IronTerm

Browser-side IBM 3270 terminal. No backend, no server-side code, no runtime build step - just static files served over HTTP. The 3270 datastream and TN3270E telnet negotiation are implemented in plain JavaScript modules running in the page; the page connects to the mainframe through any websockify-style TCP↔WebSocket relay.

Quick start

You need three things: this repo, a static HTTP server, and a WebSocket-to-TCP bridge that forwards binary frames to your mainframe.

1. Serve the static files

cd public
python3 -m http.server 8080
# → open http://localhost:8080/tn3270/

Any static server works; nothing in the project requires a build step.

2. Run a TCP↔WebSocket bridge

websockify is the simplest option. To expose a Hercules MVS turnkey listening on localhost:3270:

websockify --binary 6080 localhost:3270

For multi-target routing, websockify also supports a token file or you can put an nginx in front. The terminal sends binary as the WebSocket subprotocol; websockify accepts it by default.

Or skip running your own bridge - a public test instance of tk5-hercules (Hercules MVS 3.8j Turnkey 5) is up at:

wss://tk5.bencz.cc:6080

Drop that URL into the bridge field and connect - no setup needed. TLS is terminated at the bridge, so the page works fine when served over HTTPS. For testing only; don't use real credentials.

3. Configure the page

In the toolbar, set the bridge URL - for example one of:

ws://localhost:6080/                            (single backend)
wss://tk5.bencz.cc:6080/                        (public test server)
wss://relay.example.com/tcp?port={port}         ({port} is substituted)

Pick the model (3278-2 default), press Connect, and the OIA at the bottom turns green.

Screenshots

What works

Telnet / TN3270E (RFC 2355):

• BINARY, EOR, TERMINAL-TYPE option negotiation (RFC 1041)
• TN3270E DEVICE-TYPE / FUNCTIONS subnegotiation (BIND-IMAGE, RESPONSES, SYSREQ)
• 5-byte data-stream header on inbound and outbound records
• Outbound sequence numbers are unique and monotonically increasing (RFC 2355 §3.2)
• Dispatch by data-type - only 3270-DATA is fed to the parser; BIND-IMAGE / UNBIND / NVT-DATA / SSCP-LU-DATA are accepted and ignored.
• ALWAYS-RESPONSE / ERROR-RESPONSE handled correctly: positive RESPONSE on success, negative RESPONSE (with sense byte) when the parser rejects a record, both echoing the host's seq.
• IAC NOP keepalive after 120 s idle.

3270 datastream:

• Commands: W, EW, EWA, EAU, WSF, RB, RM, RMA (both EBCDIC- and CCW-encoded forms accepted)
• Orders: SBA, SF, SFE, SA, MF, IC, PT, RA, EUA, GE
• WCC bits: RESET-MDT, RESTORE-KEYBOARD, SOUND-ALARM, RESET-PARTITION, START-PRINTER
• 12-bit and 14-bit buffer addresses (auto-detected on input, always 12-bit on output)
• Cyclic field model - fields that wrap around the end of the buffer
• Field MDT tracking; modified-field replies use f.start + 1
• Validation attributes (mandatory-fill / mandatory-entry) - terminal refuses the AID and parks the cursor on the offending field, just like a physical 3278.
• Set Reply Mode SF (0x09) is honoured - Read Buffer responses adapt to field / extended-field / character mode with the requested attribute list.

Query Reply (sent in response to Read Partition Query):

Summary, Usable Area, Character Sets, Color, Highlight, Reply Modes, Implicit Partition.

File transfer - IND$FILE:

• Download (host → browser): user types IND$FILE GET dataset on TSO/CMS; the terminal collects the WSF data records (rectype 0x47, subtype 0x04), acknowledges each with the running buffer number, and on CLOSE triggers a browser download with the dataset name.
• Upload (browser → host): click Upload… in the toolbar, pick a local file, then run IND$FILE PUT dataset on TSO. The queued bytes are streamed back in 2 KB chunks in response to the host's 0x46 0x11 requests, terminated with an EOF error record.
• Works for both FT:DATA (binary/ASCII files) and FT:MSG (host status messages, surfaced as a flash status).
• Compressed mode (IND$FILE GET ... COMP) is detected and refused with a clear message - use the default uncompressed transfer.

Models supported:

Model	Rows × Cols	TerminalType
2	24 × 80	IBM-3278-2-E
3	32 × 80	IBM-3278-3-E
4	43 × 80	IBM-3278-4-E
5	27 × 132	IBM-3278-5-E

UI:

• Canvas-rendered display with extended color (3279-class palette), highlighting (underscore, reverse video, intensify)
• OIA status bar - connection LED, keyboard lock, insert mode, alarm flash, terminal type, cursor R/C
• Insert-mode toggle (Insert key) with overflow alarm
• Mouse selection, copy / paste / select-all (⌘/Ctrl+C/V/A)
• Rule cross-hair (toggle in toolbar)
• NVT overlay for ASCII banners before BINARY is negotiated
• Connection profiles persisted in localStorage

Encoding

Two EBCDIC code pages ship today, switchable from the toolbar dropdown and persisted per connection profile:

• CP037 - US English EBCDIC. Default. Used by classic z/OS, Hercules turnkey, pub400 (IBM i), and most legacy mainframe shops.
• CP1047 - Latin-1 Open Systems EBCDIC. The line-feed byte sits at 0x15 instead of 0x25, plus a few special-character swaps (¢/[, !/], ¬/^, etc.). Used by USS / z/OS Unix and many modern hosts that interoperate with ASCII tooling.

Switching mid-session re-renders existing cells through the new table immediately, no reconnect needed. Adding another code page is a 10-line change - drop its delta map into DELTAS in src/proto/Ebcdic.js.

Browser compatibility

Targets evergreen browsers (Chrome / Edge / Firefox / Safari). Uses ES modules, private class fields (#name), Uint8Array, Web Audio (for the alarm beep) and localStorage (for profiles). No transpilation, no bundler.

For wss:// with a self-signed certificate you have to visit https://<bridge-host>:<port>/ once and accept the cert; the browser won't surface a TLS prompt during a WebSocket handshake.

What's not implemented

This is the realistic gap list. Everything here is fixable; nothing blocks day-to-day TSO / CICS use as far as I've tested.

• Other EBCDIC code pages (CP500, CP297, CP285, …) - straightforward to add (delta maps in Ebcdic.js); CP037 + CP1047 ship today
• DBCS / SO/SI (Asian double-byte)