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

Dave McGuire mcguire at neurotica.com
Sun Apr 17 22:41:14 EDT 2016

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.


> 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

  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 McGuire, AK4HZ
New Kensington, PA

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