Editing IBM 3270 PC (section)

== Specialist Cards ==

The 3270 PC system unit is fundamentally an XT 5160 motherboard / chassis / PSU; the difference from a stock XT is in the cards that occupy nearly all eight ISA slots:

=== Keyboard Adapter Card ===

* Intercepts the '''122-key IBM 5271 Keyboard Element''' scancodes and translates them to XT scancodes.
* Two variants:
:: '''Simple''' (ROM '''6323581''')
:: '''Complicated''' (ROM '''6323582''') — adds NMI button and serial port.
* '''Critical: this card holds the video BIOS''' for the 3270 PC Display Adapter — the video cards do not work without the keyboard adapter card installed.<ref name="seasip">John Elliott, [http://www.seasip.info/VintagePC/5271.html The IBM 5271 (aka 3270 PC)], seasip.info. A community technical reference built from a working machine: the card complement and what each card does, the video BIOS living in the keyboard controller ROM (part 6323581 for the "simple" controller and 6323582 for the "complicated" one), the error codes 3201, 3202, 3203 and 2801, the 5272 display timings, and the statement that the case, motherboard, drives and power supply are all identical or very similar to the XT.</ref>
* On Models 024 and 026, which drive the 3295 Plasma Monitor, IBM fits a '''Plasma Keyboard Adapter''' in place of the standard one.<ref name="sim" />

=== 3270 PC Display Adapter ===

* Main video card, full-length 8-bit ISA.
* '''SCN2672 Programmable Video Timing Controller (PVTC)''' at I/O ports '''0180h–019Bh'''.
* '''8 KB framebuffer''' split: 4 KB for the 3270-emulation layer / 4 KB for an emulated CGA-or-MDA layer.
* Drives a unique '''720 × 350 mode''' on the IBM 5272 colour monitor.

=== All Points Addressable (APA) Card ===

* Optional add-on.
* Adds CGA-compatible 320 × 200 / 640 × 200 modes plus two 3270-PC-unique graphics modes.
* Required for graphics-capable mainframe applications.

=== Programmed Symbols (PSS) Card ===

* Optional add-on.
* '''RAM-loaded character sets''' (up to '''seven fonts''') for 3270 graphics applications.
* '''Three of the fonts (4, 5, 7) are tri-plane''' allowing 8-colour characters.

=== 3278/79 Host Connect / Emulation Adapter ===

* IBM calls this card the '''DFCA''' — Distributed Function Communications Adapter — and that is the name the diagnostics and the FRU tables use.<ref name="sim" /><ref name="mi3270">IBM, ''IBM 3270 Personal Computer Maintenance Information'', SY27-2567-2, third edition, October 1984 &mdash; hosted on this wiki as [[IBM 3270 PC Maintenance Information]]. The safety inspection checklist; the problem isolation charts, including the coaxial cable connection test and the coaxial cable resistance check; section 4, locations, including the system board diagram; section 5, removal, replacement and adjustments; and section 7, the illustrated parts catalogue for the 5271 system unit, the 5161 Expansion Unit, the 5151 Monochrome Display, the 5272 Color Display and the keyboard.</ref>
* '''Coaxial''' connection to an IBM '''3274''' (later 3174) control unit. '''It is coax, not twinax''' — twinax is the midrange 5250 attachment and will not work here — and the link is point to point, one control-unit port per workstation, with '''no terminator'''.<ref name="mi3270" /><ref name="sim" />
* Two variants exist:
:: Older — 4 KB buffer, I/O at '''2D0h–2D6h'''.
:: Newer — adds register at '''2D7h'''; reconfigurable buffer size.
* Technical reference: '''IBM 1502336''' — ''PC 3278/79 Emulation Adapter Technical Reference'' (October 1983), at <code>bitsavers.org/pdf/ibm/pc/cards/</code>.

[[File:IBM 5271 physical layout.svg|center|thumb|480px|IBM 5271 (3270 PC) physical layout diagram showing slot placement for the keyboard adapter, display adapter, host-connect card, and the optional APA / PSS expansion cards. (Image: Wikimedia Commons, public domain)]]