TN3270 to BSC bridge

This is a work in progress and is not functional at this point.

Introduction

This is all about my attempt to connect a real IBM 3174 cluster controller to the Hercules-390 emulator, running MVS3.8J (TK4-). I started with an idea to use SDLC communications to my 3174-91R controller, but decided to try a path of lesser resistance so I wouldn't have to deal with the SNA protocol.

The idea here is for this software to write and read BSC frames to a USB attached dongle (attached as a basic serial port) that then writes the data to a synchronous RS-232 interface.

flowchart TD
    mvs["Hercules emulator, running MVS3.8j (and TSO etc)."]
    tn3270[Hercules TN3270 server]
    bridge["TN3270 BSC bridge (that's this project)"]
    dongle["USB to synchronous serial (BSC) dongle, acting as DCE"]
    3174[IBM 3174-91R cluster controller]
    3179[IBM 3179 terminal]
    mvs <-->|Channel attached emulated 3274 controller|tn3270
    tn3270 <--> |TN3270 TCP/IP connection|bridge
    bridge <--> |USB serial device|dongle
    dongle <--> |RS-232 synchronous serial|3174
    3174 <--> |Coax attachment CUT|3179

The USB to serial dongle is based around an Arduino board and uses MAX232 ICs to shift voltage levels as required for RS-232/V.24 comms. The serial interface is synchronous, meaning there are clock signals for the transmit and receive data.

This project, the TN3270 BSC bridge is (or will be) responsible for --

Establish a TN3270 session for each device (real or logical terminal) attached to the 3174.
Framing the 3270 datastream, received from the TN3270 server
Sending BSC frames to the dongle
Interpreting ACKs/NAKs that come back from the dongle
Sending poll requests to the devices
Receiving BSC frames from the dongle and returning 3270 responses to the TN3270 server.

Why is this in Javascript?

In my professional paid work, I have been developing mostly back-end code in Javascript for the last 4 years. As this is what I'm currently working with, and I haven't done any C# or Java for a little while, Javascript was comfortable for me. It is not the most ideal platform for this type of work, but is adequate.

C and C++ are even longer ago, if I don't count the small amount I've been doing for the Arduino USB to BSC dongle. I may well decide to convert this code to another language at some time in the future.

What's working (as of 7/23/2022)

At this time, not too much.

The TelnetConnection class (and the TelnetOptionSet and TelnetOption classes) are mostly operational. These classes allow a connection to be made to the telnet (TN3270) server and receive the initial 3270 datastream that is sent down from MVS/TSO.

CRC calculations

I have found that the npm CRC module with function crc16 appears to give the expected BCC values for BSC.

Other ideas

I have a second IBM 3174, a model 51R. Perhaps I will try and attach this using SNA.

Another device that I have, is a INFORMER terminal, a SDLC/SNA variant. So it's pretty much the same as the above 3174 for getting this working. I think it is possible to use the V.24 port directly. If not, I do have a couple of SDLC capable synchronous modems.

Useful sources of information

IBM: General Information - Binary Synchronous Communications - GA27-3004
IBM: 3274 Control Unit Description and Programmers Guide - GA23-0061. See section on "Remote Operations -- BSC".

The Project

Major Components

Here's a more detailed view of the components of the bridge software.

flowchart TD
    bridge["Main software setup"]
    bisyncLine["The BSC line handler and is responsible for comms to the USB-BSC-dongle"]
    bscTerminal["BSC attached terminal"]
    tn3270["Communications to the telnet TN3270 server, per terminal"]

    bridge <-->|Creates the instance for the BSC line|bisyncLine
    bisyncLine -->|Creates BscTerminal instances for each attached terminal|bscTerminal
    bscTerminal -->|Creates the TerminalToTn3270 instance to handle TN3270 telnet session|tn3270

Sample BSC flows

Some sample flows are documented here

twinslow/tn3270-bsc-bridge