
        A NEW MODEM DESIGN FOR HIGH SPEED DATA TRANSFER
                               By
              Donald L. Stoner, V. P. Engineering
                   The MicroPeripheral Corp.
                         Redmond, Wash.
 
      Someone once insisted that what this country needs "is  a
 good five cent cigar".   The computer equivalent to  that migh
 be a "low cost means to transfer data at high speed".
 
      We  are  not  likely  to  see  the  former, but  the  low
 cost-high  speed modem  is a reality.  And none too soon.  The
 telephone  company  knows  about  you   folks  who  tie  up  a
 telephone  line  for hours  on end with your own version  of a
 dedicated data line.  In the near future, you are going to  be
 paying for  your telephone calls  by the minute and  by prefix
 numbers!
 
      Realizing the significant demand  for  a  low  cost- high
 speed  modem,  the  MicroPeripheral  Corporation  launched   a
 development program  in 1980.  Low  cost meant prices  similar
 to those for a  Bell 103 type of modem, that is, $300 or less.
  High speed was defined anything faster than 300 baud.
 
      It soon became apparent that the  approach  taken by Bell
 Telephone  for  high speed modems was not possible at anywhere
 near the self imposed goal of $300.  However,  a market survey
 indicated the majority  of potential users  do not demand  the
 approach  taken by  Bell.  They  simply want to  transfer data
 from  point  A to point B at the fastest  possible rate and at
 the lowest possible cost.
 
      Once  the  decision  was made to  develop our  own  modem
 approach, the project  became  an engineers  dream.  Many  new
 techniques were examined  and discarded  as the free  flow  of
 ideas  brought  out new  and  better  ways  to accomplish  the
 design goal.
 
      A New  Standard- The  final  result  culminated in  a new
 product  called "The MICRO Shuttle (tm)" which is  the subject
 of   a   patent   application.    A   communications  standard
 (MICRO-208)  was   developed   to  provide  compatibility   of
 products.  A  patent application has been made and since there
 seems to be no  prior art in  this  area,  a  grant is likely.
 The unit has  been  FCC Type Accepted  and is currently  being
 field tested at a number of beta sites.
                            FEATURES
 
      Baud   Rate-  The   MICRO  Shuttle  (tm)  is   baud  rate
 independent.   The  I/O  serial  stream can  be  at  any  rate
 between  300 and  4800 baud.  This includes  not only standard
 baud  rates   such  as  1200,  2400,  3600,   etc.   but  also
 non-standard rates.  Odd frequencies  could be used to provide
 a limited degree of network  security.  More  about networking
 later.
 
      Error  Rates- Although  it produces  slightly lower error
 rates, a  conditioned  telephone line is not required, even at
 4800 baud!   The  unit  works  quite well on  the  direct dial
 telephone network.  While  it has  not been  possible  to test
 the  device under all possible line  conditions,  errors rates
 are acceptably low for most commercial applications.
 
      In  instances  where  no errors can be tolerated (such as
 the transfer of  machine language  programs),  software  using
 the acknowledge-no acknowledge (ACK-NAK) should  be used.   If
 an   error   is  encountered,  the  software  will  request  a
 retransmission of the block.
 
      Synchonization-   The  MICRO   Shuttle   (tm)   uses  the
 asynchronous  communication  mode.  This means  that start and
 stop bits  are sent at the  beginning  and  end of each  byte.
 The  data  composition between the  start and  stop  bits is a
 function    of     the     UART     (universal    asynchronous
 receiver-transmitter) in  the  RS-232 I/O interface.   A MICRO
 208  type of  modem will function  with  16 and 32  bit UARTS,
 should they become available.
 
      It  is  possible  to speed up communications 15% or so by
 eliminating these "useless  appendages".  The price  one  pays
 for   dropping   the   start   and   stop  bits   (synchronous
 communications),  however,   is   a  tremendous  increase   in
 complexity  and  cost.   The   advantages  of  retention   far
 outweigh the slight reduction in throughput of data.
 
      For example,  since  the device is bit  synchronized, the
 terminal operator can either  keyboard send or  transfer files
 at the  maximum  baud rate.  Each  keystroke is sent out  as a
 burst of data.  Between each burst the line is quiet.
 
      Networking-    Unlike   other  modems,  no   carrier   is
 transmitted.   MICRO  208  signals  are  impressed on the line
 only during  transmission.  Consider  what would  happen if  a
 group  of Bell  103 modems were connected in parallel on a two
 wire  transmission line (not necessarily a telephone circuit).
  The carrier of each  modem would "clobber" all the others and
 no one could communicate.
 
      Using  the MICRO 208  stanadard,  virtually any number of
 terminals can  be placed in parallel  across the line so  long
 as  the  impedances  are matched  and transmission levels  are
 maintained at -10  dbm.   The  first  bit  to  arrive  from  a
 sending terminal is  used to  drive  the  CTS  (clear to send)
 line low on each receiving terminal.  This provides  a  simple
 but   effective  form  of  hardware   interrupt.    Thus,  the
 receiving terminals are locked  out  and cannot transmit while
 they are  receiving data.  Upon cessation of the transmission,
 the CTS line in each terminal goes  high after approximately 4
 milliseconds  (settling time).  Collisions are avoided  with a
 software protocol which determines who transmits next.
 
      The  time constant  of the CTS hardware interrupt is made
 sufficiently  long  to  hold the CTS  low even at  the slowest
 baud  rate.   However,  if  only  4800  baud  were  used,  the
 settling time could be reduced to less than 300 microseconds.
 
      While this  system will  certainly  not obsolete Ethernet
 (or even compete  with it), it does provide an  excellent  low
 cost way of networking a group of  terminals or  computers  in
 an office or data processing complex.
 
      Simplex- Most readers  are  familar with the  terms  half
 and full duplex.  However,  the MICRO Shuttle (tm) operates in
 the simplex mode.  This means that  only one transmission  can
 occur at a time.  The reason  is that the  modulation from the
 data stream  fills the  entire audio spectrum between  300 and
 3000 Hz.  However, since  the  settling time to enable a reply
 transmission is 4 Msecs. or less, the operation appears  to be
 full duplex to the user.
 
      For  example, consider  the case where two operators  are
 sending  data  to  each other  from  the keyboard  using split
 screen software  or terminals.  At normal typing speeds, there
 is  sufficient time  for dozens  of  characters to arrive  and
 display between each keystroke.
 
      Characters can  even be echoed just as they are with full
 duplex  transmissions.   Of  course  this   will   reduce  the
 throughput considerably.  Existing RS-232  I/O software can be
 used  with the MICRO  Shuttle  (tm)  although  most will  drop
 characters at  the  higher baud rates  if the video display is
 involved.  In  most  cases  data  transfers are  done  with an
 ACK-NAK protocol and incoming  characters are not displayed on
 the  screen.   Few  micros  are  fast  enough  to  decode  the
 character, commit  it  to memory, display  it and  scroll  the
 line.
                     HIGH SPEED LIMITATIONS
 
      The "name  of the game" is to  recreate  an exact replica
 of the local serial data  stream at some  remote location.  In
 the RS-232  convention, the  serial stream consists of  square
 wave  pulses varying  between -12  volts (mark) and  +12 volts
 (space).     These   levels    represent   ones   and   zeros,
 respectively.
 
     The square waves can be sent over  isolated wire pairs for
 reasonably  long  distances.   However, if they were impressed
 on  the telephone  line  (in  cables, with many  adjacent wire
 pairs),  the  data pulses  would  interfere  (crosstalk)  with
 voice conversations.
 
      This is  avoided  in  slow  speed modems  by  using  tone
 pairs, representing  ones  and zeros.  This  process is called
 modulation.   Even  with  tone   modulation  (frequency  shift
 keying-  FSK),  there is a limit to how fast data can be  sent
 over the telephone network.
 
      The factor limiting  the baud rate is how much  data  can
 be sent in a given amount  of time.  This is a function of the
 bandwidth of the  transmission  path.   The  bandwidth  of the
 direct  dial  telephone  network  is  limited  to  frequencies
 between approximately 300 and 3000 Hz.
 
      To increase  the rate to 1200 baud, for example, requires
 800  to  1000  Hz.  of  frequency  shift.   The  filters which
 separate the  incoming  and  outgoing  data  channels must  be
 20-30%  wider than  this to avoid  distortion.   In  addition,
 there  must  be  a  "guard  band"  between  the  incoming  and
 outgoing  channels to avoid mutual  interference.  Ultimately,
 as the baud rate is increased,  the transmission path runs out
 of  bandwidth.   There  simply  is  not  enough  room  in  the
 300-3000 Hz. voice channel of  the  telephone circuit for much
 more than  two  300 baud channels  (full  duplex) or  one 1200
 baud channel (half duplex)  with frequency  shift keying.  For
 higher  baud  rates,  a  very  expensive  method  called phase
 modulation is used.  Bell 212A modems use  this technique  for
 full duplex 1200 baud transmissions.  The typical price  of  a
 212A modem is around $1000.
                          HOW IT WORKS
 
      A single bit  at 4800  baud has the same  duration as one
 half cycle of audio  frequency at 2400 Hz.  Thus if a  one was
 followed  by a  zero,  it would  have the  same duration as  a
 complete  cycle  at 2400 Hz.   By the same token, two one bits
 (or zero  bits) in tandem would  require  the same time period
 as  a half cycle at 1200 Hz.  Extending this further, one  can
 see  that  a  typical  stream  such  a  1-0-1-1-0-0-1-0  would
 produce  a full sine  at  2400 Hz immediately  followed with a
 full sine at  1200  Hz,  followed by another full sine at 2400
 Hz.  From  this explanation, the  reader can correctly  assume
 that  any  serial bit pattern has an equivalent made up of one
 or   more  half  cycles   at  various  frequencies.    Nothing
 precludes a half  cycle at one frequency  from being  followed
 by a half cycle at another frequency.
 
      Transmission- The  modulator  in the  MICRO Shuttle  (tm)
 employs proprietary circuitry that  controls  tone generation.
 The  circuitry   examines  the  incoming  serial  stream   and
 determines  which  tone  should  be  activated.   The  control
 circuitry  creates  half  sine  waves  that correspond to  the
 steady-state period of the incoming RS-232 data stream.
 
      The pulse-to-sine  conversion circuitry must  look  ahead
 to determine the length of the  next pulse.  Thus, there is  a
 time  delay  between  the  data   stream  and  the  sine  wave
 generation.   If it "looks  ahead"  and sees no  data, no tone
 generation occurs.
 
      The  sine  wave  equivalent  of  the  digital  signal  is
 filtered  and applied  to  the  telephone line  through a line
 impedance matching transformer.
 
      Receiving- At  the receiving end,  the  incoming audio is
 coupled to  the modem  by the same line matching  transformer.
 The  signal is  then  filtered to  remove unwanted  components
 which may have been added by the telephone network.
 
      The   recovered  energy   is   applied  to  sine-to-pulse
 conversion  circuitry  which determines if a one or a zero  is
 being received.  Each time the waveform passes  through  zero,
 the  output  of the  circuit  changes  state.   The output  is
 converted to RS-232  levels  and  applied to  the input of the
 computer or terminal serial port on the RxD line (pin 3).
 
      The  data  output   is   also  coupled,  through  a  time
 constant,  to another  input  port  of  the module.  The  time
 constant of  this  port is  set for  slightly  longer than the
 widest  pulse  applied to  the modulator.  Thus,  whenever the
 RxD output  goes  high  (indicating  the  reception  of  data)
 another output line on the module goes low.  It will stay  low
 for  the period of the  time constant, after the cessation  of
 data.  This port  is connected to the  CTS line (pin 4) of the
 RS-232 I/O port.
 
      The time constant is adjustable.  As  mentioned  earlier,
 the settling or  "turn around"  time  can be  reduced  to  300
 microseconds if the modem is only used at 4800 baud.
 
      Carrier  Detect-  The reader can see  that no carrier, as
 we usually think of it, is transmitted.  In fact,  the process
 just described  is  not really modulation and demodulation  in
 the  strict sense.  We'll  continue  to use  the  name "modem"
 however,  since  this  has  become  a  genaric  term  for  the
 application rather than the process.
 
      There is  at  least one application for the MICRO Shuttle
 (tm) where  a carrier indication  is  essential.  One  of  the
 most popular applications for this device  will be as the auto
 answer  modem  for  small  data  bases.   These might  include
 community   bulletin   boards,   message   centers,   business
 terminals and so on.
 
      When  the  phone  number is called,  the modem  goes  off
 hook.  At this point  how  does the called terminal know it is
 a  MICRO  Shuttle  (tm)  calling  and  not  a  wrong   number?
 Obviously the calling  terminal must send one or two  carriage
 returns.  This information  could be  used  to establish  baud
 rate, if desired.  More important,  the  reception  of data is
 used to start the host communications  software  of the called
 terminal.
 
      So far,  so good.  But what  about the condition when the
 calling party hangs  up without exiting the program  properly?
 Some  sort of timer is required to  indicate  that no data has
 been  received  for a period  of  time.  This can  be done  in
 software, of  course, but one of the  design parameters was to
 make the MICRO  Shuttle (tm) compatible with existing bulletin
 board  software.   Almost  without  exception, these  programs
 examine the DCD line  (pin 8) to see if a carrier  is present.
 Thus the presence of a carrier must be synthesized.
 
      This  is done  by adding another timing loop to the MICRO
 Shuttle  (tm).  Upon reception  of  the first data information
 (the carriage returns), a two minute timer is started and  the
 DCD line (pin  8)  goes high.  Each time  data  arrives,  this
 timer is  reset.  Any outgoing data from the  called  terminal
 also   resets  the  timer.   This  may  be   needed  to  cover
 situations where a continuous stream of data  from  the called
 terminal exceed two minutes.
 
      If the carrier  regeneration  circuit times out,  the DCD
 line (pin  8)  goes  low  and  the modem  goes  back  on  hook
 awaiting  the  next call.   This  time  constant  can  be  set
 externally to increase or decrease  the two  minute value,  if
 desired.
 
      Those interested in  commerical sale of the MICRO Shuttle
 (tm)  should  contact  Vikki Darland, Director of  Sales,  The
 MicroPeripheral Corporation,  2643 151st Place, N. E., Redmond
 Wa. 98052, Telephone (206) 881-7544.

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