5
A N e t w o r k o f R e a l - T i m e Mini C o m p u t e r s
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WILLIAM J. LENNON Computer Sciences Department, Northwestern University, Evanston, Ill. 60201
INTRODUCTION The principal justification for having a mini or micro computer is the cost effective control and monitoring of real-time systems. A second justification is the relative ease with which independent, dedicated computers can be moulded to fit particular requirements. A common problem which can arise, however, is that when care is taken to improve user interaction or attention is paid to facilitating new program development, expensive additional resources are required. We solved this problem by building a flexible, resource distribution network to avoid two problems inherent in the routine procedure of expanding the peripheral complement of dedicated computers. First, because the truly expensive resources -printers, access to the University computer, etc. -- are required only intermittently, these resources will be used inefficiently and generally cannot benefit from economy of scale considerations. Second, and most important, each installation must invest in maintenance and development personnel eventually competing with one another for precious personnel support funds. It has been our experience that we can realize most of the economy of scale inherent in being a computing center without including most of the problems normally associated with use of such an installation* The network has been used routinely since the summer of 1971* The Computer Science Laboratory at the Technological Institute of Northwestern University is unusual, primarily because it was designed and is maintained, for the convenience of its users* It is a network of mini-computers, which has been designed to provide to remote computers easy access to commonly used peripherals and to the University's CDC 6400 and to support research in distributed resource computing (see figure 1)* The continuing growth of the system, and occasional changes that occur, are 67 Lykos; Computer Networking and Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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e f f e c t e d with minimum user inconvenience, as changes to the system do not r e q u i r e concomitant changes i n every e x i s t i n g program* Laboratory usage i n v o l v e s both real-time computing and computer system reseach* With few exceptions, software and l o c a l l y designed hardware are the work of volunteer or student p r o j e c t labor and t h e i r c o n t r i b u t i o n to the success of the l a b o r a t o r y cannot be overemphasized* They, i n t u r n , receive experience a t a nuts-and-bolts l e v e l not a t t a i n a b l e elsewhere* The s t i m u l a t i n g atmosphere of the l a b o r a t o r y f o s t e r s the development of these e n t h u s i a s t s who spend hours of t h e i r own time on p r o j e c t s f o r f a c u l t y members or fellow students* Their volunteer l a b o r , i n a d d i t i o n t o c e r t a i n design features of the system, make the network operation exceedingly economical* T e c h n i c a l l y , the laboratory houses a star-shaped network of real-time mini-computers, which i s interconnected to remote computers, most of which are DEC PDP-8 computers, although there i s no r e s t r i c t i o n as to computer type* The i n t e r c o n n e c t i o n hardware t r a n s e e i v e s characters using asynchronous s e r i a l communication a t a s u b s t a n t i a l rate (about 14,000 characters per second) with the r e c e i v i n g computer c o n t r o l l i n g the transmission of each c h a r a c t e r * This r e c e i v e r c o n t r o l e l i m i n a t e s the software overhead normally associated with such high speed communication by e l i m i n a t i n g the need f o r complex message handling and character s t r i n g b u f f e r i n g * Also, u t i l i t i e s and g e n e r a l l y needed programs are w r i t t e n to run i n the remote, rather than the c e n t r a l computer* The " i n t e l l i g e n c e " of the network i s thus d i s t r i b u t e d throughout the system, considerably s i m p l i f y i n g the maintenance and minimizing the impact of evolving s e r v i c e s and f a c i l i t i e s * The advantages of i n t e r c o n n e c t i n g a computer to the network are manyfold* The computer gains the a b i l i t y t o perform both q u a l i t a t i v e l y and q u a n t i t a t i v e l y beyond i t s own c a p a b i l i t i e s by using Network Central resources* The cost per computer i s very low — l e s s than that f o r paper tape equipment, while only under heavy load c o n d i t i o n s does i t s performance degrade to paper tape speeds* Interconnection hardware and software overhead i s minimal and the i n t e r c o n n e c t i o n i s e a s i l y accomplished* Many of the remote computers have only an instrument i n t e r f a c e , a t e l e t y p e console and a pseudo paper tape connection to the network* In t u r n , each interconnected computer strengthens Network Central by being a v a i l a b l e as a d i s t r i b u t e d i n t e l l i g e n c e resource to support other computers, when i t i s not otherwise being used* The c e n t r a l i z e d c l u s t e r of computers and p e r i p h e r a l s i s r e f e r r e d to as "Network C e n t r a l " t o d i s t i n g u i s h i t and the programs r e q u i r e d i n i t s operation from the "network" programs, which are g e n e r a l l y run i n remote computers that are supported by Network Central*
Lykos; Computer Networking and Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
5.
LENNON
Real-Time
Mini
Computers
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PROCEDURES
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Objectives The goals of the network were to f a c i l i t a t e the design and implementation of new programs as well as g e n e r a l l y provide the necessary enhancement to dedicated, real-time computers to make them more f l e x i b l e , a c c e s s i b l e and g e n e r a l l y e a s i e r to use. These goals were a t t a i n e d by meeting the following four design o b j e c t i v e s both for l a r g e r machines and minimum c o n f i g u r a t i o n c o n t r o l computers which b e n e f i t most from access to shared resources. 1. Create an economical system by pooling expensive resources and by p r o v i d i n g easy access to i t with minimum hardware cost and minimum software overhead. 2. Run a l l e x i s t i n g programs without m o d i f i c a t i o n . 3. Provide a monitor which, while transparent to most programs, would provide f u l l access to a l l network resources for newly developed programs. 4. Evolve new f a c i l i t i e s without i n t e r f e r i n g with e x i s t i n g facilities. These o b j e c t i v e s have been met for both the minimum c o n f i g u r a t i o n c o n t r o l computer, which b e n e f i t s the most from access to shared resources, and the l a r g e r c o n f i g u r a t i o n computers.
Network Hardware
The key hardware for the network i s a u n i v e r s a l s e r i a l i n t e r f a c e [1] which by s u i t a b l e l o g i c card or connector changes w i l l transceive 11-unit, 8 - b i t s e r i a l characters at rates between 110 and 153,000 baud i n t o current loops, EIA RS232 i n t e r f a c e s , c o a x i a l cable or twisted p a i r transmission l i n e s . The hardware uses a reverse channel permission to send s i g n a l (see f i g u r e 2). Thus, i r r e s p e c t i v e of baud r a t e , the r e c e i v i n g device or computer d i c t a t e s character r a t e . Time c o n s t r a i n t s are e f f e c t i v e l y removed from high-speed communication with an attendant s i m p l i f i c a t i o n of the required software. The problem of character overruns has been e l i m i n a t e d . The reverse channel i s used by each r e c e i v e r to grant i t s corresponding t r a n s m i t t e r permission to send one character. In our c o n f i g u r a t i o n , four twisted p a i r interconnect computers using differential line driver/receivers. Commonly used p e r i p h e r a l s are i n s t a l l e d along with a s i x m i l l i o n word, moving head d i s k on a c e n t r a l machine. Time c r i t i c a l or temporary p e r i p h e r a l s are i n s t a l l e d on two a d d i t i o n a l c e n t r a l
Lykos; Computer Networking and Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
COMPUTER
NETWORKING A N D CHEMISTRY
CDC 6400
(phone)
4k PDP-8/E
Network Central 8K PDP-8/I— (Graphics)
4K PDP-8/L (Editing) —32K PDP-8/E (Teaching Lab)
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12K PDP-8/E(Graphics)
Imlac PDS-1 Display
—VARIAN 73 (Microprogramming) —PDP-11 Network ( P r o t e i n Sequencing)
PDP-9 (Bio Med) D17B (Missle) 4K PDP-8/E(EE)
— 8 K PDP-12 (Chem* Eng*) 8 K PDP-8/E 8K PDP-8/E -8K PDP-8/E ( M a t e r i a l Science)
8 K PDP-8/E8 K PDP-8/E(Freshmen) [DG NOVA 8 4 0 ] — [RAYTHEON 704]lChemistry)
— 8 K PDP-8/E (Bio Med)
4K PDP-8/M— (Interfacing)
--Intel Intellec-80 (Micro- computer)
Present Configuration of the Computer Science Research Network [Systems scheduled f o r 1975 expansion are i n brackets] Figure 1.
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