Network Working Group A. Katz
Request for Comments: 798 ISI
September 1981
DECODING FACSIMILE DATA FROM THE RAPICOM 450
I. Introduction
This note describes the implementation of a program to decode
facsimile data from the Rapicom 450 facsimile (fax) machine into an
ordinary bitmap. This bitmap can then be displayed on other devices
or edited and then encoded back into the Rapicom 450 format. In
order to do this, it was necessary to understand the how the
encoding/decoding process works within the fax machine and to
duplicate that process in a program. This algorithm is descibed in
an article by Weber [1] as well as in a memo by Mills [2], however,
more information than is presented in these papers is necessary to
successfully decode the data.
The program was written in L10 as a subsystem of NLS running on
TOPS20. The fax machine is interfaced to TOPS20 as a terminal
through a microprocessor-based interface called FAXIE.
Grateful acknowledgment is made to Steve Treadwell of University
College, London and Jon Postel of Information Sciences Institute for
their assistance.
II. Interface to TOPS20
The fax machine is connected to a microprocessor-based unit called
FAXIE, designed and built by Steve Casner and Bob Parker. More
detailed information can be found in reference [3]. FAXIE is
connected to TOPS20 over a terminal line, and a program was written
to read data over this line and store it in a file. The decoding
program reads the fax data from this file.
The data comes from the fax machine serially. FAXIE reads this data
into an 8-bit shift register and sends the 8-bit byte (octet) over
the terminal line. Since the fax machine assigns MARK to logical 0’s
and SPACE to logical 1’s (which is backward from RS232), FAXIE
complements each bit in the octet. The data is sent to TOPS20 in
octets, the most significant bit first. If you read each octet from
most significant bit to least significant bit in the order FAXIE
sends the data to TOPS20, you would be reading the data in the same
order in comes into FAXIE from the fax machine.
The standard for storing Rapicom 450 Facsimile Data is described in
RFC 769 [4]. According to this standard, each octet coming from
FAXIE must be complemented and inverted (i.e. invert the order of the
bits in the octet). Thus, the receiving program did this before
Alan R. Katz [page 1]
