An amateur packet-radio installation can be divided into three parts: the terminal equipment, the packet-radio equipment and the radio equipment.

The terminal equipment provides the direct interface to the user. The user types at the terminal keyboard and the terminal displays information for the user.

The packet-radio equipment provides the packet assembly and disassembly and modulation and demodulation functions. The packet-radio equipment may consist of a separate modem and a PAD, or it may be a TNC that includes both a PAD and a modem.

The radio equipment transmits and receives packets. It includes a transceiver (or separate transmitter and receiver), an antenna and any peripheral radio equipment (amplifier, preamplifier, tuner, feed line and so on). We are going to describe these three basic components and how they are interconnected.

Terminal Equipment: The User Interface

In general, the terminal equipment that you will find in use at an amateur packet-radio station falls into one of two categories: dedicated terminals and computers that are emulating terminals.

Dedicated Terminal Equipment 

Dedicated terminals, or simply terminals for short, are devices that are designed for the single purpose of communicating with computers. Terminals are available in a wide variety. Today, the most common terminal consists of some kind of video display, a keyboard and a serial interface (typically EIA RS-232-C/EIA-232-D compatible). This type of terminal is sometimes called a video-display terminal or VDT. The VDT may be a simple device that provides basic input and output functions (commonly called a dumb terminal), or it may provide numerous support functions as well as basic input and output (this device is commonly known as an intelligent terminal).

Computers Emulating Terminals

Terminal-emulation software, which enables computers to emulate terminals, comes in a wide variety. Many computers have a number of communication programs available for them, with each program capable of emulating more than one type of terminal. With this combination of hardware and software, the variety seems infinite. 

For packet-radio applications, communication software should, at a minimum, have a few simple commands for clearing the screen, moving the cursor, backspacing and tabbing. Such software should be capable of operating at a data rate that is compatible with the serial port of your packet-radio equipment. In addition, the software should be capable of saving received data in memory and/or in storage (on tape or disk). The software should also be capable of sending data that has been previously stored. Simultaneous hard-copy printing of what the terminal receives and sends is also a good feature.

The drawback with most communications software is that it is designed to operate primarily with telephone-line modems, not packet-radio modems (TNCs). As a result, these programs include features that are useful for telephone data communications, but which are useless to the packet-radio operator. Most of these programs also lack features that the packet-radio operator would find useful. All is not lost, however. Communications software designed specifically for packet-radio applications has been written for some of the more popular computers.

Using a dedicated terminal or using a computer emulating a terminal both have advantages and disadvantages for packet-radio applications. Expense is always a consideration. On one hand, a good, used terminal may be less expensive than a new but barely adequate computer. On the other hand, a good, used computer may be less expensive than a new but barely adequate terminal.

The function of a terminal is to communicate. If you dedicate a terminal to packet-radio communications, it is doing the job it was intended to do. A computer can serve a variety of functions, however, and dedicating it to packet-radio applications may underutilize its abilities and be a waste of hardware and money.

Terminals are designed specifically for communications, while computers are not. This means that computers are not necessarily good communications tools. How good a communications tool a computer is depends on its communications software. One great advantage computers have over terminals is their ability to store data in memory and/or in a storage (on tape or disk). Most terminals have no means of storing data. All of these considerations must be weighed when you decide what use to communicate with your packet-radio equipment.

Packet-Radio Equipment: PADs, Modems, And TNCs

What follows is a breakdown of the major hardware differences between this equipment.

1) There are PADs, also known as TNCs, that require external modems. (A PAD never includes a built-in modem, but a TNC may or may not include a built-in modem.)

2) There are TNCs that include modems designed for VHF applications (1200 bauds, Bell-202 compatible). Most TNCs today are of this type.

3) There are TNCs that include modems that may be selected for 1200-baud VHF and 300-baud HF applications.

4) There are TNCs that include separate modems for VHF and applications. Separate VHF and HF radio ports permit the simultaneous connection of VHF and HF radio equipment.

a) There are TNCs in this category that permit simultaneous VHF and HF packet-radio communications.

b) There are TNCs in this category that permit gateway operation between the radio ports.

5) There are TNCs that include modems that operate at data rates other than 300 and 1200 bauds.

6) There are TNCs that are designed to be used only with specific computers.

7) There are TNCs that operate in other Amateur Radio modes. 

8) There are TNCs that include more than one of these listed features.

In addition, there are external HF modems available that may be added to those TNCs that do not include HF capabilities, or to TNCs that include HF capabilities that are barely adequate. Typically, such external modems have filters for improved HF reception and front-panel displays to facilitate HF signal tuning. A switch permits bypassing the HF modem to permit you to use your TNC with its built-in modem without changing cables.

Three commercially available HF modems are the AEA PM-I, DRSI HF*Modem and the Pac-Comm PTU-200. In addition, AEA had an HF modem (the HFM-64) for its PK-64 TNC and Heath had one (the HD-4040-2) for its HD-4040 TNC.

RF Equipment: The Radio Connection

At the RF end of a packet-radio station is a lot of equipment. Some of this equipment is of little concern to us. For example, as long as the antennas and feed line are capable of putting a signal on the desired packet-radio frequency, that satisfies our requirements. Other RF hardware needs closer inspection, however.

Our primary concern is the radio equipment's receive-to-transmit and transmit-to-receive turnaround times. This is the amount of time that it takes for a transmitter to be ready to transmit and a receiver to be ready to receive after a switch between the transmit and receive modes occurs. A TNC can switch between the transmit and receive modes very quickly, so quickly that it usually must wait for the RF equipment before it can continue to communicate. According to Tucson Amateur Packet Radio (TAPR), most amateur radios have receive-to-transmit and transmit-to-receive turnaround times between 150 and 400 milliseconds (ms), which dramatically reduces the amount of data that can be sent and increases the chance that two or more stations will interfere with one another. Such delays slow down what is intended to be a fast mode of communications.

A number of factors affect the length of this delay. The actual physical switching of an antenna, internally in a transceiver or externally with a separate transmitter and receiver, affects the turnaround time. The older the transceiver, the more likely that the switching is performed mechanically by some kind of relay. If a separate transmitter and receiver are used with one antenna, there is also likely to be a mechanical relay performing the switch. In addition, if an external power amplifier and/or receive preamplifier is used, more mechanical switching is likely to be involved.

With newer equipment, the switching is more likely to be accomplished electronically. This speeds up the process, but this improvement may be compromised by the frequency synthesizer circuitry that is used by the newer RF equipment. After switching between the transmit and receive modes, all synthesizers require some time to lock on frequency before they are ready to transmit or receive. Older RF equipment does not use frequency synthesis and does not have this delay. Some new equipment is being designed with packet radio in mind and synthesizers that can lock more quickly are now being offered. If you are out hunting for a new transceiver for packet-radio applications, keep this feature in mind.

Another problem cited by TAPR is that the modem-to-radio interface of most of the radios used for packet radio depends on audio response filters and audio levels intended for microphones and speakers. More often than not, this leads to incorrect deviation of the transmitted signal, noise and hum on the audio, etc. Splatter filters and deviation limiters distort frequency response and further reduce the performance of the packet-radio system. You are stuck in this environment unless you want to modify the radio. The problem is that trying to perform surgery on your typical VHF/UHF FM voice transceiver is difficult to impossible because of the use of LSI, surface mounting and miniaturization.

Instead of using an average Amateur Radio for packet radio (and the compromises this involves), there are alternatives that solve many of the RF equipment problems that have been discussed. The solution comes in the guise of high-speed RF equipment that is optimized for packet-radio operation.

Physical Connection

The actual physical installation of a packet-radio station is straight-forward. Basically, you connect your TNC to the terminal equipment and to the RF equipment. Figure that follows shows a typical installation.




TNC-to-Terminal Connection

Most TNCs are designed to be connected to terminal equipment by means of a serial port that is compatible with EIA standards RS-232-C or EIA-232-D. In most cases, the omnipresent 25-pin subminiature D-type connector provides the actual physical connection to the TNC. If your terminal or computer also has an EIA-compatible interface, the connection is accomplished by means of a 25-wire cable with the appropriate 25-pin connectors at each end. Appropriate, in this case, refers to connector gender. According to industry standards, a DTE has a male connector on its serial port while a DCE has a female connector. The DTE in this case is your terminal or computer and the DCE is your TNC. This means that the appropriate 25-wire cable would have a male 25-pin connector at the TNC end and a female 25-pin connector at the terminal end. This standard is not always adhered to, however.

Cables containing 25 wires are expensive. As the sidebar "EIA RS-232-C/EIA-232-D" indicates, all 25 pins of the interface are not used by your TNC. Interconnections between pins 2, 3, 4, 5, 6, 7, 8 and 20 will be adequate for almost all applications, including telephone-line data communications. An 8-wire cable is certainly less expensive than a 25-wire cable.

TNC-to-Radio Connection

In most cases, the connection of a TNC to RF equipment is a matter of making a few simple connections.

The audio output of the TNC is connected to the audio input of your transmitter/transceiver. Typically, the audio input of your radio equipment is a microphone connector, but some transceivers have separate audio inputs for AFSK tones. If such a connection is available, it's better to use that connection rather than the microphone input, because it means you will not have to disconnect the packet-radio equipment when you want to use the radio in the voice mode. In addition, the transmitter/transceiver may have circuitry that processes the AFSK input signals in some way, and such processing would probably be beneficial to your packet-radio signal as well.

The push-to-talk (PTT) line from your TNC is connected to a PTT connection on your transmitter/transceiver. Usually, PTT is available at the radio's microphone connector, but the PTT line is sometimes brought out to another jack as well. Again, connection to the optional PTT jack is preferable; this avoids cable changes when you switch modes.

The audio output of your receiver/transceiver is connected to the audio input of your TNC. Typically, the audio output of your radio is a speaker or headphone connector, but some radios have optional audio outputs (sometimes labeled "AFSK out"). Again, connection to such an optional audio output avoids cable changes, and the receiver/transceiver may provide some filtering or processing of the AFSK output signals.

In addition to these three connections, there must be a ground connection between your TNC and the transceiver (or transmitter and receiver).

Some transmitters/transceivers, typically VHF and UHF hand-held transceivers, use a common conductor for audio input and PTT. Simply connecting the TNC's audio output and PTT leads to the common conductor on the radio will not work. To make the connection successfully, a capacitor and resistor are required, as shown in the wiring diagram.