There are also character attributes associated with individual screen locations. A field attribute occupies a physical location on the screen that also determines the beginning and end of a field. The concept of formatting in these devices allows the screen to be divided into fields (clusters of contiguous character cells) for which numerous field attributes, e.g., color, highlighting, character set, protection from modification, can be set. In a data stream, both text and control (or formatting functions) are interspersed allowing an entire screen to be painted as a single output operation. Originally devices were equipped with BNC connectors, which later was replaced with special so-called DPC – Dual Purpose Connectors supporting the IBM Shielded twisted pair cabling system without the need for so-called red baluns. Cable runs of up to 1.500 meters are supported. Data is sent with a bit rate of 2.3587 Mb/s using a slightly modified differential Manchester encoding. IBM 3270 devices are connected to a 3299 multiplexer or to the cluster controller, e.g., 3271, 3272, 3274, 3174, using RG-62, 93 ohm, coax cables in a point to point configuration with one dedicated cable per terminal. Remote 3270 controllers are frequently multi-dropped, with multiple control units on a line. A remote control unit is connected to a communications line by a modem. A local control unit attaches directly to the channel of a nearby mainframe. Originally devices were connected to the control unit over coaxial cable later Token Ring, twisted pair, or Ethernet connections were available. Most 3270 devices are clustered, with one or more displays or printers connected to a control unit (the 32 included an integrated control unit).

Sample IBM 3270 cluster with one control unit connected to a printer and two displays By ensuring the CPU is not interrupted at every keystroke, a 1970s-era IBM 3033 mainframe fitted with only 16 MB of main memory was able to support up to 17,500 3270 terminals under CICS.

To do this, the 3270 was designed to minimize the amount of data transmitted, and minimize the frequency of interrupts to the mainframe. The main goal of the system was to maximize the number of terminals that could be used on a single mainframe. The 3270 series was designed to connect with mainframe computers, often at a remote location, using the technology then available in the early 1970s.

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