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Lead me gently into the land of IBM terminals

legalize

legalize

Experienced Member
Joined
Mar 24, 2006
Messages
391
Location
Salt Lake City, UT, USA
OK, some of you may know that I created and maintain the terminals wiki. I understand serial terminals pretty well, but in the IBM land, things are quite different.

Now at the lowest level, I've spent some time researching bisync, SDLC and HDLC. I know that SNA is layered on top of SDLC and that https://en.wikipedia.org/wiki/Virtual_Telecommunications_Access_Method is involved in providing services for terminals. I know that IBM 3270 protocol is often used as the basis for applications to program the terminals.

However, I don't really know anything about the software structure of IBM operating systems and how these pieces fit in historically. Ideally I'm looking for pointers to IBM documentation (bitsavers would be best) that I can read through in order to get a better handle on things. Given that it's IBM we're talking about it is most likely a set of documents and not a single document that I'm going to need to read in order to understand things. I've tried looking up stuff on IBM's web site, but I get lost too easily in stuff trying to direct me to current product offerings instead of historic offerings, hence my preference for things on bitsavers.

I have a few airline reservation terminals that I'd like to get talking at some point and that means emulating or acquiring IBM equipment for it to talk to. Interestingly enough, I'd like to develop the understanding deep enough to do emulation instead of just hooking up even more gear that I don't really understand. If I can emulate SDLC/HDLC well enough to get airline terminals to talk to a machine, then I should have a pretty good understanding of the equipment at that point.

Thanks for your help!
 
At this point I am going to scream because the questions you have asked are just so complex.

Well SNA is a layered architecture. So at the bottom we can have a physical link, so Synchronous Serial, X.25, Ethernet or Token Ring. Above this we have the DLC or datalink control protocol, and above that the application protocols which allow data to be exchanged between a program and device (or a program and a program). The protocols are different for each device type. SNA generally calls a device a "Logical Unit" (LU) and there is a partial table of types here:-

https://msdn.microsoft.com/en-us/library/ee253217(v=bts.10).aspx

There are also Physical Units

https://msdn.microsoft.com/en-us/library/ee253218(v=bts.10).aspx

We can also have Bi-Sync but bi-sync is really not part of SNA. Support was added because many folks had bi-sync terminals....
... if your terminals talk bi-sync that could be the simplest to make work as it has no control messages as described below...

Then we have the software implements SNA. On the host we have VTAM, which stands for Virtual Terminal Access Method (or TCAM which was an alternative implementation of VTAM). But its a whole lot more than that. VTAM allows the Mainframe operators to manage the network. They can enable and disable network nodes and links. Download software into the terminal controllers, etc.

The links generally don't connect directly to the host. They connect to a "Front End Processor". This generally runs something called NCP or Network Control Program. It connects to the Mainframe via something called a channel. VTAM though manages the connections on the FEP which is why the thing gets so complex.

So if you wanted to implement a TCP/IP stack you need to understand the Ethernet Layer, IP, TCP and the applications Protocol (e.g. HTTP 1.1)

In a similar way if you terminals connect via HDLC you need to be able not only to simulate the terminal messages but also handle the management protocols. You don't really need to do anything with them just send the appropriate message in return...

http://bitsavers.org/pdf/ibm/sna

has a set of introductions, so I guess

http://bitsavers.org/pdf/ibm/sna/GA27-3102-0_SNA_General_Information_Jan75.pdf

would be a good start.

http://bitsavers.org/pdf/ibm/sna/GA27-3136-10_SNA_Formats_Jun89.pdf

contains details of all the SNA messages you need to implement, and

http://bitsavers.org/pdf/ibm/3270/GA23-0059-4_3270_Data_Stream_Programmers_Reference_Dec88.pdf

contains details of the 3270 data streams that you need if your terminals talk 3270.....
 
Couple of things I missed. Many devices connect by means of some kind of remote concentrator so that also has software. What model terminals do you have? Do they have co-ax sockets?

Secondly some one has done some of the work here:-

http://www.lightlink.com/mhp/3705/

in that they don't connect physical terminals but they do emulate some components...
 
g4ugm said:
At this point I am going to scream because the questions you have asked are just so complex. [...]
Click to expand...

Yes, I know :). The complexity was preventing me from getting a good grip on where to start...

The terminals I have are airline reservation terminals:

http://terminals-wiki.org/wiki/index.php/Westinghouse_W1642
http://terminals-wiki.org/wiki/index.php/Westinghouse_W1643

Notice in this picture:

http://terminals-wiki.org/wiki/index.php/File:Westinghouse-W1643-2.jpg

the screen reads "SNA", "DSR DOWN". The airline reservation system is a derivative (or still?)
of SABRE, so they are using IBM equipment and protocols, even though the terminals
themselves are not from IBM.

You can see the connectors on the back of the terminals are DB-25 type, not coax (although
I also have a genuine IBM terminal with a coax connector somewhere). This is why I think
the physical link layer is HDLC/SDLC, although I realize that [HS]DLC can be transmitted just
fine over coax as well. In other words, physical layer is not necessarily an indicator of the
transport protocol layer.

Thanks for the pointers into the documentation, I'm going to start digging through there and
see what I can find out. I do have an HP serial protocol analyzer and I believe it says it can
detect/support BiSync, SDLC and HDLC. Would it be useful to sniff what's coming from the
terminal with such an analyzer, or do you need to hook up the terminal to an FEP in order to
see any useful chatter?
 
the characteristics of the 1642/1643 are listed in pdf/datapro/alphanumeric_terminals/Datapro_C25-010_198604.pdf

1642 is univac UTS
1643 is SDLC
 
A couple of observations..

1. The plug on the cable does not look like an RS232 cable. RS232 cables usually have pin 20 connected. I can't see pin 20 on there. Which pins are present. Given pin 7 is ground what voltages are on the other pins?
2. The documents supplied by Al indicated that these could be connected to a cluster controller. I suspect that you are missing the cluster controller. The protocol used between the cluster controller and the screen could be anything. When used standalone I would expect to config something... Any config options?
3. I assume you have no documents at all?
 
Yep, no documents at all, no other hardware either. For the W1643 I don't even have a
keyboard. For the W1642 I have multiple terminals and keyboards. Some of the keyboards
have credit card swipe readers on them -- just like you used to see in the travel agent's
office!

Thanks for that document Al, I've scraped relevant info into the terminals wiki for these various
Westinghouse terminals. It looks like the W1643 is SDLC over party-line or RS-232 as an option.
For the W1642, it looks like it was also party-line (could be SDLC or something proprietary) or
RS-232 as an option.

Regarding that connector close-up photo, it is a close-up of the connector on the keyboard :).

I haven't opened any of these up to see what's inside, but surveying the chips and boards could
yield some more information about details on these terminals.

However, let's not get too sidetracked on these specific beasts. I need to learn more about
this whole architecture anyway in order to capture information about IBM and IBM compatible
terminals (from which I gather there were sh'loads if InfoWorld and Computerworld are any
indication).
 
If that is the keyboard socket I am puzzled. The two sockets I can see are labelled "Print1" and "Aux" which I assume is short for "Auxiliary"..

As for "SNA" and "Down" well normal IBM 3270 terminals connected to a 3170/3270 display this type of status, but as symbols, as its sent to them from the controller...
 
g4ugm said:
If that is the keyboard socket I am puzzled. The two sockets I can see are labelled "Print1" and "Aux" which I assume is short for "Auxiliary"..

As for "SNA" and "Down" well normal IBM 3270 terminals connected to a 3170/3270 display this type of status, but as symbols, as its sent to them from the controller...
Click to expand...

If you're talking about this photo:

http://terminals-wiki.org/wiki/images/3/37/Westinghouse_W1642_321087256154-8.jpg

...then I'm not sure how you deduce which pins are connected from that photo.

I was referring to this photo:

http://terminals-wiki.org/wiki/images/3/3e/Westinghouse_W1642_321087256154-11.jpg

...which is a photo of the connector at the end of the keyboard cable.
 
Ok I was assuming Keyboard plug was host. Other than that I can only see 2 sockets, AUX and PRINT. AUX usually implies "Auxiliary" so some kind of secondary connection. PRINT implies a printer, so where is the host/controller connection?
 
I'll have to go look at it in the warehouse. Because I knew these terminals had weird connections,
I never investigated them too much in terms of how they hook up and whatnot. I can't remember
if there were some connectors on the underside of the terminal or not.
 
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