Terminal Emulations

In the past, you could access information, programs, processing power and functions such as printing from a host using dumb terminals, which are so called because they have no processor; all the work was being done by the host computer (the mainframe or mini computer). However, as technology has become more advanced, terminals have become increasingly intelligent, that is, more and more processing is done by the terminal rather than by the host.

In 1983, the personal computer was introduced, which has, by and large, replaced the terminal. The PC contains a Central Processing Unit (CPU) which enables it to process information. It is designed to be used by one person and can work in standalone mode, which is not connected to a host, or it can be linked to a host via a network or serial connection to a modem.

In order for a terminal to communicate successfully with a host, the terminal must speak the same language as the host. Terminal emulation software such as HostAccess enables the PC to do this. It allows a PC to act like a particular brand of terminal, for example a VT100. The PC appears as a terminal to the host computer and accepts the same escape sequences for functions such as cursor positioning and clearing the screen.

A terminal emulator enables you to connect to one or more hosts across different networks using many different emulations. To enable your terminal emulator to connect to a host across a network, you need to have the correct protocol software loaded onto both your PC and the host. Protocol software, often called a stack, gives your PC the right information to be able to send data across a network. The host also needs to have protocol software to be able to interpret the data.

A protocol is a set of formal rules, rather like a language, which enables communications between hosts and PCs. TCP/IP is a common protocol used with HostAccess.

There are two basic classes of emulation supported by HostAccess. These are sometimes known as Block mode emulation and Asynchronous, or character mode emulations. A block mode emulation is where data is edited within the terminal before it is transferred as a block to the host. These emulations tend to be those designed to work with large mainframes, for example IBM 3270 and IBM 5250. A character mode emulation, also known as asynchronous mode, is where each character is sent to the host as it is keyed into the keyboard. That is, it sends one character at a time to the host. These emulations tend to be those designed for smaller, mid-range systems such as the DEC VT100, VT220, VT320 and VT420 emulations.

When you make a connection to a host from your PC using HostAccess, you open what is called a session. In HostAccess, you can create a new session, save a session, open an existing session, have more than one session running at once and switch between open sessions. This is much like opening a new file.

When you create a new session, you have to tell HostAccess which software protocol you have installed on your PC and the host, and which emulation you are using. For example, you may want to connect to a UNIX host using TCP/IP protocol software and a VT 100 emulation.