A Data-message terminal to be used by off-shore vessels is designed to augment the capabilities of existing High Frequency (HF) radios by allowing the transmission of text messages. Sophisticated techniques of modulation and coding have been incorporated 1n the HF data link protocol to allow data communication when radio conditions. do not permit intelligible voice transmisson The results of the technical f1eld tr1als of the terminal is described in this paper.

1. Introduction

The HF Data-Message Terminal is a transportable text transmission device developed by the Department of Communications. It is designed to augment high frequency voice communication circuits to make optimal use of the limited number of frequencies available. In addition, it provides a method of reliable message exchange over poor quality circuits without skilled operators. The terminal can be interfaced to a standard HF-SSB radio so that an existing voice system can be retrofitted for transmission of hard copy messages in addition to voice. A packet radio protocol is incorporated in the design which permits a number of radios to share common frequencies and operate as a network. This terminal will find applications in shipping, mining, and remote communities for transmission of short messages.

Signals in the HF band propagate by reflection from the ionosphere. When the proper frequency is selected few watts of transmitted power is sufficient to span distances of hundreds of miles. The principle obstacle to HF data transmission is that the signal is often distorted and degraded by noise during the propagation process, (ref. 1) so that special techniques must be used to ensure the integrity of the transmitted messages. In the design of this terminal a modulation for HF especially developed to overcome distortions incurred during propagation is matched with suitable error control techniques and protocol to implement an inexpensive reliable data terminal. The estimated cost of the terminal is less than $4,000 without the radio system. The data terminal is interfaced with the HF-SSB transceiver through the audio and push to talk lines. The interface does not require modifications or special radios for operation. A block diagram depicting the system is shown in figure 1. The optional equipment can be connected to the terminal via a standard interface to suit customer requirements. This includes a CRT, line printer, disks and magnetic tapes. In addition, control can be exercised through a telephone modern allowing the terminal to be a slave to other computer controlled equipment at remote sites.

2. System Overview

The terminal consists of a keyboard, 20 character LED display, 20 column printer and two circuit boards, (see figure 1). The complete terminal is contained in a package the size of a portable typewriter and includes a self contained power supply (see fig. 2). One of the two circuit boards has the Central Processor Unit (CPU) and peripheral circuits. The second board contains a microprocessor modem, and circuitry required to interface the CPU, optional equipment and HF-SSB radio. A block diagram of the terminal is shown in figure 3.

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