Browser-side IBM 3270 terminal, plus a 5250 client for IBM i / AS/400 (beta). No backend, no server-side code, no runtime build step - just static files served over HTTP. The datastreams and TN3270E / TN5250E telnet negotiation are implemented in plain JavaScript modules running in the page; the page connects to the host through any websockify-style TCP↔WebSocket relay.

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

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

The index page lets you pick the protocol - 1 opens the TN3270 client (/tn3270/), 2 opens the TN5250 client (/tn5250/). Any static server works; nothing in the project requires a build step.

websockify is the simplest option. To expose a Hercules MVS turnkey listening on localhost:3270 (for TN3270):

websockify 0.0.0.0:6080 localhost:3270

Or to point the TN5250 client at the public IBM i at pub400.com:

websockify 0.0.0.0:6080 pub400.com:23

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 - two public test instances are already up:

wss://tk5.bencz.cc:6080        (Hercules MVS 3.8j Turnkey 5 - for TN3270)
wss://pub400.bencz.cc:6080     (pub400.com IBM i - for TN5250)

The MVS image is from tk5-hercules; the IBM i side relays to pub400.com, where you can register a free user profile. Drop the 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.

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.

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 (3270):

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

The 5250 client lives under /tn5250/ and shares the toolbar, OIA, canvas renderer, profile store, and EBCDIC tables with the 3270 side - only the wire protocol and the datastream parser are separate.

It's tagged beta because it has had a lot less mileage on real hardware than the 3270 side. Signon, WTD, input fields and AID keys work against pub400 and IBM i 7.x; ENPTUI is wired up but not all primitives have been exercised end-to-end. Expect rough edges - please file issues with a screenshot and the host you hit them on.

Telnet / TN5250E (RFC 1205, RFC 4777):

• BINARY, EOR, TERMINAL-TYPE, NEW-ENVIRON negotiation
• NEW-ENVIRON variables: DEVNAME, KBDTYPE, CODEPAGE, CHARSET, and the bypass-signon set (IBMRSEED, USER, IBMSUBSPWD)
• 10-byte GDS record header on inbound and outbound records
• GDS opcodes: NO-OP, INVITE, OUTPUT-ONLY, PUT-GET, SAVE/RESTORE-SCREEN, READ-IMMEDIATE, MESSAGE-LIGHT on/off
• ATTN / SYSREQ flags surfaced through the OIA

5250 datastream:

• Commands: WTD, WEC, WECW (write error code to window), CLEAR-UNIT, CLEAR-UNIT-ALT, CLEAR-FORMAT-TABLE, READ-INPUT-FIELDS, READ-MDT-FIELDS, READ-MDT-IMMEDIATE-ALT, READ-SCREEN-IMMEDIATE, READ-SCREEN-TO-PRINT, WRITE-STRUCTURED-FIELD, SAVE-SCREEN, RESTORE-SCREEN, ROLL
• Orders: SOH, RA, EA, ESC, TD, SBA, WEA, IC, MC, WTDSF, SF
• Basic attribute table (0x20-0x3F) - color, reverse, underline, blink, column separator, non-display
• Field Format Word - bypass, dup, MDT, auto-enter, FER, monocase, mandatory, right-adjust / zero-fill
• AID keys: Enter, Clear, Help, PF1-24, Roll Up / Down / Left / Right, Print, Attn, SysReq
• Bypass-signon: optional USER / password fields in the toolbar short-circuit the standard signon panel (RFC 4777 §5)

ENPTUI (partial):

• WDSF (Write to Display Structured Field) decoder
• Primitives: Window, ScrollBar, Selection Field
• Not yet covered: menu bars, choice presentation with cursor progression, the full set of WDSF minor structures

Models supported (5250):

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.

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.

This is the realistic gap list. Nothing here blocks day-to-day TSO / CICS use on the 3270 side, or basic IBM i signon + green-screen use on the 5250 side.

• 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)
• 5250 printer sessions - only display devices for now
• 5250 ENPTUI - full set - Window / ScrollBar / Selection Field primitives are decoded, but menu bars and the long tail of WDSF minor structures aren't wired through yet