IBM 5250 was originally a family of terminal devices sold with the IBM System/34 minicomputer systems. One model was the IBM 5251-11. It also connected to the later System/36, System/38 and AS/400 systems.
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Similar to the IBM 3270, it is a block-oriented terminal protocol, yet is incompatible with the 3270 standard. The relationship between the terminal and system was rich: The system sent a data stream containing markup bytes indicating the boundaries of data entry fields, highlighted or colored areas, and indication that the system was busy responding to the last request from the terminal. The 5250 data stream definition has been refined over time to include GUI elements like pop-up windowing, check and option boxes, mouse handling, and pull-down menus.
The term "5250" now refers to the content of the data stream itself; few physical 5250 terminals (with their bulky Twinax cables) still exist, although they are still used to provide a "connection of last resort," hard-wired to the host computer. Robustly constructed, 5250 terminals weighed roughly 36 kg (80 lbs.) The 5250 terminals generated an audible clicking sound as the user typed, similar to the electric typewriters of the era. Today, it is more common to use PC or web-based terminal emulation packages that can interpret and display 5250 data streams.
Telnet 5250, or TN5250 describes either the process of sending and receiving 5250 data streams using the Telnet protocol or the software that emulates a 5250 class terminal which communicates using that process. TN5250 allows a 5250 terminal emulator to communicate over a TCP/IP network instead of an SNA network. Standard telnet clients cannot be used as a substitute for TN5250 clients, as they use fundamentally different techniques for exchanging data.