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.)