IN Taejon, midway between Seoul and Pusan, the finishing touches are being put on the largest specialised exposition ever held. With more than 100 countries and international organisations participating, the Koreans hope Taejon Expo '93 will demonstrate the strides the country has made in the hi-tech field over the past decade. The theme of the expo is the ''Challenge of a New Road to Development''. In Samsung group's Starquest pavilion, the visitor enters into a ''cosmic voyage'', shooting through from a futuristic space centre to an outdoor ''planet park'' styled after the earth's solar system. The Daewoo group has erected an extraordinary state-of-the-art theatre to focus on how human life prospers through the power of science and technology. Many state-of-the-art products, including South Korea's largest television set and advanced computers, will be available for testing. Korea Telecom's pavilion will have an inside track to show how telecommunications can remove international barriers, including an international videophone. Taejon Expo '93 runs from August 7 to November 7.