Rapicom 450 facsimile file format
How a specific fax machine encoded image data into computer files.
Postel, J., "Rapicom 450 facsimile file format", RFC 769, DOI 10.17487/RFC0769, September 1980, <https://www.rfc-editor.org/info/rfc769>.
@misc{rfc769,
series = {Request for Comments},
number = 769,
howpublished = {RFC 769},
publisher = {RFC Editor},
doi = {10.17487/RFC0769},
url = {https://www.rfc-editor.org/info/rfc769},
author = {J. Postel},
title = {{Rapicom 450 facsimile file format}},
pagetotal = 2,
year = 1980,
month = sep,
} RFC text
Contents
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.
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
AI ELI5
On this page
§1Fax File Storage
Several organizations connected Rapicom 450 facsimile machines to computers so they could store fax images in files and retrieve them later. This memo documents the file format they adopted, so that other organizations with the same fax equipment could exchange these files reliably.
A fax machine turns a picture into numbers. This memo says how to write those numbers into a file.
§2The Record Format
The Rapicom 450 produces 585-bit data blocks. Each block is stored in a record of 8-bit bytes: a length byte (including itself and the command byte), a command byte, and the data bytes. The length value includes both the length and command bytes, so a record with a 585-bit data block is 76 bytes long (one length, one command, 73.125 rounded to 74 data bytes).
Three command codes are defined: 56 (octal 70) for a setup block, 57 (octal 71) for a data block, and 58 (octal 72) for an end-of-file marker.
Each piece of the picture gets wrapped in a labeled package: a number saying how big it is, a code saying what kind of data it is, and the picture data itself.
§3File Organization
A complete fax image file always starts with a setup block (the fax machine's configuration), followed by one or more data blocks (the actual scanned image), and ends with an end-of-file record. The first 24 bits of each data block are always a synchronization mark with the value 271141344 in octal. The next two bits of the byte after the sync mark contain a sequence number that cycles 11, 01, 10, 00 across the data records.
The 585-bit data is encoded by the Rapicom 450 hardware, so it cannot be used as a simple bitmap without understanding the machine's proprietary encoding and decoding procedure.
A fax file is like a deck of labeled cards: first comes the setup card, then picture cards in order, then a done card at the end.