A Computer Aided Design (CAD) package superficially appears similar to many drawing packages found on the Beeb. However, a CAD package stores information about each mark on the screen rather than relying on the screen data itself. For example, a straight line would be stored as two sets of co-ordinates together with a code to distinguish the type of line used. Simple art packages just produce the marks on the screen and then save the screen RAM.

A Beeb CAD package may not compete with a professional system designed to run on a much more expensive system. However, for a small outlay a CAD package on a BBC micro is ideal for tasks such as designing circuit boards, and many others.

A good CAD package has several features which distinguish it from those of lesser quality. Accuracy is important, particularly if the package allows parts of drawings to be magnified. In addition, a CAD package should be flexible and easy to use.

We will look here, at three CAD packages for the Beeb, one quite recent introduction (from Technomatic), and two aimed particularly at the educational market (Educad and Micad).

Micad is the oldest (1984) of the software reviewed here, and is supplied on disc. It is really two programs in one; Micad 2D is used for conventional CAD work whereas Micad 3D is used for 3D wire frame images. Micad 3D is not really a 'CAD' package at all, but it is a nice inclusion for the exploration of technical drawing.

All the programs that go to make up Micad (except the printout routines) are written in Basic. This makes the whole package rather slow and flickery on the mode 4 screen used.

The different drawing options of Micad 2D are selected with the cursor from a menu of single letter commands down the side of the screen. Each command is followed by a number, again Selected with the cursor. This process is very tedious. If single letters and numbers are to be used, it is much easier and quicker to enter them from the keyboard. Micad has commands for drawing straight 1lines and circles (full or dotted), and hatching (shading) areas with one of five patterns. Text can also be added to a diagram and parts of the screen 'zoomed' for detailed work.

For editing a Micad 2D drawing a separate editing program is entered. This allows any point, line or arc to be moved or deleted. Again, the unfriendly nature of the menu system and the slow operation of Micad makes editing a process to avoid whenever possible.

Another program module allows the easy addition of linear and angular dimensions to the diagram. This is a useful feature and the only really professional facility of the package.

Once a diagram has been created and edited to your satisfaction it can be printed out. Micad can cope with both simple screen dumps to Epson compatible printers, and plots to Penman and Hewlett-Packard plotters. Although the manual doesn't cover it, it would be reasonably easy to link in your own screen dump routine.

Micad 3D is similar to 2D in operation, although movement and drawing is now in three dimensions, projected on the screen. The direction of view of the object is selected from preset directions on the function keys or keyed in as three-dimensional co-ordinates. A useful feature is the 'extrusion' facility which copies a shape in one plane to another, and joins the vertices to make a solid.

The 3D program uses single key commands from the keyboard - much better than 2D's menus. However, 3D is limited and the editing crude. It is easily outshone by Interactive 3D or 3D Zicon reviewed in Beebug Vol. 4 No. 10 and Vol. 4 No. 9 respectively.

The whole Micad package seems a strange combination of different ideas bundled together without much thought as to continuity or actual use.

Although Educad is designed primarily as an introduction to CAD principles for schools, it has a decidedly homemade quality. The software is supplied on disc with a very poor 50 page manual. This lacks any useful structure and, for a product aimed at the educational market, this is a disaster.

Once the manual has been digested sufficiently to start using the package (it takes 6 pages to find CHAIN"EDUCAD" is needed), the Educad package is little better than Micad. Most of the software is in Basic, and again it is slow. Educad pictures are produced in mode 4. The cursor is moved with either the cursor keys or joystick. The cursor accelerates when a cursor key is kept pressed down. This makes long movements fast but still allows delicate manoeuvring.

Like Micad, the diagram drawing process is menu-driven and suffers accordingly. There are levels of sub-menu as well, and it is quite easy to get lost in the menu structure.

Educad produces the requisite straight lines and arcs based on construction points and lines on the diagram which can be displayed or hidden as required. A large range of options for specific operations are catered for (e.g. drawing a line as a tangent to an arc), and all work on the diagram is automatically aligned with a (variable) grid to make the final drawing more accurate. For delicate work, a zoom facility is also provided, but the grid alignment makes work difficult at high magnifications.

Menus aside, editing is reasonably easy. The cursor is moved over an element and the command to delete that element type issued. The software works out how to remove it from the drawing store.

Text of any size can be added. This is not just Beeb text but letters drawn out on the screen. Similarly, a range of preset symbols such as electronics components can be repeatedly pos1t1oned on a diagram. A character definer is supplied to produce your own fonts and symbols. For computer aided manufacturing (CAM) facilities, Edusoft also have a 'EJuCNC' package to take designs created with Educad, and generate data suitable for a CNC (Computerized Numeric Control) machine to actually make objects.

Educad uses a much more professional approach to producing technical drawings than does Micad. However, it costs over twice the price. For nearly £100 the quality of presentation of this program is well below standard.

Novacad is also expensive. It requires both a 16K ROM and a disc. Novacad is the best of the bunch, both in terms of its ease of use and its flexibility.

Drawings can be in modes 6, 1, or 4 depending on the memory available. Novacad can use B+ and Master shadow memory and an Aries board.

The screen shows only a twentieth of a complete diagram at a time. It is a window onto the complete picture. You can move between windows by pressing the Ctrl key along with a cursor direction key. The Novacad cursor is not the small 'x' of the other packages but a horizontal and vertical line running right across the screen, as used in most professional systems. This is moved with the cursor keys in large steps and can be slowed down with the Shift Lock key for fine work. In the mouse version of Novacad, control is much finer all the time.

There are eight 'levels' of a drawing, which can be displayed or hidden as required. The other levels can be used for construction lines or for, say, the reverse side design for a circuit board.

All the picture elements - line, rectangle and arc - are easily produced with a few easy-to-remember single-key commands. A useful feature is the recall of the last element at another position with the Copy key.

Novacad also has a zoom facility, but the most useful feature is the 'icon' system. Predefined sections of a diagram can be called up and deposited at any size and orientated four ways onto the screen. Even more useful, a section of the screen can be captured as a icon, stored to disc and then re-used as often as needed.

Text can be added to the diagram. Although this is only in the BBC micro's normal font and only in one size, the characters can be rotated.

As with Educad, deleting lines is a question of pointing at them with the cursor. However, Novacad uses just the Delete key to do this without the long menu sequences of Educad.

The one failing of Novacad is its production of hard copy. Only a simple screen dump is provided for dot matrix printers, but Novacad can drive a plotter to provide drawings of a high quality for those with access to such devices. This aside, Novacad is excellent. It is flexible, genuinely easy to operate without much practice, and a pleasure to use.

If money is of prime importance then Micad must be your choice. It is considerably cheaper than the others. However, you'd probably be better off with a good art package. For all the genuine CAD facilities you are likely to see or want on a BBC micro Novacad performs excellently, albeit for a high price.