Editing IBM 3270 (section)

==== 3274 controller ====
IBM introduced the 3274 controller family in 1977, replacing the 3271–2 product line.

Where the features of the 3271–2 was hardcoded, the 3274 was controlled by its microcode that was read from the 3274's built-in [[8" floppy drive]].

3274 models included 8, 12, 16, and 32 port remote controllers and 32-port local channel attached units. In total 16 different models were over time released to the market. The 3274-1A was an SNA physical Unit type 2.0 (PU2.0), required only a single address on the channel for all 32 devices and was not compatible with the 3272. The 3274-1B and 3274-1D were compatible with the 3272 and were referred to as local non-SNA models.

The 3274 controllers introduced a new generation of the coax protocol, named Category A, to differentiate them from the Category B coax devices, such as the 3277 terminal and the 3284 printer. The first Category A coax devices were the 3278 and the first color terminal, the IBM 3279 Color Display Station.<ref>{{cite web |first=Ken |last=Shirriff |title=Inside an IBM/Motorola mainframe controller chip from 1981 |date=July 2024 |url=http://www.righto.com/2024/07/ibm-3274-keystone-chip.html}} </ref>

Enabling backward compatibility, it was possible to install coax boards, so-called 'panels', in groups of 4 or 8 supporting the now older Category B coax devices. A maximum of 16 Category B terminals could be supported, and only 8 if the controller were fully loaded with a maximum of 4 panels each supporting 8 Category A devices.

During its life span, the 3274 supported several features including:

* Extended Data Stream
* Extended Highlighting
* Programmed Symbol Set (PSS)
* V.24 interfaces with speed up to 14.4&nbsp;kbit/s
* V.35 interfaces with speed up to 56&nbsp;kbit/s
* X.25 network attachment
* DFT – Distributed Function Terminal
* DSL – Downstream load for 3290 and 3179G
* 9901 and 3299 multiplexer
* Entry Assist
* Dual Logic (the feature of having two sessions from a CUT mode display).