Some months ago I posted an article to cypherpunks commenting on how easy it is to intercept the supposedly secret traffic on the Motorola mobile data terminals used by many police forces to access criminal history and other sensitive information. This data is not seriously encrypted (or encrypted at all in most systems) and illustrates the kind of security by obscurity that some people would like to see continue as the only protection for such information as it is broadcast to the world on open radio channels. Apparently someone saw my comments or had similar thoughts and today I found the following article posted to alt.radio.scanner. I do not know who the author is, and I did not write the article myself - but I think the article may interest some members of the list as it illustrates what civilian cryptography would be like if the NSA had its way... Dave Emery N1PRE die@die.com DIE Consulting Weston Mass. ----------------------- Begin quoted article ----------------------
From lord@heaven.com Mon Dec 23 23:11:43 EST 1996 Article: 44420 of alt.radio.scanner Xref: world alt.radio.scanner:44420 Path: world!blanket.mitre.org!news.tufts.edu!www.nntp.primenet.com!nntp.primenet.com!howland.erols.net!newsfeed.internetmci.com!news.ro.com!news From: lord <lord@heaven.com> Newsgroups: alt.radio.scanner Subject: MDT stuff Date: Mon, 23 Dec 1996 16:04:39 -0600 Organization: screw Lines: 765 Message-ID: <32BF01F6.2742@heaven.com> Reply-To: lord@heaven.com NNTP-Posting-Host: ts2p5.ro.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii; name="Mdt.txt" Content-Transfer-Encoding: 7bit X-Mailer: Mozilla 3.0 (Win95; I) Content-Disposition: inline; filename="Mdt.txt"
Greetings one and all, Have you ever lusted to decode Mobile Data Terminal (MDT) tranmissions? Have you ever wanted to see the same NCIC and motor vehicle information that law enforcement officers see? Have you ever wanted to see what officers send to each other over "private" channels? And all this with an interface you can build with only a few dollars worth of parts from your local radio shack? If so this posting might be your rendevous with destiny. The tail end of this posting includes the source code of a program that decodes and displays MDT messages. It stores roughly 30k of messages in a buffer and then writes the whole buffer to a file called "data.dat" before terminating. The program may be interrupted at any time by pressing any key (don't use control-c) at which point it writes the partially filled buffer to "data.dat". This program only works for systems built by Motorola using the MDC4800 tranmission protocol. This accounts for a large fraction of public service MDT systems as well other private systems. The existence of this program is ample evidence that Motorola has misrepresented its MDT systems when it marketed them as a secure means of communcications. The interested reader will soon discover that these systems do not use any form of encryption. Security concerns instead have been dealt with by using a code. "And what might this code be called?" asks the reader. The code turns out to be plain ASCII. What follows is a brief description of how the program and the MDC4800 protocol work. If you don't understand something go to your local library and check out a telecommunications theory book. 1. The raw transmission rate is 4800 baud. The program's interrupt service routine simply keeps track of the time between transitions. If you're receiving a perfect signal this will be some multiple of 1/4800 seconds which would then give you how many bits were high or low. Since this is not the best of all possible worlds the program instead does the following: transitions are used to synchronize a bit clock. One only samples whenever this clock is in the middle of the bit to produce the raw data stream. This greatly reduces jitter effects. 2. Whenever a tranmitter keys up the MDC4800 protocol calls for bit synchronization (a sequence of 1010101010101010....). In the program this will result in receive bit clock synchronization. There is no need to specifically look for the bit sync. 3. Look for frame synchronization in raw bit stream so that data frames can be broken apart. Frame synchronization consists of a 40 bit sequence : 0000011100001001001010100100010001101111. If this sequence is detected (or 35 out of 40 bits match up in the program) the system is idling and the next 112 bit data block is ignored by the program. If the inverted frame sync is detected one immediately knows that 112 bit data blocks will follow. 4. Receive the 112 bit data block and undo the bit interleave. This means that one must reorder the bits in the following sequence : {0,16, 32,48,64,80,96,1,17,33,49,65,81,97,2,18,34,...} if the orignal sequence were received as {0,1,2,3,4,5,6,7,8,...}. 5. Check the convolutional error correcting code and count the number of errors. The error correcting code is rate 1/2 which means we will be left with 56 data bytes. The encoder is constructed so that the output consists of the original data bits interspersed with error correcting code. The generating polynomial used is : 1 + X^-1 + X^-5 + X^-6 Whenever an error is detected a counter is incremented. An attempt is made to correct some errors by finding the syndrome and looking for the bogus bit. 6. Keep receiving 112 bit data blocks until either a new frame sync is found or two consecutive data blocks have an unacceptably large number of errors. 7. Each data block consists of six data bytes; the last 8 bits are status bits that are simply ignored. The program shows the data in two columns - hexadecimal and ASCII. This data is kept in a buffer and is written to the file "data.dat" when the program terminates. 8. What the program doesn't do: As a further check on the data there can be CRC checks. This varies from protocol to protocol so this program does not implement the CRC checks. Nonetheless, it is a relatively trivial matter to find the generating polynomial. The addresses, block counts, and message ID numbers are also quite easy to deduce. As you can see, there is no encryption. The bit interleave and the error correcting coding are there solely to insure the integrity of the ASCII data. Since any moron could have figured this stuff out from scratch one could argue that MDTs do not use "...modulation parameters intentionally witheld from the public". Therefore the Electronic Communications Privacy Act may not prohibit receiving MDT tranmissions. However, consult your attorney to make sure. The total disregard for security will no doubt annoy countless Motorola customers who were assured that their MDT systems were secure. Since Federal law states that NCIC information must be encrypted your local law enforcement agency might be forced to spend millions of dollars to upgrade to a secure MDT system - much to the delight of Motorola and its stockholders. Cynics might conclude that the release of a program like this is timed to coincide with the market saturation of existing MDT systems. Also, this program is completely free and it had better stay that way. What's to prevent you from adapting this into a kit and selling it
from classified ads in the back of Nuts and Volts? Nothing. But take a look at Motorola's patents sometime. You'll notice that this program does things that are covered by a shitload of patents. So any attempt to take financial advantage of this situtation will result in utter misery.
Please keep the following in mind: this program only works with the first serial port (COM1). If your mouse or modem is there too bad. If you don't like this rewrite the program. ------------------------------------------------------------------------ What equipment do I need? RADIO SCANNER: A scanner that can receive 850-869 MHz. For those of you who don't know, this is the band where most business and public service trunked radio systems can be found along with the mobile data terminal transmissions. Chances are excellent that if your local authorities have a motorola trunked radio system and mobile data terminals that this is the frequency band in use. Very rarely will one find mobile data terminals in other frequency bands. Now for the fun part - your scanner should probably be modified to allow you to tap off directly from the discriminator output. If you wait until the signal has gone through the radio's internal audio filtering the waveform will likely be too heavily distorted to be decoded. This is exactly the same problem that our friends who like to decode pager transmissions run into - some of them have claimed they can only decode 512 baud pager messages using the earphone output of their scanner. These mobile data terminal messages are 4800 baud so I don't think you have a snowball's chance in hell without using the discriminator output. If you don't know where to begin modifying your scanner you might want to ask those who monitor pagers how to get the discriminator output for your particular radio. COMPUTER/SCANNER INTERFACE Those of you who have already built your interface for decoding pager messages should be able to use that interface without any further ado. For those starting from scratch - you might want to check out packages intended for pager decoding such as PD203 and the interfaces they describe. The following excerpt gives an example of a decoder that should work just fine (lifted out of the PD203 POCSAG pager decoder shareware documentation):
0.1 uF |\ +12v ---||-----------------------|- \| AF IN | |741 \ ---- | | /--------------------- Data Out | \ ------|+ /| | CTS (pin 5/8) | / 100K | |/-12v | or DSR (pin 6/6) | \ | | GND / ----/\/\/\---- GND ------ GND (pin 7/5) | | 100K | \ N.B. Pin Numbers for com port are GND / given as x/y, where x is for a 25 \ 10K way, y for a 9 way. / | GND
The above circuit is a Schmitt Trigger, having thresholds of about +/- 1v. If such a large threshold is not required, eg for a discriminator output, then the level of positive feedback may be reduced by either reducing the value of the 10K resistor or by increasing the value of the 100K feedback resistor.
The +/- 12v for the op-amp can be derived from unused signals on the COM port (gives more like +/- 10v but works fine !):-
TxD (2/3) --------------|<-------------------------------------- -12v | | RTS (4/7) --------------|<-------- GND - - | | _ + 10uF --------->|------- - - | Diodes 1N4148 | - + 10uF GND | | DTR (20/4) ------------->|-------------------------------------- +12v
If I were building this circuit I would strongly suggest tying the non-inverting (+) input of the op-amp to ground since you are working directly with the discriminator output and don't need a Schmitt trigger. All these parts or equivalents are easily available (even at your local Radio Shack which stocks the finest collection of components that have failed the manufacturer's quality control checks and supported by a sales staff that's always got the wrong answers to your questions). Also: DO NOT use the RI (ring indicator) as an input to the computer. ------------------------------------------------------------------------- How do I check things? As a first step, I would get a package such as PD203 and use it to decode a few pages. If you can get that working you know that that your interface circuit is functioning correctly. If you are in a reasonably sized town you might be part of the ardis network. The ardis network is a nationwide commercial mobile data terminal network where one can send/receive E-mail messages from one's portable computer. It has been exclusively assigned the frequency of 855.8375 MHz. Therefore, if you can hear digital bursts on this frequency you are basically guarranteed that these are MDC4800 type messages. You should be able to get stuff popping up on your screen although a lot of the messages will not be plain english. If your interface works but you can't seem to get any messages on the screen for a channel you know is a Motorola MDT system then it might be possible that your scanner/interface is putting out data with the polarity reversed. To check for this run the program with a command line arguement. When it runs you should an initial "Polarity reversed" message and hopefully then things will work out for you. Other than that: if this program doesn't work pester someone else who has got it working. Don't bother pestering the author(s) of this posting; the shit(s) aka "she/he/it (s)" are afraid of a thousand lawyers from Motorola descending like fleas to infest their pubic hair and accordingly have decided to remain forever anonymous. No doubt someone on the usenet who sees this post will know what to do with this program and also hopefully rewrite into a more user friendly form. When you do please don't forget to release the source code. ------------------------------------------------------------------------- Future projects/nightmares you might want to think about: Certain MDT systems embed formatting information in the text in the form of ESC plus [ plus a few more bytes. Someone might want to decode these on the fly and format the output so it looks exactly the same way as the user sees it. Make it so that this program works with com ports other than COM1. Make it user friendly? Enlarge the data buffer from the current 30k. Give the output data file an unique name each time the program is run instead of "data.dat". How about sorting through the past traffic so that you only see traffic to a specified user? The program does not cut data blocks off in the display but it might add an extra one or two (which will display as all zeroes). Someone might want to make all those zeroes be shown as blanks instead. Write some real instructions. Now the more ambitous stuff: Are you half-way competent with RF engineering? Then listen in to the tranmissions from the mobile units back to the base station. That way you get everyone's password and user IDs as they log on to the MDT system. By this point you will no doubt have been able to figure out all of the appropriate communications protocols so you should think about getting your own transmitter up and running along with the necessary program modifications so that you can transmit MDT messages. The required transmitter can be very simple - for example, those masocists who want to start from scratch might want to special order an appropriate crystal (pulling the frequency with the computer's tranmit signal), building a frequency multiplier chain, and adding a one watt RF amplifier to top it all off (see the appropriate ARRL publications for more information on radio techniques). Now you can log in and look at the criminal records and motor vehicle information on anybody you can think of. Find out what your neigbors are hiding. Find out who that asshole was that cut you off downtown. Find out where your former girl/boy friend is trying to hide from you. And on and on... Those with simpler tastes might want to simply transmit at the base station's frequency to any nearby MDT terminal - now you too can dispatch your local law enforcement agencies at the touch of your fingers!!! See your tax dollars at work tearing apart every seam of your neighbor's house. Or create strife and dissension in your local law enforcement agency as more and more officers come out of the closet using their MDTs trying to pick up fellow officers. There are municipalities that have put GPS receivers on all of their vehicles. Should it happen that the information is sent back over one of these networks you could have your computer give you a real-time map showing the position of every vehicle and how far away they are from you. Extend your knowledge to other data networks. Here you will want to look at the RAM mobile data network. It uses the MOBITEX protocol which is really easy to find information on. Since it is an 8 kilobaud GMSK signal there is a decent chance that you can use the interface described here. This transmission mode demmands much more from your equipment than MDT tranmissions. At the very least you must be much more careful to make sure you have adequate low frequency response. Despite this it is possible to receive and decode MOBITEX transmissions with a simple op-amp circuit! This just goes to show you what drivelling bullshit RAM's homepage is filled with - they explain in great detail how hackers will never be able to intercept user's radio tranmissions (incidentally explaining how to decode their tranmissions). The necessary program will be the proverbial exercise left for the reader. For better performance buy a dedicated MOBITEX modem chip and hook it up to your computer. ----------------------------------------------------------------------- A few words about the program: Remember - you must have your decoder hooked up to COM1. The RTS line will be positive and the DTR line negative but if you built the decoder with a bridge rectifier you really don't have to worry about their polarity. Stop the program by punching any key; don't use control-c or control-break! If you must reverse polarity run the program with any command line arguement (example: type in "mdt x" at the command line if your program is mdt.exe). You should then see the "Polarity Reversed." message pop up and hopefully things will then work. As far as compiling this - save the latter portion of this posting (the program listing) and feed it to a C compiler. Pretty much any C compiler from Borland should work. If you (Heaven Forbid) use a Microsoft C compiler you might need to rename some calls such as outport. Follow any special instructions for programs using their own interrupt service routines. This program is not object oriented. It also does not want anything whatsoever to do with Windows. Please don't even think about running this program under Windows. Finally, here it is: Good Luck and may God be with you! ---------------------- C u t H e r e ! ! ! -------------------------- /* start of program listing */ #include <stdio.h> #include <conio.h> #include <dos.h> #include <math.h> /* Purpose of program: receive messages using the Motorola MDC4800 */ /* protocol and show them on the screen */ /* */ /* WARNING TO ALL : This program is free. Please distribute and modify*/ /* this program. I only request that you always include the source */ /* code so that others may also learn and add improvements. The */ /* status of this program at the time of the original release is : */ /* it doesn't have much in the way of a user interface or options but */ /* it should work if you follow the procedure in the text file. Don't */ /* expect any sort of support (you get what you pay for - nothing in */ /* this case). Finally, here's a special message to all of you who */ /* might have the urge to try to make money with this information: */ /* I know where you live. I will shave your pets and pour rubbing */ /* alcohol all over them (unless said pet happens to be a Rottweiler).*/ /* I will have sex with your wife while you off at work; on the rare */ /* occasions when you have sex with your wife she will in the throes */ /* of passion cry not your name but mine. I will sell drugs to the */ /* demented spawn you refer to as your children. And if that's not */ /* enough for you let a thousand lawyers from motorola descend on you */ /* and pound your fat rear end so far into the ground that it finally */ /* sees daylight again somewhere in Australia. */ /* */ /* */ /* General tidbits (a few of those "Why were things done this way */ /* questions). */ /* 1. Why is captured data kept in an array and only written to a */ /* disk file at the very end? Because disk access seems unreliable */ /* when so much time is taken up by the background interrupt service */ /* routine. */ /* 2. Why is the array storing this so small? Because yours truly was */ /* too damn lazy to use a pointer and allocate a huge chunk of memory.*/ /* (Hint for those of you who would like to improve this. */ /*--------------------------------------------------------------------*/ /* global variables */ int lc=0; char fob[30000];/* output buffer for captured data to be sent to disk */ int foblen=29900; int fobp=0; /* pointer to current position in array fob */ char ob[1000]; /* output buffer for packet before being sent to screen */ int obp=0; /* pointer to current position in array ob */ static unsigned int buflen= 15000; /* length of data buffer */ static volatile unsigned int cpstn = 0; /* current position in buffer */ static unsigned int fdata[15001] ; /* frequency data array */ void interrupt (*oldfuncc) (); /* vector to old com port interrupt */ /**********************************************************************/ /* this is serial com port interrupt */ /* we assume here that it only gets called when one of the status */ /* lines on the serial port changes (that's all you have hooked up). */ /* All this handler does is read the system timer (which increments */ /* every 840 nanoseconds) and stores it in the fdata array. The MSB */ /* is set to indicate whether the status line is zero. In this way */ /* the fdata array is continuously updated with the appropriate the */ /* length and polarity of each data pulse for further processing by */ /* the main program. */ void interrupt com1int() { static unsigned int d1,d2,ltick,tick,dtick; /* the system timer is a 16 bit counter whose value counts down */ /* from 65535 to zero and repeats ad nauseum. For those who really */ /* care, every time the count reaches zero the system timer */ /* interrupt is called (remember that thing that gets called every */ /* 55 milliseconds and does housekeeping such as checking the */ /* keyboard. */ outportb (0x43, 0x00); /* latch counter until we read it */ d1 = inportb (0x40); /* get low count */ d2 = inportb (0x40); /* get high count */ /* get difference between current, last counter reading */ tick = (d2 << 8) + d1; dtick = ltick - tick; ltick = tick; if ((inportb(0x3fe) & 0xF0) > 0) dtick = dtick ^ 0x8000; else dtick = dtick & 0x3fff; fdata[cpstn] = dtick; /* put freq in fdata array */ cpstn ++; /* increment data buffer pointer */ if (cpstn>buflen) cpstn=0; /* make sure cpstn doesnt leave array */ d1 = inportb (0x03fa); /* clear IIR */ d1 = inportb (0x03fd); /* clear LSR */ d1 = inportb (0x03fe); /* clear MSR */ d1 = inportb (0x03f8); /* clear RX */ outportb (0x20, 0x20); /* this is the END OF INTERRUPT SIGNAL */ /* "... that's all folks!!!!" */ } void set8250 () /* sets up the 8250 UART */ { static unsigned int t; outportb (0x03fb, 0x00); /* set IER on 0x03f9 */ outportb (0x03f9, 0x08); /* enable MODEM STATUS INTERRUPT */ outportb (0x03fc, 0x0a); /* push up RTS, DOWN DTR */ t = inportb(0x03fd); /* clear LSR */ t = inportb(0x03f8); /* clear RX */ t = inportb(0x03fe); /* clear MSR */ t = inportb(0x03fa); /* clear IID */ t = inportb(0x03fa); /* clear IID - again to make sure */ } void set8253() /* set up the 8253 timer chip */ { /* NOTE: ctr zero, the one we are using*/ /* is incremented every 840nSec, is */ /* main system time keeper for dos */ outportb (0x43, 0x34); /* set ctr 0 to mode 2, binary */ outportb (0x40, 0x00); /* this gives us the max count */ outportb (0x40, 0x00); } /****************************************************************/ int pork(int l) { static int s=0,sl=0x0000,t1,asp=0,ll,k,v,b,tap,synd=0,nsy; static char line[200]; /* array used to format 112 bit data chunks */ static int lc=0; /* pointer to position in array line */ if (l == -1) { /* printf (" %2i\n",asp); */ sl = 0x0000; s = 0; synd = 0; if ((asp <12) && (lc > 50)) { ll = 12 - asp; for (ll=0; ll<6; ll++) { v = 0; for (k=7; k>=0; k--) { b = line[ (ll<<3) +k ]; v = v << 1; if ( b == 49) v++; } ob[obp] = (char) v; if (obp < 999) obp++; } } lc = 0; tap = asp; asp = 0; return(tap); } else { s++; if (s==1) { line[lc] = (char) l; lc++; } /* update sliding buffer */ sl = sl << 1; sl = sl & 0x7fff; if (l == 49) sl++; if (s >1) { s = 0; if ((sl & 0x2000) > 0) t1 = 1; else t1 = 0; if ((sl & 0x0800) > 0) t1^=1; if ((sl & 0x0020) > 0) t1^=1; if ((sl & 0x0002) > 0) t1^=1; if ((sl & 0x0001) > 0) t1^=1; /* attempt to identify, correct certain erroneous bits */ synd = synd << 1; if (t1 == 0) { asp++; synd++; } nsy = 0; if ( (synd & 0x0001) > 0) nsy++; if ( (synd & 0x0004) > 0) nsy++; if ( (synd & 0x0020) > 0) nsy++; if ( (synd & 0x0040) > 0) nsy++; if ( nsy >= 3) /* assume bit is correctable */ { printf ("*"); synd = synd ^ 0x65; line[lc - 7] ^= 0x01; /**********************************************/ } } } return(0); } void shob() { int i1,i2,j1,j2,k1; /* update file output buffer */ i1 = obp / 18; if ( (obp-(i1*18)) > 0) i1++; fob[fobp] = (char) (i1 & 0xff); if (fobp < 29999) fobp++; for (i2 = obp; i2<=(obp+20); i2++) ob[i2] = 0; for (j1 = 0; j1 < i1; j1++) { for (j2 = 0; j2 < 18; j2++) { k1 = j2 + (j1*18); printf("%02X ", ob[k1] & 0xff); fob[fobp] = (char) (ob[k1] & 0xff); if (fobp < 29999) fobp++; } printf (" "); for (j2 = 0; j2 < 18; j2++) { k1 = j2 + (j1*18); if (ob[k1] >=32) printf("%c",ob[k1]); else printf("."); } printf("\n"); } obp=0; printf("BUFFER: %3i percent full\n",(int)(fobp/299.0)); } /**********************************************************************/ /* frame_sync */ /**********************************************************************/ /* this routine recieves the raw bit stream and tries to decode it */ /* the first step is frame synchronization - a particular 40 bit */ /* long sequence indicates the start of each data frame. Data frames */ /* are always 112 bits long. After each 112 bit chunk this routine */ /* tries to see if the message is finished (assumption - it's finished*/ /* if the 40 bit frame sync sequence is detected right after the end */ /* of the 112 bit data chunk). This routine also goes back to hunting */ /* for the frame sync when the routine processing the 112 bit data */ /* chunk decides there are too many errors (transmitter stopped or */ /* bit detection routine skipped or gained an extra bit). */ /* */ /* inputs are fed to this routine one bit at a time */ /* input : 48 - bit is a zero */ /* 49 - bit is a 1 */ void frame_sync(char gin) { static unsigned int s1=0,s2=0,s3=0,nm,j,t1,t2,t3,ns=0,st=0,n,m,l,chu=0,eef=0; static char ta[200]; if (st == 1) { ta[ns] = gin; ns++; if (ns >= 112) /* process 112 bit chunk */ { chu++; ns = 0; for (n= 0; n<16; n++) { for (m=0; m<7; m++) { l = n + (m<<4); pork(ta[l]); } } if (pork(-1) > 20) eef++; else eef=0; if (eef > 1) /* if two consecutive excess error chunks - bye */ { st = 0; shob(); eef = 0; } /* else eef = 0; */ } } /* s1,s2,s3 represent a 40 bit long buffer */ s1 = (s1 << 1) & 0xff; if ((s2 & 0x8000) > 0) s1++; s2 = (s2 << 1); if ((s3 & 0x8000) > 0) s2++; s3 = (s3 << 1); if (gin == 49) s3++; /* xor with 40 bit long sync word */ t1 = s1 ^ 0x0007; t2 = s2 ^ 0x092A; t3 = s3 ^ 0x446F; /* find how many bits match up */ /* currently : the frame sync indicates system id / idling / whatever */ /* inverted frame sync indicates message follows */ nm = 0; for (j=0; j<16; j++) { if (t1 & 1) nm++; if (t2 & 1) nm++; if (t3 & 1) nm++; t1 = t1 >> 1; t2 = t2 >> 1; t3 = t3 >> 1; } if (nm < 5) { st = 1; ns = 0; } else if (nm > 35) { if (st==1) { shob(); } st = 0; ns = 0; } } void main (int argc) { unsigned int n,i=0,j,k,l,cw1=49,cw0=48; FILE *out; char s=48; double pl,dt,exc=0.0,clk=0.0,xct; if (argc > 1) { printf ("Reverse Polarity.\n"); cw1 = 48; cw0 = 49; } /* dt is the number of expected clock ticks per bit */ dt = 1.0/(4800.0*838.8e-9); oldfuncc = getvect(0x0c); /* save COM1 Vector */ setvect (0x0c, com1int); /* Capture COM1 vector */ n = inportb (0x21); /* enable IRQ4 interrupt */ outportb(0x21, n & 0xef); set8253(); /* set up 8253 timer chip */ set8250(); /* set up 8250 UART */ while ((kbhit() == 0) && (fobp<29900)) { if (i != cpstn) { if ( ( fdata[i] & 0x8000) != 0) s = cw1; else s = cw0; /* add in new number of cycles to clock */ clk += (fdata[i] & 0x7fff); xct = exc + 0.5 * dt; /* exc is current boundary */ while ( clk >= xct ) { frame_sync(s); clk = clk - dt; } /* clk now holds new boundary position. update exc slowly... */ /* 0.005 sucks; 0.02 better; 0.06 mayber even better; 0.05 seems pretty good */ exc = exc + 0.020*(clk - exc); i++; if( i >buflen) i = 0; } } outportb (0x21, n); /* disable IRQ4 interrupt */ setvect (0x0c, oldfuncc); /* restore old COM1 Vector */ /* save captured data to disk file */ i = 0; out = fopen("data.dat","wt"); if (out == NULL) { printf ("couldn't open output file.\n"); exit(1); } i = 0; while ( (i < fobp) && (i < 29800)) { j = ((int)fob[i] & 0xff); i++; for (k=0; k<j; k++) { for (l=0; l<18; l++) { fprintf(out,"%02X ",((int) fob[i + l]&0xff)); } fprintf(out," "); for (l=0; l<18; l++) { n = ((int)fob[i]&0xff); i++; if (n >=32) fprintf(out,"%c",n); else fprintf(out,"."); } fprintf(out,"\n"); } fprintf(out,"\n"); } fclose(out); } /* end of program listing */