Nonprogrammable devices

GDDM V3R2 System Customization and Administration

Nonprogrammable devices

GDDM V3R2 System Customization and Administration
SC33-0871-02

In many ways, devices such as the IBM 3179-G, 3192-G, and 3472-G are equivalent to a workstation running in non-retained mode. GDDM sends individual vector commands to the device and the device performs the vector to raster conversion to determine which pixels to activate to show the picture.

The device, however, has only one default vector-symbol set and has only four image symbol sets, so symbols have to be rastered out by GDDM. It also has no local clipping capability, so GDDM may have to do extra clipping in the host (for example, when a page is scrolled or an operator window is sized). GDDM tries to minimize the amount of the picture that needs to be updated and will only redraw a small section of the screen if this is possible.

Dot-matrix printers such as the IBM 4224 and IBM 4234 are also in the nonprogrammable device category. The vector-to-raster conversion process is performed in the printer, and so a 4224 printer generally requires less host-processor resource than, for example, a 3287 printer.

Because the paper moves through the printer in only one direction, all the GDDM data that describes the picture must be sent to the printer before the rastering process can begin. The 4224 has no resident vector symbol sets, so all graphics text referencing such symbol sets must be expanded into its constituent lines and arcs by GDDM. GDDM does not send arcs to the device. They are split into short vectors, resulting in more host processing and longer data streams.

Plotters such as the IBM 737x and 618x are also in the nonprogrammable, order-driven class.

They are similar to the programmable devices in that they can translate orders into pictures. However, the set of orders that they can draw is limited:

Area fill is done by sending vector definitions of each of the lines that make up the shading pattern.
Arcs are drawn as several very short vectors. Each vector is drawn by moving the pen by no less than half a pen width.
Symbol sets are expanded into vectors in the host.

Because anything drawn on a plotter stays drawn, overlaid segments need to be processed by GDDM before the drawing orders are sent to the plotter so that hidden parts of objects are not drawn. This process is referred to in GDDM as reverse clipping, and applies only to GDDM support of these plotters.

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