Introduction to IBM 3770 SNA/RJE Communications
Who Uses 3770 SNA/RJE Communications Today?
What Did 3770 SNA/RJE Terminals Look Like?
What is SNA/RJE?
3770 SNA/RJE Communications Frequently Asked Questions
3770 SNA/RJE Communications Glossary
Things to Consider When Purchasing 3770 SNA/RJE Emulation
This list of frequently asked questions about
3770 SNA/RJE is derived from one company's
nearly fifteen years experience selling and
supporting RJE emulation products. Where
appropriate, embedded links have been included
to take you outside the www.3770-emulation.com
site to other resources for more detailed
||I know what IBM 3270 emulation is. How is
3770 emulation different from 3270 emulation?
||It is easy to be confused when talking about
the various IBM terminals and emulators that
bring the functionality of these terminals
to the PC. Although they may both use one
of the same underlying SNA protocols, IBM
3270 emulation and 3770 emulation are as
different as night and day. A 3270 terminal
was an interactive display station and keyboard
often with an attached Model 3287 printer.
Such terminals are designed for an operator
to make queries and enter data interactively.
A 3770 terminal is designed to transmit and
receive data in large batches -- originally
via punch cards and printed output, now via
disk files. One key clue to the fundamental
differences between the two types of terminals
is the large display found on 3270 terminals,
while a 3770 terminal had only a small console
display, or no display at all.
||We've been told to get a 3770 emulator and
we don't understand why.
||That seems to happen a lot. Need for 3770
connectivity often sneaks up on some organizations
-- particularly customs brokers, freight
forwarding firms, and anyone seeking to exchange
files with mainframe systems over established
SNA protocols. Often purchasing agents in
these organizations are sent blindly out
onto the Internet to locate something vaguely
referred to as a "3770 emulator."
Obviously 3770 connectivity is not cutting
edge technology and therefore most people
have never heard of it, but it is still frequently
used in many specific remote printing and
business-to-business communications applications
-- most of which have been in place for many
years. In a great many of these situations
3770 emulation is the best, even the only
way, to move vital data from point A to point
B -- that's why you've been asked to obtain
a 3770 emulation package.
||How is SDLC different from async communications?
||The Synchronous Data Link Control, SDLC,
protocol often used by 3770 SNA/RJE terminals
is a specific form of synchronous communications.
Although both synchronous and async (asynchronous)
communications are used to send data over
modems, the similarity stops there.
Asynchronous communications sends and received
each data byte individually; synchronous
communications establishes a timing pattern between the transmitter and receiver and
then data streams from one to the other without
the overhead of start and stop bits -- and
usually without a parity bit -- which are
required by, and add overhead to, asynchronous
Almost all computers sold today
communications capability. They
by different names: COM ports,
and TTY ports. Almost no computers
synchronous ports -- that's why
of a synchronous communications
like a 3780 emulator, almost
acquiring both hardware and software.
communications is most commonly
used in connectivity
with IBM mainframes. See the glossary and links on that page for more details
on synchronous vs. asynchronous communications.
||What is RJE and how does it relate to file
||RJE is an acronym for Remote Job Entry. In
the early days of computing submitting jobs
(i.e., programs and data) to a mainframe
computer was typically done by transmitting
punch cards from an RJE terminal. The mainframe
would process the job, usually taking its
own sweet time in doing so, and the results
would eventually come to the terminal in
the form of output printed to the printer
and/or punched out onto a new set of cards.
Back then disk storage was extremely expensive
and therefore "data files" as we
know them today did not exist outside of
the mainframe. But as computers got smaller
and their cost came down, there was an increasing
need to move data files more from one computer
to another. SNA/RJE was already in place
and lent itself perfectly to performing file
transfers as well as remote printing. It
is still used for that purpose today particularly
between PCs and mainframes.
||SNA/RJE is such an old technology. Isn't
3770 emulation going away?
||Well, yes and no. Yes, in the sense that
eventually the need for 3770 connectivity
will disappear. But, no in the sense that
it has been around for a long time and there
is so huge an investment in a SNA/RJE infrastructure,
especially in the financial and government
sectors, that the phasing out of such a reliable
system is often prohibitively expensive.
The same thing has been said about the COBOL programming language -- it was supposed
to disappear years ago but so much of the
software in the mainframe world is written
in COBOL that it is simply too expensive
to do away with it. There are no new large
scale RJE systems being installed today,
but until existing systems are phased out,
which may take another ten years or more,
there will continue to be a need for 3770
||Why are 3770 emulation products so expensive?
||A basic 3770 emulation package for Windows
PC is likely to cost between US $1,000 and
US $5000 including both emulation software
and hardware (a sync adapter and/or synchronous
capable modem. With some companies technical
support may cost extra. Often the purchasers
of 3770 emulation suffer from "sticker
shock" when prices in this range are
first mentioned. However, when you understand
that your organization will never be called
upon to install 3770 emulation on everyone's
desktop, and that specialized knowledge is
required to produce, maintain, and support
a product that is not sold in huge quantities,
the higher price is easier to justify and
||Why can't I use the modem that came with
||This is one of the most frequently asked
questions. The modems that comes as standard
equipment in PCs are asynchronous modems.
3770 RJE emulation requires the use of a
modem that can operate in synchronous mode. Originally the modems used with 3770
terminals were only synchronous -- namely
the Bell 201C (2400 bps) and the Bell 208B
(4800 bps). Nowadays many modems are capable
of both synchronous and asynchronous operation
(i.e., Hayes Optima, 3Com Courier) but these
are higher priced products than the relatively
inexpensive Win-modems that come preinstalled
||Why do I need to put an adapter in my PC?
||It is not always a requirement to put an
adapter in your PC in order to use a 3770
emulation product. There are other solutions
such as using a specific modem which incorporates
a feature named AutoSync (see Q & A on
that subject for more information.) But the
general answer to this question is that PCs
do not come with hardware that supports synchronous
communication protocols such as SDLC used
by a 3770 emulator. (The built-in COM port
of your PC is asynchronous only.) So it is
necessary to either add a synchronous port
by the addition of an adapter or provide
a way to do an asynchronous - synchronous
conversion (e.g., AutoSync) to support 3770
RJE emulation on a given PC.
||What is AutoSync?
||AutoSync is a feature built into Hayes Optima Business Modems. When AutoSync is enabled, the Optima modem
performs an "on the fly" asynchronous
to synchronous conversion of data being sent
by the modem and a synchronous to asynchronous
conversion of data received by the modem.
In effect, then, the Optima modem permits
a connection to a synchronous remote system
via a built-in asynchronous COM port.
||When I load the 3770 emulator that my company
purchased, it doesn't seem to do anything
-- even after I dial out. What am I missing?
||A 3770 RJE communications session is not
interactive like other terminal connections
such as 3270 or Telnet. There is not going to be a sign on screen
or prompts for an operator to send a user
ID and password. RJE by definition is a batch
process which entails sending and receiving
files rather than character oriented interaction
with a user. Even after a physical connection
has been established, most RJE mainframe
computers wait for the terminal to send something
-- usually commands and/or data to run a
particular program. Therefore, if you do
not have things set up for your emulation
product to send anything, the connection
may remain idle. For this reason, 3770 emulation
products are typically script driven in order
to automatically send commands and/or data
to the mainframe.
||How is 3770 SNA/RJE emulation different from
3780 BSC/RJE emulation?
||Fundamentally 3770 SNA/RJE and 3780 BSC/RJE terminals were designed to do the same job:
namely, submit batch jobs from a remote location
to an IBM mainframe computer. (Oddly the
3780 terminal came before the 3770 family.)
The difference is in the underlying communications
protocols used by the terminals. 3780 BSC/RJE
uses the IBM Binary Synchronous Communications
(BSC or bisync) protocol while a 3770 SNA/RJE
terminal uses the SDLC protocol (or other protocol supported by IBM's SNA network). These protocols are mutually exclusive
and therefore 3770 and 3780 terminals are