DX3270 — Free TN3270 & TN5250 Terminal Emulator for macOS

v1.7.3 — TN5250 / IBM i (AS/400) support

DX3270

TN3270E + TN5250 Terminal Emulator for macOS

Native Cocoa. CoreText rendering. Full TN3270E and TN5250 negotiation.

Connect to IBM Mainframes (z/OS, z/VM, z/VSE) and IBM i (AS/400) — for free.

Free & Open Source<br>TN3270E<br>TN5250 / IBM i<br>Apple Silicon<br>Intel<br>No Java<br>No X11<br>TLS 1.2+<br>ISPF / TSO<br>AS/400 Logon<br>GDDM Graphics

Download for Mac

View on GitHub

ISPF 8.1 Primary Option Menu — connected to IBM ZExplore at 204.90.115.200:623

Why DX3270

No Java.<br>No X11.<br>No license fee.

Every decent TN3270 or TN5250 client for macOS either costs $50–$100+, requires Java, or<br>hasn't been touched in a decade. DX3270 handles both protocols in one native app.

🍎

Truly Native

Built on Cocoa, CoreText, and CoreGraphics. Renders like a Mac app because it is a Mac app. Apple Silicon and Intel binaries.

🔒

TLS Out of the Box

OpenSSL powers implicit TLS (TLS 1.2+) on any port. Connect to z/OS, z/VM, z/VSE or IBM i — encrypted or plain Telnet.

🖥️

TN5250 — IBM i / AS400

Full TN5250 engine: GDS framing, WTD command, 32-entry IBM colour table, non-display password masking, field navigation, AID transmission. One app, two protocols.

📊

GDDM Vector Graphics

Full GOCA order-stream decoder renders charts, diagrams, and topology maps as CoreGraphics overlays — no plugin needed.

🆓

Free Forever

MIT license. Source on GitHub. No subscriptions, no dongles, no activation keys. Just a terminal emulator that works.

⌨️

Correct Protocol

Full TN3270E negotiation, proper AID key codes per IBM GA23-0059, Query Reply with colour/highlight structured fields. ISPF and AS/400 logon just work.

🖥️

Five Screen Models

Model 2–5 plus a non-standard 62×160 large model. Dynamic Usable Area Query Reply, 14-bit buffer addressing.

Technical Details

Every feature you need,<br>nothing you don't.

Feature<br>Details

ProtocolTN3270E (RFC 2355) with automatic fallback to classic TN3270 and TN5250 for IBM i — selectable per connection<br>SecurityPlain Telnet and implicit TLS (TLS 1.2+) on any port via OpenSSL<br>Screen ModelsModel 2 (24×80) · Model 3 (32×80) · Model 4 (43×80) · Model 5 (27×132) · Large (62×160)<br>EBCDICCP037 (US), CP500 (International), CP1047 (Open Systems)<br>5250 ColoursFull IBM SA21-9247 32-entry table — Green, White, Red, Turquoise, Yellow, Pink, Blue × Reverse, Underline, Blink, NonDisplay, ColSep modifiers<br>5250 FieldsFFW1/FFW2/FCW field attributes, inline attribute bytes, non-display password masking, MDT tracking, Tab/BackTab, field-length enforcement<br>3270 ColoursIBM 3279 seven-colour palette, extended attributes (SA/SFE), per-cell background fill, reverse video, underscore<br>GDDM/GOCAFILRECT · FULLARC · CGPOS/CLCS · SCOL · SMIX · BSEG/ESEG · LNPOS/LNAT — CoreGraphics vector overlay<br>KeyboardPF1–PF24, PA1–PA3, Clear, Reset, Tab/BackTab, ErEOF, Insert, arrows — correct IBM AID codes for both 3270 and 5250<br>FontMenlo (default) or bundled IBM 3270 font by Ricardo Bánffy (SIL OFL). Switchable live in Preferences.<br>Query ReplyRead Partition Query with Colour, Highlighting, and GOCA Data Streams structured fields<br>ExportSave Screenshot as PNG (⌘⇧P) · Export Screen as UTF-8 Text (⌘⇧T)<br>DiagnosticsTraffic Monitor — live hex/ASCII dump of all Telnet bytes (⌘⇧D)<br>Connection HistoryLast 20 hosts stored in NSUserDefaults; selects port, TLS, CA bundle, code page automatically<br>macOS12 Monterey and later · Apple Silicon (ARM64) + Intel (x86_64)

Screenshots

See it in action

ISPF 8.1 Primary Option Menu on z/OS

TN5250 — IBM i (AS/400) logon at pub400.com — full colour, password masking, field navigation

GOCA view — Monthly CPU Utilisation bar chart

GOCA view — Throughput vs Response Time scatter plot

Download

Pick your architecture.

Pre-built DMGs are published automatically on every push to main via GitHub Actions.

Apple Silicon · ARM64

DX3270 for M-series Macs

M1, M2, M3, M4 and later. Native ARM64 binary — instant launch, no Rosetta. TN3270E and TN5250 in one app.

Download DMG

Intel · x86_64

DX3270 for Intel Macs

2019 Macs and earlier. Cross-compiled from Apple Silicon using Clang's native x86_64 support. Full TN3270E and TN5250 support.

Download DMG (Intel)

First launch — macOS Gatekeeper<br>DX3270 is currently unsigned. After dragging to /Applications, open System Settings → Privacy & Security and click Open Anyway . Or run once in Terminal:

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

Getting Started

Connect in 60 seconds.

01

Download & Install

Download the DMG for your Mac architecture. Open it and drag DX3270.app to /Applications.

02

Bypass Gatekeeper (first time only)

macOS will block an unsigned app. Go to System Settings → Privacy & Security → Open Anyway , or run the xattr command above.

03

Open the Connect dialog — choose your protocol

Select TN3270E for IBM Mainframes (z/OS, z/VM, z/VSE) or TN5250 for IBM i (AS/400). IBM ZExplore is at 204.90.115.200:623; a free public IBM i is at...