[vcf-midatlantic] IBM mainframe terminal concepts, was Re: Anyone bringing IBM 3270 related items to VCF?

Dean Notarnicola dnotarnicola at gmail.com
Mon Apr 18 07:41:57 EDT 2016

Thank you, o guru and infinite source of large systems wisdom! :-)

On Sunday, April 17, 2016, Dave McGuire via vcf-midatlantic <
vcf-midatlantic at lists.vintagecomputerfederation.org> wrote:

> On 04/14/2016 02:39 PM, Dean Notarnicola via vcf-midatlantic wrote:
> > Just a thought: You may be able to run the Hercules​ system/390 emulator
> > with an IRMA 3270 or similar coax terminal card and attach to that.
>   No.
> > SIMH can emulate some IBM minis as well,
>   3270 is a mainframe protocol, unrelated to minicomputers.
> > but I'm not sure that they can interact with the card.
>   They cannot.
>   Here's how this works.
>   One does not connect terminals to an IBM mainframe directly, even
> console terminals.  Devices called "establishment controllers", usually
> of the 3174 family, connect to the mainframe via one of several types of
> interfaces.  Common interfaces for this are parallel channel (a.k.a.
> "bus and tag") and ESCON, which are interfaces present on most such
> machines, both new (current) and old.  The original establishment
> controllers were 3274s...they are pretty tough to find now, but they're
> similar (but much more limited) to what I'm describing here.
>   3174 controllers come in several models, implemented in a few
> different form-factors.  There are floor-standing models about the size
> of a living room end table, tabletop models about the size of a larger
> PeeCee chassis, and rackmount models of similar dimensions.
>   3174s have from one to several terminal ports, which are BNC
> connectors for connection to terminals using coaxial cable.  The
> specific type of cable used is RG-62, 93-ohm impedance.  There are
> single-terminal and multiplexed ports; essentially one of those BNC
> connectors can connect to either a single terminal or a terminal
> multiplexer box (3299 or similar) which allows the connection of up to
> 32 separate terminals to each of the 3174's BNC ports.  On most models
> of 3174, the terminal ports, the host interface(s), and any other
> optional accessories come in the form of plug-in modules that slide into
> slots in a backplane/card cage arrangement.  These modules are about the
> size of a paperback book, but a bit longer and skinnier.  They are
> boards with protective plastic shrouds wrapped around them.
>   3174 controllers boot (or "IML", for "Initial Microcode Load") from
> either an internal 1.2MB or 2.4MB (yes, 2.4MB) 5.25" floppy disk, or
> from an optional internal MFM hard drive.  What the rest of us call
> "firmware" IBM calls "microcode".  Actual "microcode" (as the rest of us
> use the term) is also called "microcode" in the IBM world, so IBM's use
> of the term is overloaded a bit.
>   Once up and running, a 3174 is an appliance.  You don't really talk to
> it, either from a terminal or from the mainframe, but you do talk
> through it.  It just sits there quietly doing its job, supporting
> terminals.
>   3270 is not a peer-to-peer protocol, nor is it a networking protocol.
>  3270 terminals (which includes a microcomputer with a 3270 interface
> card plugged into it) cannot talk to other 3270 terminals.
>   Parallel channel and ESCON ports on a mainframe have channel
> addresses, or blocks of such addresses, associated with them.  A 3174 is
> configured to answer at one or more such addresses, and whatever
> operating system you're booting ("IPLing", in IBM parlance: "Initial
> Program Load") is configured to access its primary console at a specific
> channel address.  That console I/O goes through the establishment
> controller and to the terminal that you've configured.
>   On a 3174 running from a floppy, which is by far the most common
> configuration, there's a regular IML disk and a "utility disk".  There's
> usually a little "pocket" inside the machine where the utility disk and
> spare IML disks live.  3174s have a little keypad and display on the
> front; one can tell the 3174 to boot the utility disk by inserting that
> disk and using the keypad to tell it that it's about to boot that disk.
>  That utility disk system brings up a self-contained set of menus that
> can be used to configure ("customize") the regular IML disk in terms of
> terminal types, host attachment configuration, addressing, languages of
> keyboards attached to its terminals, and lots of other stuff.  It's
> extremely elaborate, flexible, and well thought-out.
>   There are two basic classes of 3270 terminals: CUT ("Control Unit
> Terminal") and DFT ("Distributed Function Terminal").  The former is
> very simple and older; the latter can be much more complex.  Some DFT
> terminals are so complex that they don't have enough ROM to store all of
> their microcode (firmware).  When they power up, they request a firmware
> download ("DSL", for "Down Stream Load") from the 3174 controller
> they're connected to.  The 3174 will then check to see if it has the
> requested microcode file (identified by terminal model, which is
> embedded in the DSL request) and if so, sends the microcode to the
> terminal, which then initializes and becomes a functional terminal.  The
> 3290 multi-session plasma terminal is an example of such a device.
>   The DSL files for specific terminals are typically delivered on
> separate floppies.
>   The hard drive option for some 3174 models allows one to store the
> normal IML code, the utility IML code, and any DSL modules you may need.
>  The only option is a 20MB MFM disk, a Seagate ST-225.
>   There are three main "families" of 3174 microcode: Configuration
> Support A, Configuration Support B, and Configuration Support C, in
> increasing level of complexity, capability, and demands (amount of RAM,
> etc) on the 3174's hardware configuration.  Configuration Support C
> includes an IP stack.  If one has a 3174 with 6MB of RAM (the maximum
> for the top-end models) and an Ethernet or Token Ring option module
> installed, and run Configuration Support C microcode, one can bring up
> its IP stack on a network and it will establish tn3270 connections
> (tn3270 encapsulates a 3270 datastream within a TCP connection) which
> one can then use to connect to a networked IBM mainframe, either real or
> emulated.
>   If you're running the Hercules emulator and have everything configured
> properly, including the stuff mentioned in the paragraph immediately
> preceding this one, you can get real IBM 3270-family terminals, which
> are amazingly nice to work on, running against the mainframe emulated by
> Hercules.  I've done this, and can provide assistance. (you'll need it)
>  You can also see (and use) stuff like this up and running at the Large
> Scale Systems Museum.
>              -Dave
> --
> Dave McGuire, AK4HZ
> New Kensington, PA

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