ARCNET: The Sleeping Giant

from the May/June 1984 issue of Two/Sisteen magazine

by Bro. Gary Eck, S.M.

Since its introduction, very little has been published to describe the use of ARCNET in concrete situations. This article describes the experiences of one ARCNET installation.

A giant of a computer system lies hidden in the Radio Shack product line. Based on the Model II and Model 12 workhorse computers, the ARCNET system allows for the linkingof many computers into a powerful, effective local area network system. Actually, ARCNET provides an efficient way of linking many more computers than XENIX.

How ARCNET Was Chosen

Early successful applications of individual TRS-80 Model II computers in a few administrative offices of Chaminade High SchooL a large Catholic boys’ school on Long Island, led to a decision to extend computers to all administrative offices in order to streamline office procedures. One of these early applications was the formation of a comprehensive data base on each student, using Profile Plus (and all its extensions). With an enrollment of approximately 1580, the data base was quite large, requiring four floppy disks to handle all the data, such as names, addresses, birth dates, eighth grade schools, parents’ occupations, standardized testing results, and so on. In addition to the necessary summaries, mailing labels, and similar uses, this data base has also allowed the easy preparation of special analyses and detailed data which was previously too demanding to research.

The large number of students generated a constant flow of data changes which were communicated using both the traditional paper memo and frequent copying of all four disks needed for each computer using this data. The need for some type of network linking the several computers soon became evident. Cost considerations and the favorable experiences of the school personnel using Model Us gave preference to expanding the TRS80s into a network rather than purchasing an entirely new system.

The XENIX operating system was carefully considered. To oversimplify, XENIX uses the Model 16’s power and memory to service three users. By partitioning memory and processor activity, XENIX supports three (and more recently six) users simultaneously. The particular needs and administrative responsibilities within the school divided up nicely into four groups of three related users, each which XENIX could have served well, except for the lingering problem of communicating changes and data among the various systems. The chore of transferring updated disks and paper notices still remained if this arrangement were adopted.

The ARCNET system, on the other hand, allows for the linking of more than three computers into a common network . Based on token passing technology licensed from Datapoint Corporation, Tandy’s ARCNET allows for an almost instant networking of Model IIs and 12s.

System Components

The system consists of three major parts: applications processors, hubs and cables, and file processors.

Applications Processor

Each user of ARCNET is considered an applications processor. An ARCNET communications board must be installed in each user’s computer; when in use, the machine must operate with TRSDOS 4.3 which is very sirni)ar to TRSDOS 4.2, with the addition of a few commands to coordinate the ARCNET communications.

After executing a command to activate the ARCNET board, the user must use the MOUNT command which enables the local computer to access the ARCNET files as though they were located on one of the local disk drives. In our ap plication, a brief DO file automatically executes these commands; once this file has executed, the user sees the same Model II or 12 he is accustomed to, with the addition of the contents of an entire hard disk which happens to be located elsewhere in the building.

For example, a commonly used BASIC program looks up the class schedule for a particular student; this program runs as usual at the application processor, but instead of finding the desired files on the local Drive 0 or hard disk, it will find the files on the remote disks provided by ARCNET.

Hubs and Cables

The applications processors are joined to common hubs by ordinary RG-62 coaxial cable. Active hubs have eight ports to link to applications processors, file processors, or other hubs. Passive hubs can be used to more cheaply link three users into one active port (a simple coaxial “Tee” connector is even cheaper when only two users are involved).

Figure 1 shows the arrangement of active hubs and cables connecting the school’s ARCNET equipment. In our setting the longest single cable run is 400 feet, although the system can extend up to 2000 feet.

The File Processor

The third major component of an ARCNET system is the file processor. There must be at least one file processor, although more may be added as needed. The file processor is an hard disk based computer which is totally devoted to serving the system by making its files available to any user who wants to access them. To simplify explanations, we describe these as “public” files which are accessed on ARCNET. When the power is first turned on at the file processor computer, it is an ordinary TRSDOS 4.3 machine and can be used as usual when not activated for ARCNET. The command ARC80FP causes a new operating system to load, overwriting TRSDOS with ARCDOS. After a second entry of date and time, the command START FP is given to complete the transformation of the machine into a file processor for ARCNET. No further attention is needed at the file processor until it is time to turn it off at the end of the day (with the command DO ENDARC). Once ARCDOS is loaded, there is no way back to TRSDOS except the reset switch.

What ARCNET Does

Practically, what the individual user experiences with ARCNET is very simple. Instead of many disks to keep organized and put into various expansion drives, there is only one floppy disk which goes into Drive 0 and does everything. The local computer turns on as usual, except for a couple of brief messages which display as the DO STARTARC file automatically connects to the system after the date and time inquiries. The user runs whatever programs he wants without having to worry about changing floppy disks or having the most recently updated disks at hand. When the work is done the user effectively has a hard disk full of current programs and data, without having the equipment located in the office.

Although the capacity of the hard disk is given through ARCNET, its spe_ed is not; the system runs at floppy speeds in order to accommodate all the necessary communications. Sometimes this can be aggravating as a file intensive program such as Profile Plus repeatedly reads a directory of several hundred items looking for multiple files. Once the user has the files, things do work quite rapidly.

Our System Set-Up

According to the product literature, ARCNET can support up to 255 computers, including one or more file processors. Our application has grown to fourteen users and one file processor. Four users have their own hard disks and rarely connect to the system; these computers are in the administrative offices which maintain the official school records, such as grades, the student data base, schedules, etc. On a regular basis, these official files are simply copied over to the “public” files, bringing along all updates without requiring paper memos or transferring of disks. The maintenance of the official files distinct from the “public” system prevents any tampering or mischief along the long of the movie “War Games.” The limited use of passwords within Profile Plus and TRSDOS adds additional protection where appropriate.

Eventually twenty-four administrators or secretaries will have access to ARCNET. Each has a Model II or 12 computer, a local printer, and a single disk to access ARCNET. Without ARCNET, a three drive expansion bay or hard disk would be needed at each location. Not only is the financial savings substantial, but there is no need to find room for the additional equipment in each office. With only one disk to gain access to the entire ARCNET public files, there are no longer the problems such as having the right disk in the wrong drive or the disk from one data base being mixed with those from another data base.

ARCNET has proven to be very easy to expand. Adding another user consists of placing the equipment, connecting the single coaxial cable, providing one disk with a few simple files, and updating some identity code files at the file processor.

Hard Disk Backup

In addition to these obvious conveniences and savings, ARCNET has provided a simple, convenient way of doing the necessary tasks of backing-up hard disk files. Instead of saving files onto floppy disks (a tedious task which often involved fifteen or twenty disk swaps), we simply copy critical files onto the “public” ARCNET file and then from the “public” file onto another hard disk. A few DO files further automated this process so that no intervention is needed once the process begins, freeing the use to do other work or to update files of other machines. There have been times when one person was simultaneously backing-up files on four machines without using any floppy disks beyond those needed to access ARCNET.

ARCNET Weaknesses

While ARCNET is very effective in our setting, it is not without its weaknesses. ARCNET is designed to be invisible to the user, which it usually is; when there are problems, however, it is also rather invisible to the person trying to diagnose the problems. Most reference manual suggestions amount to “check the file processor” but give no explanation of what to check for or how to check for it. A good diagnostic manual and software is very much needed.

Another weakness results from the inability of Profile Plus or BASIC to share the same file between two programs. Once a program has claimed a file, even just to read it, no other user can access it. If a second user attempts to open a file which is in use, a “File Busy” error message occurs. Since most of our users are working on different tasks, this is not a serious limitation; when it does occur, a five or ten minute wait usually solves the problem.

A third difficulty arises in that ARCNET is slow in reading the directory for the “public files” at the file processor; the wait can seem rather long if a program needs to read the directory many times. A new, faster version of ARCDOS would be a major improvement in the system.

Conclusions

All in all, ARCNET has been effective in our setting. Already the system has grown far beyond the capabilities of XENIX. With Profile Plus, SCRIPSIT, and BASIC all on ARCNET, we have far outgrown the biggest XENIX system and we have not needed to make any changes to our already existing data bases. By using DO files to automate the connection of a user to ARCNET, only the most minimal training has been necessary for an operator to move from a stand-alone computer to ARCNET.

ARCNET is indeed a powerful system with many capabilities and an enormous capability of expansion. Certainly, not every situation needs to network computers, and even fewer situations need to network as many computers as we have. For places with the larger number of computers to network, ARCNET is effective and workable.

Tandy has developed a sleeping giant of a local area network system. A few improvements may help to awaken the giant to even greater power.