AID – Attention Identifier.  3270 terminals buffer all keystrokes at the client system until an Attention Identifier (AID) key is pressed.  AID keys include Enter, Clear, PF1 - PF24 and PA1 - PA3.  These keys provide commands to the host system.

ASCII – American Standard Code for Information Interchange; the most common format for text files in personal computers and on the Internet.  In an ASCII file, each alphabetic, numeric, or special character is represented with a 7-bit binary number (a string of seven 0s or 1s).  128 possible characters are defined.  ASCII was developed by the American National Standards Institute (ANSI).

Block Mode – An application setting that limits copy, paste and cut operations to a block of data independent of beginning or end of lines.

Code Page – The collection of graphical characters required to support a specific language..

COM Automation - The Component Object Model is a specification and supporting systems software that enables the creation of software components that are programming-language independent and location transparent.

DDE – Dynamic Data Exchange was originally called Dynamic Data Linking, or DDL.  It is a Microsoft Windows 3 hotlink protocol that allows application programs to communicate using a client-server model.

DNS - The domain name system (DNS) is the way that Internet domain names are located and translated into Internet Protocol addresses.  A domain name is a meaningful and easy-to-remember "handle" for an Internet address.  Because maintaining a central list of domain name/IP address correspondences would be impractical, the lists of domain names and IP addresses are distributed throughout the Internet in a hierarchy of authority.  There is probably a DNS server within close geographic proximity to your access provider that maps the domain names in your Internet requests or forwards them to other servers in the Internet.

EBCDIC – EBCDIC is an extension to 8 bits of BCDIC (Binary Coded Decimal Interchange Code), an earlier 6-bit character set used on IBM computers. EBCDIC is used on IBM mainframe systems.

Entry Assist – A set of functions for data entry that make a session with an IBM host act like a typewriter.  You can set margins, tabs, wrap-around, end-of-line signal, etc.

GDDM - GDDM (Graphical Data Display Manager) is a computer graphics system for the IBM System/370 which was developed in IBM's Hursley lab, and first released in 1979. GDDM was originally designed to provide programming support for the IBM 3279 colour display terminal and the associated 3287 colour printer. The 3279 was the first colour graphics terminal that could be used in a general business environment.

HLLAPI – Short for High Level Language Application Program Interface, an IBM API that allows a PC application to communicate with a mainframe computer.  HLLAPI requires a PC to run 3270 emulation software and then defines an interface between a PC application and the emulation software.

Host Name - A host or domain name locates an organization or other entity on the Internet.  An example of a host name would be  The name is a representation of the IP address of the host in a form that is simpler and easier to remember than the IP address itself, and is translated to an IP address using the Domain Name System (DNS).

IND$FILE – Developed in the early 1980s to transfer files between an IBM mainframe and a system emulating a 3270 terminal.  Variations of the IND$FILE program were developed for MVS/TSO, VM/CMS and CICS (both under MVS and DOS/VSE).

IP Address - In the most widely installed level of the Internet Protocol (IP) today, an IP address is a 32-bit number that identifies each sender or receiver of information that is sent in packets across the Internet.  An IP address has two parts: the identifier of a particular network on the Internet and an identifier of the particular device (which can be a server or a workstation) within that network.  On the Internet itself - that is, between the router that move packets from one point to another along the route - only the network part of the address is looked at.

IPv6 - Internet Protocol version 6 (IPv6) is a version of the Internet Protocol that is designed to succeed Internet Protocol version 4 (IPv4).  IPv6 was developed by the Internet Engineering Task Force (IETF) to deal with the long-anticipated IPv4 address exhaustion, and is described in Internet standard document RFC 2460.

Line Mode An application setting that allows copy, paste and cut transactions to be work on whole or partial lines of text, including the carriage return/line feed characters.

Logical Unit (LU) – In IBM's Systems Network Architecture (SNA), a logical unit (LU) is a port through which an end user accesses the SNA network in order to communicate with an application program.  This connection depends on a physical connection being established through associated physical units (PUs).  Typically, a logical unit is a unique connection to an application program. A physical unit is usually a hardware device, such as a terminal control unit.

Mainframe - Mainframe is an industry term for a large computer, typically manufactured by a large company such as IBM for large-scale computing purposes.  Historically, a mainframe is associated with centralized rather than distributed computing.

Null On IBM mainframes, blanks can be represented by either zeros (NULL) or blanks.  Even though the difference is not visible on the screen, it can be important for copying/pasting data.

Port - A port is a "logical connection place" and specifically, using the Internet's protocol, TCP/IP, the way a client program connects to a particular server program on a computer in a network.  Higher-level applications that use TCP/IP such as the Web protocol, Hypertext Transfer Protocol, have ports with preassigned numbers.  Other application processes are given port numbers dynamically for each connection.  Port numbers are from 0 to 65536. Ports 0 to 1024 are reserved for use by certain privileged services.  For the Telnet service, port 23 is defined as the default.

Programmed Symbols - A programmed symbol is a character on the 3270 device in which certain pixels are illuminated to produce a desired shape in a position (cell) on the display. A loadable programmed symbol set is a terminal character set that contains these application-defined programmed symbols.

Raster graphics - Raster graphics are computer graphics in which a display image is composed of an array of pixels arranged in rows and columns.

Session – A communications term that describes a live interactive connection between two network nodes, that in the case of 3270 emulation is the connection between  the PC and the 3270 host.  In this application, the session is also the unit around which all display and connection parameters are stored.

SNA – SNA is a proprietary IBM architecture and set of implementing products for network computing within an enterprise.  It existed prior to and became part of IBM's Systems Application Architecture (SAA).  SNA itself contains several functional layers and includes an application program interface called the Virtual Telecommunications Access Method (Virtual Telecommunications Access Method), a communications protocol for the exchange of control information and data, and a data link layer, Synchronous Data Link Control (SDLC).  SNA includes the concepts of nodes that can contain both physical units that provide certain setup functions and logical units, each associated with a particular network transaction.

SSL Security - The Secure Sockets Layer (SSL) is a commonly-used protocol for managing the security of a message transmission on the Internet.  SSL has recently been succeeded by Transport Layer Security (TLS), which is based on SSL.  SSL uses a program layer located between the Telnet Protocol and Transport Control Protocol (TCP) layers.  Developed by Netscape, SSL also gained the support of Microsoft and other Internet client/server developers as well and became the de facto standard until evolving into Transport Layer Security.  The "sockets" part of the term refers to the sockets method of passing data back and forth between a client and a server program in a network. SSL uses the public-and-private key encryption system from RSA, which also includes the use of a digital certificate. Click here for more detailed information on SSL security.

TCP/IP The Transmission Control Protocol/Internet Protocol is the basic communication language or protocol of the Internet.  It can also be used as a communications protocol in a private network (either an intranet or an extranet).  When you are set up with direct access to the Internet, your computer is provided with a copy of the TCP/IP program just as every other computer that you may send messages to or get information from also has a copy of TCP/IP.  TCP/IP is a two-layer program.  The higher layer, Transmission Control Protocol, manages the assembling of a message or file into smaller packets that are transmitted over the Internet and received by a TCP layer that reassembles the packets into the original message.  The lower layer, Internet Protocol, handles the address part of each packet so that it gets to the right destination.  Each gateway computer on the network checks this address to see where to forward the message.  Even though some packets from the same message are routed differently than others, they'll be reassembled at the destination.

Telnet – Telnet is a text-based way of connecting to other computers and networks.  It is one of the oldest forms of the Internet.  When you telnet to another computer, it is like you are using a terminal of that system.  Telnet operates in a client/server environment in which the computer you are using is the Client negotiates opening a session on another computer (the remote host or Server).

Terminal - In data communications, a terminal is any device that terminates one end (sender or receiver) of a communicated signal.  IBM's 3270 Information Display System was a widely-installed system of such terminals in corporations.  Many applications designed for the 3270 or other "dumb" terminals are still in use at PCs that emulate or act like a 3270.  The term is sometimes used to mean any personal computer or user workstation that is hooked up to a network.

Terminal Type – Terminal type refers to the type of display on the host you would like to use.  Each terminal type has different options, i.e. display size and the use of extended attributes.

TN3270 –A widely used protocol for encapsulating 3270 data streams in TCP sessions.

TN3270E (Enhanced) - A protocol that more fully supports 3270 devices than do traditional tn3270 practices.  Specifically, it defines a method of emulating both the terminal and printer members of the 3270 family of devices via Telnet; it provides for the ability of a Telnet client to request that it be assigned a specific device-name (also referred to as "LU name" or "network name"); finally, it adds support for a variety of functions such as the ATTN key, the SYSREQ key, and SNA response handling.

Triple Plane - A triple-plane symbol set is a programmed symbol set that possesses a portion of a symbol or the whole symbol defined in each of the primary color planes (red, blue, and green), allowing you to display or to print a whole symbol in one color, multiple colors, or a blend of colors.

Vector Graphics - Vector graphics is the use of geometrical primitives such as points, lines, curves, and shapes or polygons, which are all based on mathematical equations, to represent images in computer graphics.

VTAM - VTAM (Virtual Telecommunications Access Method) is an IBM application program interface (API) for communicating with telecommunication devices and their users.  VTAM was the first IBM program to allow programmers to deal with devices as "logical units" without having to understand the details of line protocol and device operation.  Prior to VTAM, programmers used IBM's Basic Telecommunications Access Method (BTAM) to communicate with devices that used the binary synchronous (BSC) and start-stop line protocols.