3770 SNA/RJE Emulation FAQs

3770 SNA/RJE Communications FAQs


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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 information.


Q: I know what IBM 3270 emulation is. How is 3770 emulation different from 3270 emulation?
A: 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.


Q: We've been told to get a 3770 emulator and we don't understand why.
A: 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.


Q: How is SDLC different from async communications?
A: 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 communications.

Almost all computers sold today have asynchronous communications capability. They are called by different names: COM ports, serial ports, and TTY ports. Almost no computers have built-in synchronous ports -- that's why the purchase of a synchronous communications product, like a 3780 emulator, almost always involves acquiring both hardware and software. Synchronous 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.


Q: What is RJE and how does it relate to file transfer?
A: 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.


Q: SNA/RJE is such an old technology. Isn't 3770 emulation going away?
A: 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 emulation.


Q: Why are 3770 emulation products so expensive?
A: 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 to accept.


Q: Why can't I use the modem that came with my PC?
A: 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 in PCs.


Q: Why do I need to put an adapter in my PC?
A: 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.


Q: What is AutoSync?
A: 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.


Q: 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: 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.


Q: How is 3770 SNA/RJE emulation different from 3780 BSC/RJE emulation?
A: 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 not interchangeable.

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