Network Working Group                                          J. Postel
Request for Comments: 769                                            ISI
                                                       26 September 1980



                   Rapicom 450 Facsimile File Format
                   ---------------------------------

Introduction:

Several  organizations  in the ARPA Internet  community have RAPICOM 450
facsimile   machines   interfaced   to  computers.   This  allows  these
organizations  to enter a facsimile  representation  of a  page  into  a
computer  file, and to produce a page from stored facsimile data.  These
organizations  can exchange  stored facsimile data via file transfer and
other protocols.   The purpose  of this note is to document  the  format
used for  these  files  so  that  other  organizations  with  compatible
facsimile devices can join in this information exchange procedure.

The Rapicom 450:

The Rapicom  450 has a built in encoding/decoding  scheme.   It produces
data blocks  of 585 bits.   There are "set up" blocks and "data" blocks.
The machine sends/receives several copies of the set up block, but since
they are identical only one set up block is stored in the file.

Records:

Each 585 bit block  is placed  in a record  of 8-bit  bytes.  The record
format is a length byte, a command byte and the data bytes.  Each record
is an integral  number  of bytes.   The length value includes the length
byte and the command  byte.   The command describes the data in the data
field.

                                    
   0        1        2        3                              length 
   +--------+--------+--------+--------+---//---+--------+--------+ 
   | length | command|   data                                     | 
   +--------+--------+--------+--------+---//---+--------+--------+ 

                      Rapicom 450 Facsimile Record

Commands:

56 - SET-UP

  The command code 56 (70 octal) indicates the following data field is a
  set up block.





Postel                                                          [page 1]


                                                       26 September 1980
Rapicom 450 Facsimile File Format                                RFC 769



57 - DATA

  The command code 57 (71 octal) indicates the following data field is a
  data block.

58 - END

  The command  code 58 (72 octal) indicates that this is the last record
  in the file.   In this case the length may be 2, indicating that there
  is no data in this record.

Conventions:

In the files exchanged  to date,  each record  contains one block.  This
means the data field is 74 bytes long  (585/8=73.125),  and  the  length
field  has the value  76 (114 octal),  except  the last record which may
carry no data and have a length of 2.

The first  record  of a file is always  a SET UP record,  the  following
records are DATA records, until the last record which is an END record.

Details:

The 585 bit data  block  is encoded by the Rapicom 450 and so can not be
used a bit map unless the encoding/decoding procedure is known and used.

The first 24 bits of the block is always a synchronization mark with the
value 271 141 344 in octal or 101110010110000111100100 in binary.

The low order  two bits of the  next  byte  contain  a  sequence  number
(modulo 4).  The sequence number bits cycle in the order 11, 01, 10, 00,
starting with the first DATA record (not the SET UP record).

The line below represents  a DATA record,  where L represents  a  length
bit,  C represents a command bit, M represents the synchronization mark,
S represents  a  sequence  bit,  F  represents  a  fill  bit,  the  dash
represents 68 other data octets, and an D represents a data bit.

LLLLLLLLCCCCCCCCMMMMMMMMMMMMMMMMMMMMMMMMDDDDDDSSDDDDDDDD-DFFFFFFF

In the line below  the normal values have been filled in for the length,
the command, the synchronization mark and fill bits.

0100110000111001101110010110000111100100DDDDDDSSDDDDDDDD-D0000000






[page 2]                                                          Postel