram The whole circuit, minus the power supply section, is spread out over two pages (Fig.1) and looks pretty daunting but really, most of it just shows all the connections to and from the big black chips. At the top lefthand cor- ner is all the circuit to do with the interface to the phone lines - the LIU or line isolation unit. Below that is an amplifier and speaker which enables you to hear dial tone and all the other beeps and cheeps that faxes and modems make when they are operating. In the bottom lefthand corner are the key devices, U10 and Ul 1. These are the fax/modem chipset, made by Exar (XR2901 and XR-2902). Ul 1 is the DSP or digital signal processor while U10 is the analog processor. To the right of these is U3, another Exar chip which is presently the only option with this kit. U3 provides V21 and V23 modem standards which are 300 and 1200/75 baud rates. In the bottom righthand corner of the circuit is U4 (the microcontroller) and its associated RAM and EPROM chips, U2 and U9, respectively. U8, to the left of U4, is a nonvolatile RAM chip, a so-called NOVRAM. This chip has all the information the processor needs at power up. At the top righthand corner is all the circuitry to do with the RS232 port and the front panel LEDs. It consists of address decoding chips U17 and U7, together with U5 (to the right ofU9), plus U6 (an octal buffer/driver for the LEDs) and U14, U15 and U16 which are two 1488s and a 1489 to provide the RS232 port. Well, to be candid, we can't say a lot about how most of this circuitry works since it is all locked up inside the three key chips already mentioned. In fact, when you come right down to it, we can't say anything except that all the signal levels will be 5V logic signals. The exception is the LIU which we'll now proceed to describe. Line isolation unit At the top left of the circuit are shown two sockets, Jl and J2 . These are US modular sockets which are connected in parallel, so that a phone can be connected together with the Fax/Modem. From the modular connectors, the incoming lines go via linking connectors JM9, JM10 and JMl 1. These allow correct connection for Australia or New Zealand. After these, the lines go though inductors 11 and 12 to bridge rectifier D4-D7 and also to the ring detect circuit consisting of 10kQ resistor R35, lµF capacitor C52, 27V zener Z7, diode Dl and optocoupler U21. U21 couples through the de- +13V 14 U16 C31 0.1 ! 14 U15 C5 0.1 ! DB 1N4001 t---------voo + C37 2200 _ C3B 2.2 + - + CB 0.1 C34 0.1 09 1N4001 C36 1000 + - C35 2.2 wave rectifiers D8 & D9 and associated capacitors to provide positive and negative rails of ±13V to ±18V. These in turn are fed to 3-terminal 5V regulators to give ±5V rails. While the power supply circuit is shown on a separate page, that was purely because we did not have space for it on the main circuit. It is on the PC board along with all the other circuitry. Construction -13V C1 01! 1 U15 1 U16 C2 01I . ~ 123 Fig.2: the power supply uses a 9-12VAC plugpack to drive half wave rectifiers D8 & D9 to produce ±13-18V rails. These rails in turn drive positive & negative 3-terminal regulators to give ±5V supplies. t ected ring signal to the microcontroller U4, to tell it that the phone is ringing. After the diode bridge, the line sig. nal goes via the relay contacts for relay RLl. This relay "seizes" the line when it is energised via latch IC U7 which is controlled by that master of events U4 (yep, the micro). When the line is "seized" - a fancy way of saying that it is connected - it is coupled to a gyrator circuit consisting ofQl , QZ, 10µF capacitor C55 and associated resistors. QZ and Ql look like a Darlington transistor pair which is exactly what they are but they act as a current sink controlled by the voltage across the 10µF capacitor. The whole circuit thereby acts as a gyrator or quasi inductance. The gyrator acts in concert with 4. 7µF capacitor C54 and transformer Tl to provide the required 600!.1 load impedance to the incoming lines. Back to back zener diodes ZZ and Z3 provide clipping of the input signal so that it does not damage the following circuitry which is the "hybrid". Op amps U13a and U13b form · the "hybrid", a circuit which formerly was provided by a bridge transformer. Its purpose is to separate the incoming signal from the outgoing signal. The core of the circuit is really op amp U13b together with the resistors at its input and its 22kQ feedback resistor. U13a acts simply as an inverting amplifier with a gain of 1. It takes the output signal from U10 (and U3 if fitted) and feeds them to transformer Tl via 464!.1 resistor R16 and lµF capacitor C51. U13b discriminates against the outgoing signal from U13a by virtue of the resistors at its input. As far as the incoming signal is concerned, it sees U13b as a non-inverting amplifier with a gain of 2, as set by resistors R18 and R19. The incoming signal is also loaded by 4640 resistor R16 and the virtual zero output impedance of U13a. Because of the low impedance of U13a, virtually none of the incoming signal appears at pin 5, the noninverting input ofU13b, so it operates as a straight inverting amplifier. Outgoing signals from U13a, on the other hand, see U13b as having no gain because the signal appears at both inputs. So both the incoming and outgoing signals are passed in the correct directions through the circuit and they do not interact. Critical to the circuit operation are the values of the resistors around U13b - they are all 1% tolerance. Virtually, the only other analog part of the circuitry is that associated with U18, the audio amplifier which drives the speaker for dial tones and so on. Power supply Compared with the rest of the circuitry, that for the power supply is dead simple. It consists of a 9-1 ZVAC plugpack transformer feeding half Since this project is all based on one compact PC board, by the time you have finished assembly of the board you have finished the project ! Start the assembly by carefully checking the board for any etching errors, track breaks or shorts. There should not be any but check thoroughly anyway because once all the parts are on, fault-finding is very difficult. This done, install all the resistors , diodes and zener diodes - see Fig.2. Check each resistor with your digital multimeter before it is installed and soldered into place. Next, solder in all the capacitors, making sure that the polarity of the electrolytics is correct, then install the sockets. These will be supplied for all except the 48-pin chip, U10. For this, machine pin IC socket strips will be supplied. You may to have snap these off to length before soldering each section in. Install the sockets according to the wiring diagram. Ignore the fact that the photographs of our prototype show vacant sockets - these are not necessary, so don 't install them. This means that you don 't need to install sockets for Ul, U3 or UZO. These components, plus transistor Q3, relay RLYZ and some resistors and capacitors, are reserved for future enhancements of the product and also the NZ version. Note also that the board has provision for two overlapping sockets, U10 and U12. Only U10 is used in this circuit, so don't put in the socket strips for U12. Do not put any of the ICs in yet. That has to wait until you've completed the initial power checks. Now install the 2-link, 3-link and 4-link jumper blocks. Note that unless you are building a version for New Zealand, jumper JM11 is not needed. JM4 is also not needed unless U3 is to be installed. The miniature loudspeaker is inSEPTEMB ER 1991 63 w ...J <I: ::E w u.. XR2902/XR2402 Fig.3: this wiring diagram shows the top PC pattern only. U10 & U12 are shown overlapping but only U10 is used here. Refer to the text regarding the various jumper link options & for details on component omissions. stalled on a couple of pillars and connected by two wires which drop down onto the board. You can now install the rest of the hardware , including the two crystals (don't get them accidentally swapped around), the two regulators, the US modular sockets and DC input socket, the female 25-pin D socket and the miniature toggle switch. Finally, you can install the eight rectangular LEDs. Note that there is provision for 12 LEDs in all and 12 are shown in the photographs but this circuit uses only eight. Those labelled Al to A4 on the wiring diagram are omitted. The LEDs need to be installed flat on the PC board so that they just touch the back of front pan el when it is 64 SILICON CHIP mounted. The best way to do this is to bend the leads of each LED at rightangles 3mm from the body. This is done by holding the LED so that the anode lead is on the right and then bending both leads down. Don't make a mistake here otherwise all the LEDs will be installed back to front! Warning! The Fax/Modem is not an Austel approved device. It has been designed so that when it is submitted as a commercial product, fully assembled , it should pass all Austel requirements. In the meantime, connection to Telecom lines is an offence. The front panel is a piece of screen printed Mylar film with double side adhesive tape. You peel off the backing and then stick it to the moulded front panel escutcheon. This whole assembly can then be pop riveted or secured to the board with screws, nuts and lock washers. There are also two rightangle metal brackets which need to be pop riveted to the board. At this stage, your Fax/Modem is almost complete but don't rush along to finish it. Carefully check your work against the wiring diagram and the circuit. Any mistakes need to be fixed now. Power up Now connect the AC plugpack and check the voltages to the inputs of the Although not shown here, inductors L1 & L2 will be supplied with kits. Note, however, that only eight LEDs are required, so omit LEDs A1-A4 (top right). Use a fine-tipped temperature controlled iron to solder the parts to the PC board. 3-terminal regulators. They should lie in the range of ±13V to ±18V, depending on the particular plugpack supplied. Now check the outputs of the 3-terminal regulators. They should be +5V or -5V, as the case may be. Now check that the correct supply voltage (+5V or -5V) is present at the supply pins for each IC socket and at the anodes of the eight LEDs. Note that the positive supply on some chips is labelled Vdd or Vee and is +5V. Vpp, pin 1, on the EPROM (U9) is also +5V. Vss, the negative supply, is -5V. On U15 and U16 (the 1488 RS232 line drivers), pin 14 should be the unregulated positive supply rail (+13V or above) , while pin 1 is the unregulated negative supply (-13V or below). Similarly, on U14, the 1489 line receiver, pin 14 should be +13V. There is no negative supply to U14. With all the supply voltage checks complete and correct, you can insert all the ICs. Make sure, whatever you do, that they all go in the right way around. They all face to the front of the board except for U21, the optocoupler. Now power up the circuit. Nothing should happen except that LEDs MR and HS should light up. If not, turn off the power again and check your work very carefully. If you can't get the two LEDs to light up, check that they are definitely wired in correctly and that the +5V rail is present at their anodes and at pin 20 of U5. You should have done this before but never mind. Failing that, you can always return it to the suppliers, PC Marketplace, who will be happy to fix it for their prescribed fee. On the other hand, we expect that the unit will perform exactly as it should and so you can proceed to connect the Fax/Modem to your computer and install the software. The software comes supplied on two 360Kb floppy discs. To load the software, insert the first disc in the drive and type install. After that, it's simply a matter of answering the questions. Don't try simply copying the files to your hard disc, as they are all compressed. The software, by the way, features drop-down menus, is easy to operate, and can be either mouse or keyboard driven. Have fun. SC Where to buy the kit The full kit of parts and the software is available from PC Marketplace Pty Ltd, PO Box 1100, Lane Cove, NSW 2066. Phone 02 418 6711. The price is $299.00. The kit will be available early in September. SEPTEMBER 1991 65