Classic Computer Magazine Archive ANTIC VOL. 2, NO. 2 / MAY 1983

Talking to Big Brother

by Jordan Powell

Communicating with large IBM computers is remarkably easy with ATARI, and relatively inexpensiveŅat least in mainframe terms. Besides the usual required equipment you will need a modem and some special software at your end, and a device called a "protocol converter" at the IBM's end of the phone line. Of course, that means you must have some control, influence or special knowledge concerning the receiving end, but for those of you whose employers have IBM mainframes, that shouldn't be too difficult.

The problems confronted are not unlike the cultural and language barriers between two nationsŅa big one and a small one. The big one doesn't know the small one's language, and isn't about to learn it. The small one, in this case the ATARI has to "come up." And just because the ATARI is a real computer doesn't mean the IBM will treat it like one. In fact, it will be more readily recognized if it plays dumb, and even pretends to be a member of the family.

You probably know that the ATARI doesn't speak IBM's language. IBM speaks EBCIDIC (Extended BinaryCoded Decimal Interchange Code), and doesn't much care that nobody else does. It does realize, however, that lots of other computers speak ASCII, so it will take on a henchman to translate, when necessary, and that's mostly what the protocol converter is and does.

Unfortunately, poor cousin ATARI doesn't even speak ASCII, it speaks a dialect called ATASCII, so it has to have an interpreter too, to tell it what to say and how to say it.

The interpreter of choice at the ATARI end is the Chameleon CRT Emulator from the Atari Program Exchange. Chameleon translates ATASCII into ASCII, and makes the ATARI appear to be a well-known, if not IBM, computer terminal. It also makes the 40-column screen simulate an 80-column screen by scrolling.

Chameleon does not solve the problem entirely. We still must translate ASCII to EBCIDIC, and appear to the IBM as if we are an acceptable terminal. The protocol converter does this. Recently, the PCI Protocol Converter has become available at about $3,000, and makes the ATARI look like an IBM 3270 terminal. It also bestows some 3270 features in the bargain. Obviously, this is more than poor cousins can afford, but many mainframe owners either have installed converters, or can be cajoled into doing so. One converter can recognize up to 20 terminal types, so its usefulness extends beyond the ATARI application.

The final problem -- hardware, specifically modems and cables. Modems make it possible to send digital information created in computers across phone lines which were meant to transmit voice data. There are two points to be made here. The first is that the modem should be at least 1200 baud (bits per second). This is because 3270 devices work in screens of data rather than a line at a time, and at speeds slower than 1200 baud, filling a screen takes forever. Still, 300 baud modems can be used if the receiving modem supports that rate. Speeds faster than 1200 baud may be possible, depending on the quality of the phone lines used.

The second point is that modems have protocols too, so make sure that the modem you have and the one at the mainframe use the same modem protocol. The modem must be a directconnect type, which connects to your phone jack via a regular phone cord. A common modem protocol used for 1200 baud mainframe communication is the BELL 212A protocol. The U.S. Robotics Micro Link 1200 works well and is relatively inexpensive ($500).

Connecting the modem to the 850 via an ATARI cable presented a problem. While the signals coming out of the 850 interface are RS-232 standard, the positions of the pins on the modem side of the Atari cable are not in the RS-232 standard positions. I had to strip the connector off the cable and place the wires in the correct positions in a new connector. Remember, SEND-DATA from the 850 goes to RECEIVE-DATA on the modem, and vice versa.

The rules for communicating from one device to an IBM requires either Bisynch or SDLC j both foreign to most non-IBM equipment. Both of these translations were handled properly by the PCI protocol converter. This device i s attached at the mainframe, between it and the modem, and "filters" all of the signals going to and from the mainframe. It can convert from ASCII to EBCIDIC and vice versa. It can also convert from IBM data-communications protocols to the asynchronous protocols used by most terminal equipment.

The protocol converter can enable asynchronous terminals, or an ATARI that looks like one, to use the IBM 3270 terminal features. We connected the ATARI with Chameleon emulating an ADM-3A terminal to the modem via the 850. We then dialed the protocol converter, which allowed us to simulate an IBM terminal and gain access, as a 3270, to the Time Sharing Option of IBM's MVS operating system.

One more warning: the protocol converter is sensitive to noise on the phone line just before you connect up, so if your Modem is a manual one try to turn it on as soon as possible before the computer answers the phone. You should consult with your modem dealer or factory technical contact about how best to do it.


Communications protocols are becoming increasingly complex, though the idea behind them is simple: if two or more devices are going to communicate, there must be some rules. An analogy is a town meeting. If everyone spoke at once, nobody would hear anyone else; or,~if two people spoke to each other at the same time, neither would hear the other. Communicating devices have the same problems, so sets of rules define the way communicating devices interact. If the devices don't all follow the same set of rules (protocols), communication is impossible.

There are many layers of protocol in data communications. The first layer is at the physical (modem) level. Fortunately, Bell modem protocols are the industry standard and most modems use them. The 212a 1200-baud protocol is the one we used. The next layer up is the datalink layer. This layer defines the method by which data elements (characters, messages) are sent and received, while the physicallayer protocols define the method by which the signals carrying the data elements are sent. Asynchronous communications means that each character is preceded by a start bit, which signals that a character is coming, and followed by one or more stop bits signalling end of character. The sender and receiver are not synchronized (clocked at the same rate in tandem). Synchronous protocols send data in larger units. The message, which could be the contents of an entire screen, is preceded by a start bit (or by start characters) and continues on until the sender appends the message-end character(s).

IBM uses synchronous protocols. The information utilities (the SOURCE, CompuServe) use Async, teletype-like protocols for communicating with micros. Programs which allow access to these services do not readily communicate with IBM mainframes unless special provisions are made at the mainframe. It may be possible to get a protocol converter to translate for these programs.

IBM uses "small" computers as front-ends to the large mainframes. These computers handle the communications tasks, allowing the mainframe to work on applications programs. There is software for such a communications computer (IBM 3705) which allows Async devices to communicate with the mainframe, but it is apparently too costly, complex and demanding of the 3705's resources for most companies to want to use it. IBM should be coming out with a new communications computer with more power and flexibility in the near future.

You will need:

Atari 800 computer
Atari 810 Disk Drive
Atari 850 Interface
Atari Modem Cable
From your local Atari dealer

Chameleon Terminal Emulator
Atari Program Exchange
P.O. Box 3705
Santa Clara, CA 95055
in CA 800-672-1850

U.S. Robotics
Micro Link 1200 Modem
U.S. Robotics Inc.
1035 W. Lake St.
Chicago, IL 60607

PCI Protocol Converter
(at the mainframe)
6150 Canoga Ave.
Woodland Hills, CA
91367 800-423-5904
in CA 213-716-5500