: p.19 Later 3270s had an Attention key, a Cursor Select key, a System Request key, twenty-four PF keys and three PA keys. The operator console keyboard had twelve PF keys and two PA keys. The data entry keyboard had five PF keys and two PA keys. Both versions had two Program Attention (PA) keys. The typewriter keyboard came in both a 66 key version, with no programmed function (PF) keys, and a 78 key version with twelve. This is mainly used for debugging purposes to preserve the application program screen contents while replacing it, temporarily, with debugging information.Įarly 3270s offered three types of keyboards. There is also a read buffer capability that transfers the entire content of the 3270- screen buffer including field attributes. Some users familiar with character interrupt-driven terminal interfaces find this technique unusual. This technique enhances the terminal throughput of the CPU, and minimizes the data transmitted. Using a technique known as read modified, a single transmission back to the mainframe can contain the changes from any number of formatted fields that have been modified, but without sending any unmodified fields or static data. 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 m (4,900 ft) are supported, although IBM documents routinely stated the maximum supported coax cable length was 2,000 ft (610 m). Data is sent with a bit rate of 2.3587 Mbit/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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