57 - DATA
The command code 57 (71 octal) indicates the following data field is a
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.
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.
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.
In the line below the normal values have been filled in for the length,
the command, the synchronization mark and fill bits.
[page 2] Postel