tn3270

tn3270 is a pure TypeScript implementation of the Telnet protocol necessary to negotiate a connection from a software 3270 emulator and a host that accepts 3270 connections.

I use tn3270 as the basis for my own 3270 emulator EL-3270. I factored out the connection code to better expose it for others to use in their own emulators. I hope that it also exposes the protocol in an easy to understand way.

• References
• Installation
• API
  • Example
• 3270 Font

References

These two documents explain the protocol exhaustively.

• RFC1756 TN3270 Current Practices
• RFC1647 TN3270 Enhancements

I also found this summary of Telnet commands by Dave Marshall very helpful.

Installation

npm install --save tn3270

API

tn3270 negotiates a session with the host and if successful returns an Observable stream of data from the host to the emulator. A write API supports data transfer from the emulator back to the host.

All the other complex details of the 3270 data stream are delegated to the emulator, of which EL-3270 is a good example.

Because the API surface is so small, I've reproduced it here in full.

import { Observable } from 'rxjs/Observable';
export declare class Tn3270 {
  host: string;
  port: number;
  model: string;
  stream$: Observable<Buffer>;
  constructor(host: string, port: number, model: string);
  write(bytes: any): void;
}

write accepts either an array of bytes or a Node.js Buffer.

Example

Here's how I use tn3270 in my Electron 3270 emulator.

import { Tn3270 } from 'tn3270';

ipcMain.on('connect', (event: any,
                       host: string,
                       port: number,
                       model: string) => {
  theTn3270 = new Tn3270(host, port, model);
  theConnection = theTn3270.stream$.subscribe({
    next: (data: Buffer) => {
      const view = new Uint8Array(data.length);
      for (let i = 0; i < data.length; i++)
        view[i] = data[i];
      theWindow.webContents.send('data', view);
    },
    error: (error: Error) => theWindow.webContents.send('error', error.message),
    complete: () => theWindow.webContents.send('disconnected')
  });
});

3270 Font

Many thanks to Ricardo Bánffy for his 3270 Font, republished here for convenience. I actually grabbed the TTF versions from the S3 bucket https://s3.amazonaws.com/3270font/3270_fonts_4cfe95c.zip that Ricardo references.

The 3270 font can be included in your app simply via the fonts/3270.css file. Here's how I use it in my Angular CLI projects, just like any other font in angular.json.

"styles": [
  "node_modules/roboto-fontface/css/roboto/roboto-fontface.css",
  "node_modules/tn3270/fonts/3270.css"
],