Decoding facsimile data from the Rapicom 450 (RFC0798)

Publishing Venue

Internet Society Requests For Comment (RFCs)

Related People

A.R. Katz: AUTHOR

Abstract

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.

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<...