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IBM introduced the 3270 family of block mode terminals in 1971, for use with IBM mainframes (System/370 and successors). Then, in 1977, IBM introduced the 5250 family of block mode terminals for use with System/34 minicomputers, which was also used by the System/38 (1978), System/36 (1983), and AS/400 (1988).

The 5250 terminal protocol is incompatible with 3270, although obviously inspired by it.

My question is, why did IBM introduce a new incompatible range of terminals for System/34 rather than just using the existing 3270 terminals? (I can imagine that political infighting and NIH syndrome between different IBM divisions could have had a role to play in it, but that's just speculation, and I was hoping someone might have a more concrete knowledge of the history of this.)

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    (No time for a full answer) Beside the obvious intend to keep markets seperated, the 5250 block mode does carry a wider variety of input control (field types, cursor handling, mandatory fields etc.) incompatible with 3270 block mode. Squeezing this into 3270 mode would end up as a secondary protocol anyway, as there was no room for a compatible extension in coding, as well as in host handling. – Raffzahn Apr 11 at 7:31
  • I too am inclined to the answer being "because it's better". Older protocols get replaced. – another-dave Apr 11 at 12:14
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    New features don't require a new protocol. New features have been added to 3270 through new orders, for example introduction of the "Start Field Extended" (SFE) order to enable setting many more field attributes than the original "Start Field" (SF) order supported. Mandatory fields, for example, are supported in extended 3270 through field validation extended attributes set by SFE. SFE itself is extensible since new extended attributes can be defined. – Simon Kissane Apr 11 at 23:38
  • Don't forget that IBM was in business to sell stuff, and they sold stuff to major companies that had lots of money. The desire of companies to get you on an upgrade cycle was just as big then as it is today - just that the timeframe of the upgrade cycle was longer back then. – davidbak Jul 5 at 20:54
  • I thought most of iBM 's revenue came from leases. Or was that only for big iron¿ – Walter Mitty Jul 6 at 0:44
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I've used both 5250 and 3270 terminals and the main differences were the twinax cabling and a little local smarts that allowed the 5250 to download a screen format form that could perform some data validations in the terminal

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    Are those "reasons" rather than incidental differences? – another-dave Jul 5 at 15:44

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