X3270 — Free TN3270 Terminal Emulator for macOS

A native macOS (ARM - Apple Silicon + Intel) TN3270/TN3270E terminal emulator for connecting to IBM Mainframes (z/OS, z/VM, z/VSE).
Built entirely in C++ and Objective-C++ on top of native Cocoa, CoreText and OpenSSL.
No license fee. No Java. No X11.

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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 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

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Features

Feature	Details
Protocol	TN3270E (RFC 2355) with automatic fallback to classic TN3270
Security	Plain Telnet and implicit TLS (TLS 1.2+) on any port
Screen models	IBM 3278 Model 2 (24 × 80)
EBCDIC code pages	CP037 (US), CP500 (International), CP1047 (Open Systems)
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)
Rendering	CoreText glyph metrics for pixel-perfect character grid
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 a new DMG via GitHub Actions.

1. Download X3270-<version>.dmg
2. Open the DMG and drag X3270.app to your /Applications folder
3. On first launch: right-click → Open (macOS Gatekeeper; the app is unsigned)

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Connecting to a Mainframe

1. Launch X3270 — 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 / Shift+Tab	Next / previous field
Insert	Toggle insert mode
Option+Delete	Erase to End of Field
Option+E	Erase Input (all unprotected fields)
↑ ↓ ← →	Cursor movement

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Building from Source

Prerequisites

# Xcode Command Line Tools
xcode-select --install

# Homebrew + OpenSSL
brew install openssl@3 cmake

Build

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

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

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

Package a DMG for distribution

./package.sh
# produces: dist/X3270-1.0.0-build1.dmg

Pass an optional build number:

BUILD_NUMBER=42 ./package.sh
# produces: dist/X3270-1.0.0-build42.dmg

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License

X3270 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.