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