From: "Dave Emery" <die@pig.die.com>
Date: Mon, 23 Dec 1996 20:50:44 -0800 (PST)
To: cypherpunks@toad.com
Subject: Lack of security of police Mobile Data Terminals (MDTs)
Message-ID: <9612240450.AA24404@pig.die.com>
MIME-Version: 1.0
Content-Type: text/plain


	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
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From: lord <lord@heaven.com>
Newsgroups: alt.radio.scanner
Subject: MDT stuff
Date: Mon, 23 Dec 1996 16:04:39 -0600
Organization: screw
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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  */