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Radio modems ( 19 ) :

General information

Data Radio Standard Test Methods Project - A project to establish Standard Test Methods for the evaluation of radios used in data communications especially aimed at Amateur Packet Radio.
Federal Standard 1052 - This standard is one of a series of standards pertaining to automatic high frequency (HF) radio equipment and operation. The basic standard is Federal Standard (FED-STD) 1045A, Telecommunications: HF Radio Automatic Link Establishment.
Introduction to Packet Radio - Packet has three great advantages over other digital modes: transparency, error correction, and automatic control. The operation of a packet station is transparent to the end user; connect to the other station, type in your message, and it is sent automatically. The data on acket radio system is usualy sent using radio modems. AX.25 (Amateur X.25) is the communications protocol used for packet radio.
Introduction to Packet Radio
Internet Gateways to Amateur Radio tcp/ip Servers
Selecting a Radio Modem Technology forIndustrial Automation Applications - Article from Instrumentation &Control Systems Magazine (July 1997).
The Protocol AX.25 - The name AX25 originates from the recommendation X.25 of CCITT, adding letter A that stands for Amateur; AX25 is therefore Amateur packet radio link layer protocol. The purpose of this protocol is to define the frame structure and to set the requirements of the station that sends or receives that frame or packet.
Using Radiometrix modules operating on 418 or 433.92MHz in North America under FCC Regulation Part 15, Section 231, Paragraph (e)
RFC 1226 - Internet protocol encapsulation of AX.25 frames This memo describes a method for the encapsulation of AX.25 (the Amateur Packet-Radio Link-Layer Protocol) frames within IP packets. This technique is an Experimental Protocol for the Internet community.

Radio modem project

Baycom Modem - A BAYCOM Packet Radio modem is the cheapest way to get on the air, here is the circuit diagram of it
Packet Radio Modem Schematics
Yet Another 9k6 Modem - multi-standard modem for packet radio using FPGA chip

Radio modem interfacing

Interfacing the TAPR 9600 bps Modem with the Ottawa PI2 Packet Interface Card
PSK31 & SSTV Interface Home Page - radio interfacing details
Radio Interface Schematics
Radio-TNC Wiring Diagrams Index - Here you can fidn wiring and pinout information for very many communications radios.

Soundcard to radio interfacing

Sound Card Interface with Tone Keyer - This is an improved version of the audio interface commonly used to connect a PC's soundcard to a transceiver's receive and transmit audio circuits for PSK31, SSTV, . The usual version of this type of interface (including the commercial "RigBlaster") requires the use of a serial port to provide PTT (push-to-talk) control for the radio's transmitter. This version includes an audio tone detector that keys the transmitter whenever transmit audio is generated by the application running on the PC.
Sound Card to Radio Interface Home Page