DX3270 — Free TN3270 & TN5250 Terminal Emulator for macOS

A native macOS (ARM - Apple Silicon + Intel) TN3270E and TN5250 terminal emulator for connecting to IBM Mainframes (z/OS, z/VM, z/VSE) and IBM i (AS/400, iSeries) — from the same app, with the same native rendering pipeline.
Built entirely in C++ and Objective-C++ on top of native Cocoa, CoreText and OpenSSL.
No license fee. No Java. No X11.

For more informations about sponsoring and supporting the project, see the Support section below.

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Why this exists

If you work with IBM Mainframes on a Mac, you've probably noticed that every halfway-decent TN3270 terminal client costs money — sometimes a lot of money. We're talking $50–$100+ for software that essentially emulates a 1970s text terminal. One popular commercial option charges a recurring subscription just to type on a green screen. That's absurd.

There are a handful of free alternatives, but they either require Java (slow, ugly, a security nightmare), run inside X11 (no thanks), or are abandonware that hasn't been touched in a decade and breaks on every macOS release.

So I built one from scratch. Native Cocoa. Native CoreText rendering. OpenSSL for TLS. Full TN3270E negotiation including ISPF Query Reply so the menus actually appear. It took a weekend of frustration and a lot of reading ancient IBM manuals — but the result is a clean, fast, free terminal that feels like it belongs on a Mac.

If you work in Mainframe or IBM i and you're tired of paying for the privilege, this is for you.

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Screenshot

ISPF 8.1 Primary Option Menu on z/OS — connected to IBM ZExplore mainframe at 204.90.115.200:623

Two sessions side by side — z/OS on the left, IBM i on the right. Both running ISPF Primary Option Menu.

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TN5250 — IBM i / AS400 Support

Version 1.7.0 adds full TN5250 protocol support alongside the existing TN3270E engine. One app — two green screens.

Select TN5250 from the Protocol drop-down in the Connect dialog, point it at your IBM i (AS/400) host on port 23 (or your configured port), and you are in. The IBM i logon screen appears in the same Cocoa window you already know from 3270.

What works

Capability	Details
GDS framing	10-byte GDS record header (length, class 0x12, type 0xA0, flags, opcode) decoded for every inbound record
Write To Display (WTD)	Full command parser: SBA (Set Buffer Address), SF (Start Field with FFW1/FFW2/FCW pairs), RA (Repeat to Address), inline attribute bytes
Field attributes	FFW1 bits mapped to 3270-style protection/MDT flags; inline attribute bytes (0x20–0x3F) create coloured sub-field markers without clobbering the governing input-field definition
IBM 5250 colour table	Full 32-entry IBM SA21-9247 attribute table: Green / White / Red / Turquoise / Yellow / Pink / Blue with Reverse, Underline, Blink, NonDisplay, and Column-Separator modifiers
Non-display fields	Password and other non-display fields (attr 0x27/0x2F/0x37/0x3F) rendered as blanks — characters are stored internally and transmitted correctly on Enter
Cursor & navigation	Home, Tab/BackTab between unprotected input fields (inline attribute sub-fields correctly skipped), arrow keys, Escape = Attention
AID transmission	SBA + data record sent for every modified unprotected field (MDT=1) on Enter; PF1–PF24, PA1–PA3
Keyboard lock	System lock while host processes input; OErr lock when typing in a protected field or past field-end; Escape resets
EBCDIC codec	Shared codec with the 3270 engine — CP037/CP500/CP1047 all work
TLS	Same OpenSSL transport layer as 3270 — connect encrypted with the same CA-bundle workflow

3270 not broken

The two protocol engines are completely separate (DataStream5250Parser / KeyboardState5250 vs. DataStreamParser / KeyboardState). Switching protocol in the Connect dialog tears down and re-creates the engine; the rendering path is shared. No 3270 regression.

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GDDM / GOCA Graphics

Version 1.6.0 adds full support for GDDM vector graphics sent by applications running on VM/CMS or z/OS. Charts, diagrams and topology maps are rendered as a CoreGraphics overlay on top of the text screen — no plugin, no browser, no Java.

GOCA View 1/4 — Monthly CPU Utilisation (%). Vertical bars coloured by threshold: green < 60 %, yellow 60–79 %, red ≥ 80 %. Axes, tick marks and value labels all rendered as GOCA vector objects.

GOCA View 3/4 — Throughput vs Response Time scatter plot. Coloured FULLARC dots, blue FILRECT grid lines, and a red trend line. All 25 data points decoded from a single GOCA Write Graphics Object structured field.

Supported GOCA orders: FILRECT · FULLARC · CGPOS / CLCS · SCOL · SMIX · BSEG / ESEG · LNPOS / LNAT.
Coordinate space: AW = 9, AH = 12 GOCA units per character cell; Y = 0 at the top-left of the text area, increasing downward (flipped for Cocoa internally).
IBM 3279 palette colours (0xF1–0xF7) map to the same CoreGraphics colours used for text attributes.

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IBM 3270 Font

DX3270 ships with the authentic IBM 3270 terminal font by Ricardo Bánffy, bundled directly in the app. It is off by default so the familiar Menlo monospace is used out of the box.

TSO/E Logon screen rendered with the IBM 3270 font — notice the characteristic terminal typeface.

How to activate

1. Open DX3270 → Preferences (⌘,)
2. Check "Use IBM 3270 font (by Ricardo Bánffy)"
3. All open terminal windows switch instantly — no reconnect needed

The setting is saved and restored on every launch.

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Features

Feature	Details
Protocol	TN3270E (RFC 2355) with automatic fallback to classic TN3270 and TN5250 for IBM i — selectable per connection
Security	Plain Telnet and implicit TLS (TLS 1.2+) on any port
Screen models	Model 2 (24 × 80) · Model 3 (32 × 80) · Model 4 (43 × 80) · Model 5 (27 × 132) · Large custom (62 × 160) — selectable per connection
EBCDIC code pages	CP037 (US), CP500 (International), CP1047 (Open Systems)
5250 colours	Full IBM SA21-9247 32-entry attribute table: Green, White, Red, Turquoise, Yellow, Pink, Blue + Reverse/Underline/Blink/NonDisplay/ColSep modifiers
5250 fields	FFW-based field protection, inline attribute bytes, non-display password masking, MDT tracking, Tab/BackTab navigation
UI	Native Cocoa window, green-on-black phosphor, 600 ms cursor blink
Keyboard	PF1–PF24, PA1–PA3, Clear, Reset, Tab/BackTab, ErEOF, Insert, arrows
Query Reply	Responds to IBM Structured Field Read Partition Query (required for ISPF); advertises GOCA graphics capability
GDDM / GOCA graphics	Full GOCA order-stream decoder: filled rectangles, full arcs (circles), absolute and relative line sequences, character strings at absolute position, set-colour, set-mix, and segment boundaries. Rendered as a CoreGraphics vector overlay on top of the text layer. Coordinate space AW=9/AH=12 units per cell, Y-flipped for Cocoa. IBM 3279 palette (0xF1–0xF7) colours. See GDDM / GOCA Graphics.
Rendering	CoreText glyph metrics for pixel-perfect character grid; CoreGraphics vector overlay for GOCA graphics
App icon	Native macOS squircle icon — white gradient, bold DX3270 lettering with green terminal cursor, bundled as AppIcon.icns
Shortcuts reference	Built-in keyboard shortcuts window — DX3270 → Keyboard Shortcuts… (⌘/)
Screenshot	Save the terminal screen as a PNG image (File → Save Screenshot… ⌘⇧P)
Text export	Export the screen content as a formatted UTF-8 text file (File → Export as Text… ⌘⇧T)
macOS	12 Monterey and later (Apple Silicon + Intel)

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Download

Pre-built DMG releases are available on the Releases page. Every push to main automatically builds and publishes two DMGs via GitHub Actions — one for each architecture.

DMG	For
DX3270-<version>-build<N>.dmg	Apple Silicon Macs (M1/M2/M3/M4, 2020 and later)
DX3270-<version>-build<N>-Intel.dmg	Intel Macs (2019 and earlier)

1. Download the DMG that matches your Mac
2. Open the DMG and drag DX3270.app to your /Applications folder
3. On first launch macOS will block the app because it is unsigned — see below

First-launch: bypassing macOS Gatekeeper

DX3270 is currently unsigned (a Developer ID certificate is planned for a future release once the remaining bugs are ironed out). macOS will refuse to open it directly. There are two ways around this:

Option A — System Settings (GUI)

1. Try to open DX3270.app — dismiss the "damaged or can't be opened" alert
2. Open System Settings → Privacy & Security
3. Scroll down — a banner appears: "DX3270 was blocked because it is not from an identified developer"
4. Click Open Anyway
5. Confirm in the following dialog — the app launches and macOS remembers the choice

Option B — Terminal (one-time command)

sudo xattr -r -d com.apple.quarantine /Applications/DX3270.app

This strips the quarantine flag that triggers Gatekeeper. After running it, DX3270 opens normally from Finder or the Dock without any further prompts.

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Connecting to a Mainframe

1. Launch DX3270 — the Connect dialog opens automatically
2. Fill in:

   Field	Example
   Host	204.90.115.200
   Port	623 (plain) · 992 (TLS) · 23 (standard Telnet)
   SSL/TLS	check for encrypted connections
   CA Bundle	path to a PEM file if using a private CA (optional)
   Code Page	CP037 (US default) · CP500 · CP1047

3. Click Connect

The terminal window opens. Type your credentials at the logon screen. ISPF and TSO sessions are fully supported.

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Keyboard Map

Key	3270 Function
F1–F12	PF1–PF12
Shift+F1–F12	PF13–PF24
Option+1/2/3	PA1 / PA2 / PA3
Return	Enter (AID)
Escape	Reset (unlock keyboard)
Option+Escape	Clear screen
Tab / Alt+Tab	Next / previous field
Insert	Toggle insert mode
Option+Delete	Erase to End of Field
Option+E	Erase Input (all unprotected fields)
↑ ↓ ← →	Cursor movement

Application shortcuts

Shortcut	Action
⌘N	New Connection
⌘,	Preferences
⌘⇧P	Save Screenshot
⌘⇧T	Export Screen as Text
⌘⇧D	Traffic Monitor
⌘/	Keyboard Shortcuts window
⌘Q	Quit DX3270

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Building from Source

Prerequisites

# Xcode Command Line Tools
xcode-select --install

# Homebrew + OpenSSL + CMake
brew install openssl@3 cmake

Build (Apple Silicon)

git clone https://github.com/el-dockerr/X3270.git
cd X3270
cmake -B build -DCMAKE_BUILD_TYPE=Release
cmake --build build
open build/DX3270.app

To set an explicit build number (useful in CI):

cmake -B build -DCMAKE_BUILD_TYPE=Release -DBUILD_NUMBER=42
cmake --build build

Package DMGs for distribution

Apple Silicon only:

./package.sh
# produces: dist/DX3270-1.5.0-build1.dmg

Intel only (cross-compiled from Apple Silicon — see prerequisites below):

./package_intel.sh
# produces: dist/DX3270-1.5.0-build1-Intel.dmg

Both architectures in one step:

BUILD_NUMBER=42 ./package_all.sh
# produces: dist/DX3270-1.5.0-build42.dmg
#           dist/DX3270-1.5.0-build42-Intel.dmg

Cross-compiling for Intel from Apple Silicon

Apple's Clang toolchain supports cross-compilation natively. The only prerequisite is an x86_64 OpenSSL library, which lives in the Intel Homebrew at /usr/local.

One-time setup:

# 1. Install Rosetta 2
softwareupdate --install-rosetta --agree-to-license

# 2. Install the x86_64 Homebrew (runs under Rosetta)
arch -x86_64 /bin/bash -c \
  "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"

# 3. Install x86_64 OpenSSL
arch -x86_64 /usr/local/bin/brew install openssl@3

Then run ./package_intel.sh or ./package_all.sh as shown above.

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Version History

v1.7.3 — 2026-06-02

Field navigation & Hercules / CP1047 bracket compatibility

• Fixed: Tab / Alt-Tab stopped on protected fields — KeyboardState::advanceToNextField skipped only auto-skip cells (protected + numeric), so Tab/BackTab landed on plain protected fields too. The 3270 keyboard now skips any field whose attribute has the FA_PROTECTED bit set, matching the existing KeyboardState5250 behaviour. Tab and Alt-Tab now move the cursor only between unprotected (input) fields.
• New: Hercules-style EBCDIC bracket compatibility option — Preferences → Compatibility → Display Hercules-style EBCDIC brackets as [ ]. When enabled, the EBCDIC codec maps inbound 0xAD / 0xBD (CP1047 native bracket bytes) and 0x4A / 0x5A to [ and ] on display, and outbound keystrokes for [ / ] are sent as 0xAD / 0xBD so they are stored natively on a CP1047 host. Wired through EbcdicCodec::setHerculesBrackets, kPrefHerculesBrackets (NSUserDefaults), and applied in both the TerminalView rendering codec and the TerminalWindowController engine codec; both observe NSUserDefaultsDidChangeNotification and update live.
• Note on ISPF EDIT under TK5 — ISPF EDIT scrubs [ and ] out of its data row before transmission because they fall outside its "displayable character set", emitting SBA + SF protected orders at those positions. The toggle does the right thing at the codec level (anything actually on the wire as 0xAD/0xBD/0x4A/0x5A renders as a bracket), but it cannot recover bytes the host never sends. Use BROWSE or HEX ON to confirm the file's stored bytes; compilers and other tools that read the file directly see the brackets correctly.

v1.7.2 — 2026-06-01

Keyboard fix

• Fixed: back-tab did not step to the previous input field — advanceToNextField walked the screen by a single position and stopped at the first field-attribute byte it hit. When the cursor sat at the first character of a field, the FA immediately before it was that field's own start byte, so the function set the cursor right back to where it already was. Both KeyboardState::advanceToNextField (3270) and KeyboardState5250::advanceToNextField (5250) now skip an FA whose successor equals the current cursor when scanning backward, so back-tab correctly lands on the first character of the previous unprotected field.
• Changed: back-tab keystroke is now Alt+Tab — Shift+Tab is intercepted by NSWindow for its built-in key-view focus cycling and cannot be bound reliably. Back-tab is now Alt+Tab (the ⌥ Option key), which is delivered to the view unmodified. The Keyboard Map and the in-app Keyboard Shortcuts window have been updated to match.
• Fixed: crash on closing a terminal window (-[TerminalView drawRect:] EXC_BAD_ACCESS) — TerminalView holds raw pointers into the ScreenBuffer, KeyboardState / KeyboardState5250, and GraphicsBuffer objects owned by TerminalWindowController via std::unique_ptr. When the window closed, the controller could be deallocated (and those engine objects destroyed) before AppKit issued its final drawRect: pass as part of the close transaction, leaving the view dereferencing freed memory inside _screen->at(...). windowWillClose: now detaches the view from all three engine pointers before any teardown, so the trailing draw safely sees _screen == nullptr and returns early.

v1.7.1 — 2026-06-01

Multi-session stability

• Fixed: crash when reconnecting after closing a terminal window — TerminalWindowController did not register itself as the window's delegate, so windowWillClose: never fired. The session and its read loop kept running in the background, and the next connect attempt (to the same or a different host) raced against the still-live controller, crashing the app. The controller now adopts <NSWindowDelegate> and disconnects cleanly on close.
• Fixed: dealloc running on the network thread — The detached read-loop thread held a strong self capture, so the controller's final release (and the AppKit teardown it triggers) could land on a background thread. The thread now hands its reference back to the main queue at exit.
• Fixed: stale terminal references in the Connect dialog — Closed terminal windows are now removed from ConnectionWindowController's session list via a new onClosed callback, and the spurious "Connection closed" error from the read loop's parting message no longer surfaces back to the connect screen when the close was user-initiated.
• Multiple concurrent sessions — With the lifecycle fixed, you can keep several terminal windows open against different hosts at the same time and open new ones from File → New Connection… (⌘N) without restarting the app.

v1.7.0 — 2026-05-31

Full TN5250 support — IBM i / AS400

• TN5250 session (TN5250Session, DataStream5250Parser, KeyboardState5250) — complete TN5250 engine added alongside the existing TN3270E engine. Both protocols share the same ScreenBuffer model and CoreText rendering pipeline; switching is a single drop-down in the Connect dialog.

• GDS record framing — 10-byte GDS header decoded; PUT_GET (0x03) and INVITE (0x01) opcodes immediately unlock the keyboard before processing stream content, matching IBM i behaviour.

• Write To Display parser — SBA, SF (with FFW1/FFW2/FCW pairs and 5250 display attribute byte), RA, inline attribute bytes (0x20–0x3F), and WTD command flags all handled.

• Full IBM colour table — All 32 entries of the IBM SA21-9247 5250 attribute table decoded: seven colours (Green, White, Red, Turquoise, Yellow, Pink, Blue) × modifiers (Normal, Reverse, Underline, Blink, NonDisplay, Column-Separator). Mapped to IBM 3279 CoreGraphics colours shared with the 3270 renderer.

• Non-display fields — Password fields (attribute 0x27/0x2F/0x37/0x3F NonDisplay modifier) render as blank space; characters are stored in the buffer and transmitted correctly on AID.

• Inline attribute byte handling — Inline attribute bytes within the data stream create sub-field colour markers without clobbering the governing SF field definition (including fieldLen and protection bits). This was the root cause of the password field being incorrectly classified as protected.

• AID transmission — Outbound record format: [row][col][AID][SBA+data per MDT field]; trailing nulls trimmed. Enter, PF1–PF24, PA1–PA3 all generate correct AID codes per IBM SA21-9247.

• Keyboard — Home (first unprotected field), Tab/BackTab (skips inline-attr sub-fields), arrow keys, Insert, Backspace/Delete, Erase Field (⌥E), Escape (Attention AID), Erase to End of Field.

• Keyboard lock — System lock while host is processing; OErr on protected-field or field-overflow entry; Escape resets. Status bar shows ERR indicator on OErr.

v1.6.0 — 2026-05-29

GDDM / GOCA graphics support

• GOCA Query Reply — The Structured Field Query Reply now advertises Data Streams support (type 0x84, stream 0x02 = GOCA), causing GDDM on VM/CMS to send vector-graphics structured fields instead of falling back to text-only mode.

• GraphicsBuffer — A new GraphicsBuffer data model accumulates decoded GOCA drawing commands (GocaMoveTo, GocaLineTo, GocaArc, GocaFilledRect, GocaSetColor, GocaSetMix, GocaCharString, GocaBeginSegment) as a typed C++17 std::variant list, using the same dirty-flag/callback pattern as ScreenBuffer.

• GocaParser — A new stateful FSM (GocaParser) decodes the GOCA order byte stream inside Write Graphics Object structured fields. Supported orders: SPOS (set position), LNPOS / LNAT (line to absolute / relative), FULLARC (circle), FILRECT (filled rectangle), SCOL (set colour), SMIX (set mix/blend mode), CGPOS / CLCS (character strings at given / current position), BSEG / ESEG (segment boundaries). All unrecognised orders are safely skipped using the GOCA implied/explicit length table.

• DataStreamParser WSF routing — handleWSF() now routes GOCA-bearing structured fields: SF type 0x0D (Begin/Reset graphics) resets the parser; 0x0E (Write Graphics Object) feeds the GOCA payload to GocaParser; 0x0F (Erase Graphics) clears the buffer.

• CoreGraphics overlay in TerminalView — A new drawGraphicsOverlay: pass renders the GraphicsBuffer command list via Core Graphics after the text layer and before the OIA bar. GOCA coordinates (in device units matching the Usable Area QR cell dimensions AW=9, AH=12) are mapped to Cocoa pixel coordinates with Y-flip. IBM 3279 palette colours reuse the existing colorFor3270Code() function. EBCDIC character strings are decoded via EbcdicCodec and rendered with the current terminal font. Mix mode 0x04 (XOR) maps to kCGBlendModeXOR.

• No PC3270G/GX extensions — IBM PC3270 Graphics Adapter proprietary extensions are out of scope for this release.

v1.5.0 — 2026-05-28

App renamed to DX3270

• The app is now called DX3270 (Dockerr's X3270) across every visible surface — menus, window titles, DMG filenames, bundle identifier (com.dx3270.macos), OIA status bar, and default export filenames — to avoid confusion with the separate open-source x3270 project.

Native app icon

• A purpose-built macOS icon is now bundled as AppIcon.icns (all 10 iconset sizes, 16 px – 1024 px). Design: Apple squircle shape, white-to-light-indigo gradient background, bold DX3270 lettering in the system SF font with a green terminal-cursor block, and a TN3270 Terminal subtitle in secondary-label gray.

Keyboard Shortcuts window

• DX3270 → Keyboard Shortcuts… (⌘/) opens a floating, read-only reference window listing every terminal key and application shortcut, organised into four sections: Function Keys, Session Control, Navigation & Editing, and Application.

v1.4.0 — 2026-05-27

Screenshot and text export

• Save Screenshot — File → Save Screenshot… (⌘⇧P) captures the live terminal view as a pixel-perfect PNG image using NSBitmapImageRep and writes it to a user-chosen file. Useful for documenting session output or sharing screen content.
• Export as Text — File → Export as Text… (⌘⇧T) reads the current screen buffer, decodes every cell from EBCDIC to UTF-8, and saves a fixed-width plain-text file that preserves the exact column layout. Field-attribute positions are written as spaces so column alignment is maintained.
• Both actions are only enabled when an active session exists (validateMenuItem: guard).

Donation link in Connect dialog

• A ♥ Support this project link is now shown in the header of the Connect dialog, opening the Stripe donation page in the default browser. (No ads, no tracking, just a simple way to say thanks if you find the app useful. And since this app is not affiliated with IBM, there are no corporate sponsorships or licensing fees to worry about.)

v1.3.0 — 2026-05-25

Multi-model screen support

• Five screen models selectable per connection — Model 2 (24 × 80, default), Model 3 (32 × 80), Model 4 (43 × 80), Model 5 (27 × 132), and a non-standard Large model (62 × 160). The choice is presented as a Screen Model drop-down in the Connect dialog and is saved/restored per host in connection history.
• TN3270E DEVICE-TYPE negotiation — The negotiated terminal type string (IBM-3278-2-E through IBM-3278-5-E) now matches the selected model, so hosts that honor DEVICE-TYPE will configure the session to the correct size automatically.
• Dynamic Usable Area Query Reply — The structured-field Query Reply (0x80) now reports the actual grid dimensions and character cell sizes derived from the selected model, replacing the previous hard-coded 24 × 80 values. Hosts such as ISPF use this to determine wrapping and field layout.
• 14-bit buffer addressing — The 62 × 160 Large model requires 9 920 cells, exceeding the 12-bit address limit (4 095). encodeAddress / decodeAddress now transparently use 14-bit binary addressing for positions above 4 095, and the Query Reply advertises 14-bit capability accordingly (addrMode = 0x00).
• Dynamic screen buffer — ScreenBuffer internal storage is now a std::vector<Cell> sized to the selected model; the compile-time SIZE / ROWS / COLS constants have been removed in favour of instance methods size(), rows(), cols().
• Adaptive terminal window — TerminalView derives its cell grid and preferred window size from the attached ScreenBuffer, so the window grows or shrinks to fit the chosen model on every connection.

v1.2.0 — 2026-05-23

IBM 3270 font support

• Bundled IBM 3270 font — The authentic 3270font by Ricardo Bánffy is now shipped inside the app bundle (three variants: Regular, SemiCondensed, Condensed).
• Optional via Preferences — A new checkbox in DX3270 → Preferences ("Use IBM 3270 font") switches the terminal between the default Menlo font and the 3270 font at runtime. The setting persists across app launches.
• Live switching — Changing the preference immediately redraws all open terminal windows and resizes them to match the new cell dimensions.
• Attribution — The 3270font is the work of Ricardo Bánffy and contributors, released under the SIL Open Font License 1.1. See Acknowledgements below.

v1.1.0 — 2026-05-22

IBM 3279 color rendering

• Extended attribute support (SA / SFE) — Set Attribute (0x28) and Set Field Extended (0x29) structured fields are now fully parsed. Foreground color (type 0x42), background color (type 0x45), and highlighting (type 0x41) attributes are stored per cell and carried through startField / writeChar.
• IBM 3279 colour palette — Seven standard IBM colors rendered correctly: blue (0xF1), red (0xF2), pink (0xF3), green (0xF4), turquoise (0xF5), yellow (0xF6), white (0xF7). Default field color derived from the field attribute Protected/Numeric/MDT bits (green / red / blue / white quadrant).
• Intensified fields — Unprotected-intensified fields now render in red (IBM default) instead of white.
• Reverse video (highlight 0xF2) — Foreground and background swapped at render time.
• Underscore (highlight 0xF4) — 1 px bottom stroke drawn per cell.
• Per-cell background fill — Non-default cell backgrounds are filled before drawing the character.
• Fixed FA_DISP_LP constant — Was 0x08, corrected to 0x04 per IBM GA23-0059.

Keyboard / function key fixes

• Fixed PF10 / PF11 / PF12 AID codes — The emulator was sending 0xFA / 0xFB / 0xFC for these keys. The IBM GA23-0059 standard mandates 0x7A / 0x7B / 0x7C. ISPF (and all other 3270 hosts) do not recognise the wrong codes, so F12 (and F10/F11) had no effect. Fixed by replacing the broken 0xF0 + n arithmetic with the correct IBM lookup table. PF22–PF24 (Shift+F10/11/12) were similarly wrong (0xCA–CC → 0x4A–4C).
• Added performKeyEquivalent: override in TerminalView — macOS routes some function-key events through the key-equivalent path (menu shortcut resolution) rather than keyDown:, silently dropping them. The override mirrors the full PF1–PF24 / Shift+F1–F12 mapping so those events are consumed by the terminal regardless of which path the OS uses.

Cursor and OIA improvements

• Block cursor — Replaced the thin underline cursor with a full block cursor (cell filled with cursor color, character re-drawn in background color). The cursor is now visible at all times including when the keyboard is locked.
• OIA layout — Version string moved to the lower OIA row to avoid overlapping the status and cursor-position indicators.

Connect dialog — connection history

• The Host field is now an editable drop-down combo box. Every successful connection is saved to a history list (up to 20 entries, most recent first, deduplicated by host:port).
• Selecting a previous entry from the list automatically restores the paired Port, SSL/TLS, CA Bundle, and Code Page settings — no need to re-enter them.
• History is persisted in NSUserDefaults across app launches.

v1.0.3 — 2026-05-22

ISPF / 3270 data stream fixes

• Fixed: ISPF screen input error code 23 on protected fields — Read Modified responses were including protected fields that had MDT=1 (host-written output fields). Per IBM GA23-0059, Read Modified must return only unprotected (input) fields; sending protected field data back caused ISPF to reject the input with error code 23. Fix: getModifiedFields() now skips any FA cell with the Protected bit set.
• Fixed: Read Modified All now correctly returns all MDT fields — CMD_READ_MODIFIED_ALL (0x0E/0x6E) was handled identically to CMD_READ_MODIFIED, so the protected-field filter was incorrectly applied to host-solicited "all fields" polls as well. Per spec, Read Modified All must include both protected and unprotected modified fields. The two commands are now handled separately; buildReadModifiedRecord accepts an includeProtected flag.

v1.0.2 — 2026-05-19

Traffic Monitor panel

• New floating Traffic Monitor window (Debug → Traffic Monitor, ⌘⇧D) showing all raw inbound and outbound Telnet/TN3270 bytes as a colour-coded hex dump (TX blue, RX green) with timestamps, byte counts, and a printable ASCII column.
• Clear button wipes the log; Save to File… exports the full session as plain text.
• Captures traffic from the moment a connection is initiated so the full negotiation is always visible.

TN3270 / z/VM protocol fixes

• Fixed: z/VM stuck at NVT "PRESS BREAK KEY TO BEGIN SESSION" — The client was proactively sending WILL BINARY, DO BINARY, WILL EOR, DO EOR during the opening handshake. z/VM responds with DONT BINARY / DONT EOR, which per RFC 854 permanently disables those options for the session. z/VM then committed to NVT mode and never offered 3270 negotiation. Fix: remove proactive BINARY/EOR offers from connect(); let the server drive binary/EOR negotiation after the terminal-type exchange.
• Fixed: Duplicate WILL TN3270E confusing TN3270E-capable servers — When the server confirmed our initial WILL TN3270E by echoing DO TN3270E, the response handler was sending a second WILL TN3270E, causing some servers to reject TN3270E entirely. Fix: added sentWillTN3270E_ / sentDoTN3270E_ guards (same pattern as the existing sentWillBinary_ guards).

Other fixes

• WILL TN3270E server offer not handled → fixed
• enterDataMode() guard was too strict (required willBinary_/willEOR_) → fixed
• Write command reset buffer address to 0 → fixed
• FUNCTIONS REJECT from server not handled → fixed
• Keyboard locked permanently after a failed AID send (SendRecordCallback now returns bool) → fixed
• Keyboard started unlocked instead of locked-while-connecting → fixed (LockReason::Connecting initial state)
• DEVICE-TYPE REJECT did not call enterDataMode() → fixed
• Duplicate WILL/DO for BINARY/EOR during re-negotiation → fixed (sentXxx_ flags)
• TERMINAL-TYPE SEND sub-negotiation did not set doTermType_ → fixed
• Query Reply was missing Colour and Highlighting structured fields (required for ISPF menus) → fixed

v1.0.1 — 2026-05-13

Initial public release — basic TN3270E support, TLS support, ISPF Query Reply support, CoreText rendering, keyboard input, and a simple Connect dialog.

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Support this project

DX3270 is free, open-source software — no license fee, no subscription, no Java.
If you find it useful, any support is genuinely appreciated and helps keep the project going.

Individual support

A one-time donation via the Stripe link above is the simplest way to say thanks.

Corporate sponsorship

If your company relies on DX3270 for day-to-day mainframe or IBM i work, consider becoming a corporate sponsor. Sponsorship helps fund ongoing development, bug fixes, new features, and long-term maintenance.

Interested? Reach out by email: kalski.swen@gmail.com

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Acknowledgements

IBM 3270 Terminal Font
The optional terminal font bundled with DX3270 is 3270font, designed and maintained by Ricardo Bánffy and contributors.
It is derived from the classic x3270 bitmap font, redrawn as a modern vector typeface in OTF/TTF format.
The font is distributed under the SIL Open Font License, Version 1.1 — see LICENSE.txt in the upstream repository.
Many thanks to Ricardo and all contributors to that project for their meticulous work keeping this piece of mainframe history alive.

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License

DX3270 for macOS is released under the MIT License.
See LICENSE for the full text.

Written by Swen Kalski, 2026.

IBM, z/OS, ISPF, and 3270 are trademarks of IBM Corporation.
This project is not affiliated with or endorsed by IBM.