tn3270, what and where


Barry Shein (bzs%bu-cs.bu.edu@bu-it.BU.EDU)
Mon, 7 Mar 88 14:56:34 EST


>Hi, people here are just starting to mumble about 3278 type terminals,
>connected to (yeuch) IBM type main(larf larf)frame machines, connected to
>unix boxes over the ethernet.
>Thanks in advance, Ross Wakelin

You're talking about sitting at a 3278 talking to an IBM mainframe and
then telnet'ing to a Unix (or other full-duplex ASCII system.)

Basically, you can't *really* do it, although I can tell you how...

The problem is that the 3278 is a half-duplex device. When you type in
characters they are not sent to the host until an ENTER or other PF
key is struck, the plain text chars are buffered locally in the
terminal controller. Much of the software on Unix and other hosts
depend on seeing every character as it is typed to perform properly
(although full-screen editors are the most extreme case even simply
deleting a character on a line-oriented command expects this
capability.)

There is TCP software/hardware that supports Telnet available on the
IBM mainframe, for example Wiscnet (the current IBM TCP/IP product)
for VM or ACC's products for MVS.

You can telnet with this to a Unix or similar host. It will be very
similar to logging in from a paper terminal. No full screen (eg.
editors) software will be useable. I would imagine there are people
for whom this is satisfactory (eg. you just want to recompile and run
a program who's input/output is all to disk files.) We all did live
with this sort of environment for years.

The only other way I know of defeating this are black boxes which
splice into your 3278 coax and present an RS232 out the side. There's
a switch (similar to an A/B port selector), in one position you are
using the 3278 as such onto the IBM via the coax, in the other
position the black box uses the 3278's screen/keyboard more or less
directly to emulate a VT100 over the RS232 line which must then be
attached to something else (eg. a terminal server.)

If there are any other solutions (eg. mods on the 3270 controllers) I
too would be curious, time marches on, who knows, someone may have
something new.

        -Barry Shein, Boston University



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