Choosing DEC-compatible terminals Points to look out for

by CHRIS KING

M ost DEC plug-compatible ter- minals on the market today have one thing in common.

They were created with the intention of offering the buyer a viable alterna- tive to DEC systems both in terms of price and performance. Despite up- sets such as the arguments between DEC and suppliers of emulators of DEC VT200 terminals, which has sent some suppliers back to the draw- ing board, plug-compatible terminals should be available for a long time yet.

choosing terminals

Assuming the right to choose remains with buyers, how exactly do they go about selecting the right system and how do they arrange them to work properly?

In the case of most DEC-compat- ible equipment - and not just ter- minals - the options presented to users are generally devices which offer DEC emulation, plus a number of additional advantages. In some cases there will be a saving of up to 20%

Abstract: Plug-compatible terminals offer some advantages over those offered by DEC. When buying terminals, check for value fm money and reliability of the supplier. Consider how you want to cluster the terminals before purchase.

Keywords: data processing, p&compatible manufacturers, visual d~p~ay terminals.

Chris King is sales manager of Sinrrom Ekc- tronics.

against DEC’s prices. However, you have to understand

fully what the differences between DEC terminals and their emulators are and what the considerations are before the purchase. So, what do you look for in a pIug-compatible ter- minal?

As with anything, price largely determines what you will get, so it is important to evaluate performance against cost. Will you be getting good value for money?

The most important point is that cheapest is not necessarily best. The buyer must aim to achieve the best value for his or her company. The features and ~rformance included in the package cost vary slightly among manufacturers, but the following are reasonable prices to expect.

For around &500 you should be able to find a basic terminal which is limited to VT52 compatibility, with a one page display memory, 12 in, 80 column screen, a detachable keyboard and little more.

At around &850 you really start to see improvements in terms of per- formance and overall facilities. You should expect VT100 and VT52 emu- lation, with a swivel and tilt monitor, a choice of phosphor colours and a low profile design for maximum com- fort. Typical of this type of terminal is the Model T4, which is manufactured in the UK by Cifer.

At the El200 - &I 500 level of terminal you should expect muhiemu- lation characteristics encompassing VT521100 and VT200. The VDUS offer a larger screen size of around 15

ins, plus improved ergonomics and design features.

Graphics capability

However, the buyer must also take note of ~phanumerics and graphics, as well as keyboards. The terminal should incorporate a dual-plane dis- play, which should allow both alpha- numerics and graphics to be handled independently and also provide stan- dard RS232C printer and line ports.

In the alphanumeric mode, the buyer should have the option of a 132 or 80 column format and 24 lines with a 25th status line. Of particular im- portance is the screen memory, and one should expect to see as many as five or seven pages of scrollable dis- play memory. In addition, software loadable character fonts should be provided, as should the facilities to blink, embolden, underline, inverse, italic and highlight characters in any combination.

In the case of graphics, Tektronix 4010 and 4014 graphics compatibility is the norm, 1024 x 300 point resolution is typical and you should expect a graphics dump-to-pr~ter facility as well. Alpha, Plot and GIN modes should also be included in the price. Keyboards should be detach- able, lightweight and low-profile. In addition, a local editing facility should be provided as should the provision of multiple user loadable function keys.

Ergonomics

The most recent development of ter- minal technology is, not surprisingly,

32 0011-684X/85/060032-03$03.00 0 1985 Buttenvorth & Co (Publishers) Ltd. data processing

systems

ergonomics. People are now spending a large part of their working days sitting in front of screens. A number of additional ergonomic features are

beginning to creep in, apart from tilting and rotating screens, display intensity controls, detachable key- boards and textured non-reflective

screens in a bid to respond to the criticisms levelled at VDUs and their

regular use. The prime hazard is eye strain

brought about by the actual colour of

the display. IResearch has shown that

some colours, notably green and

white, induce fatigue over long periods. In Sweden there are now laws banning the manufacture of ter- minals with anything other than an

amber display.

Clustered terminals

Having established what constitutes a good DEC-compatible terminal, buy- ers must also look at how they can arrange them to work efficiently, and must therefore look at how to cluster

terminals around DEC systems. There are many reasons for con-

sidering clust.ering, such as to reduce terminal cabling or computer expan- sion, or to provide access to a multi- host environment or fault tolerant reliable communications, or simply to increase system security.

There are a surprising number of alternative solutions, each of which has different Imerits. The various solu-

tions offered by different companies specializing in data communications, DEC includeId, can be categorized as:

l communications and statistical

multiplexers, l port selectors and data switches, l local area networks, l front-end processors.

But whatever method is chosen for a particular application, it is essential that it provides a complete, compre- hensive and cost-effective solution to

the problem.

~0127 no 6 julyiaugust 1985

The use of communications multi-

plexers has the advantage of offering the lowest cost per additional line, providing that access to only one host computer is required and it is in- tended to locate the terminal cluster within a maximum of 300 ft of the

computer room. Statistical multiplexers can be used

to combine several users on a single concentrated line, thereby effectively reducing total line costs. Statistical

multiplexers do not negate the need for multiplexer boards in the host

computer, so they therefore have the same expansion requirements and need all the same distribution panels and cable connections. In effect, the statistical multiplexer is only a suit- able solution when communications are limited to one host computer, unless used in conjunction with a data switch, and if the terminal cluster is to

be located at another site or even in another town.

‘Switch’ or ‘port selector’ are the

names normally given to units which act as wiring multiplexers, such as data switches, terminal switches, data PABXs and data concentrators. They enable different terminals to be switched to different hardware ports on one or more host systems.

There are several attractive features of data switches worth mentioning, particularly their ability to provide load balancing and port contention as standard. Terminal user queueing is available, as are gateways to remote

networks, with multicomputer ven- dor support over RS232 links pro- vided, as well as the support of both synchronous and asynchronous ter-

minals . A further alternative approach to

the problem of clustering terminals around DEC systems is the adoption of a communications front-end pro- cessor to offload the communications protocol overhead of one or more host processors.

Despite the try standard, also provide a

lack of any real indus- LAN technology can solution. The primary

purpose of a LAN is to provide resource sharing while reducing over- all system expansion overheads and

cabling costs. However, system

length, and consequently communica- tions, are usually confined to less than

5 km although it will provide a moderate to high data rate for com- munications of l-20 Mbit/s.

Naturally, DEC has products on offer for whatever approach best suits your application. In addition, there are over 20 suppliers with DEC host- compatible products capable of sup- porting asynchronous terminals , which offer various solutions as dis-

cussed above. There is one product, the Able

Attach, which defies categorization. It combines the best features of the data switch, multiplexing and long line drive capabilities. Most important, it

needs only a single host interface board to the DEC computer. It can:

offer the lowest cost per line for

clustering terminals t.han any of the other methods, maintain data throughput at ter- minal speeds without sacrificing any long line capability, if it is needed, save much needed space in the computer room, eliminate cabling congestion.

So, as can be seen, there are a variety of alternative approaches to the prob- lems of clustering terminals around a DEC system, all of which have rela-

tive merits and demerits in the way in which they provide a solution.

Suppliers

When choosing DEC-compatible terminals do not forget the reliability of the supplier itself. After all, you will want to be sure that your sup- plier, whether manufacturer or distri- butor, is going to be able to support the product you have just purchased.

Look for companies which specia- lize in DEC plug-compatible equip- ment, with dedicated sales forces

33

systems

The Able Attach. A typical configuration for up to 184 terminals comprises three Attach subsystems interconnected with each other and connected to three Unibus The decision-making process should

computers via three host interfaces, providing much improved flexibility by virtue of be made easier through elimination.

a full backup facility in cases where the main computer fails. Any of the 184 cl

terminal users may be assigned dynamically to any of the computers although no more than 128 uws can be assigned to any one computer

Sintrom Electronics, Arkwright Rd, Reading, Berks RG2 OLS, UK.

which are actually able to discuss technical data and applications in depth.

There are then, advantages in buy-

ing DEC plug-compatible equipment - terminals in particular. They exist for the good of the DEC user, who deserves the right to choose the best for his or her company.

Yet, how, with such a wide choice of alternatives, does he or she make

the best decision? It is not easy, but look at:

l price versus performance, l availability, l suppliers’ reputation, l suppliers’ support capability, 0 ergonomics l versatility.

microprocessors and microsystems

Special Issue on aids for visually handicapped - December 1984

There are 44 million blind people in the world - but only 20,000 electronic aids known to be in use. What is the reason for this shortfall: lack of production facilities, lack of funds, or lack of interest? What are the advances in technology - so beneficial and even vital to the business community - contributing to the job prospects and private lives of the visually handicapped?

These, and other questions about provisions for the visually disabled are addressed in this special issue of Microprocessors and Microsystems. For further information contact:

Anne Browne, Editor, Microprocessors and Microsystems, PO Box 63, Westbuy House, Buy Street, Guildford, Surrey GU2 5BH, UK. Tel: (0483) 31261 Telex: 859556 SCITEC G

data processing