Signal multiple driver

When there are multiple drivers on a net, a resolution function is needed to resolve the final value on the net. In VHDL, one can create resolution functions and associate it with a datatype or a signal object. std ulogic is an example of an unresolved datatype, while stdjogic is an example of a resolved datatype. The resolution function is used to figure out the final value on a signal when there are more than one driver on it. In Verilog, one can t have user defined resolution functions. Instead one can use net types with built in resolution functions (pre-defined in Verilog) such as wand, wor, and so on. 

A signal widi multiple drivers must be of a type that can be resolved. 

When synthesized, multiple drivers for one signal are connected together in one of three ways ... 

The output of the adder is connected to the internal bus D. As the multiplier can also drive this bus, the output must be made three state. Only when the ADD, SUB or CMP instruction signals are active should the result be placed on the bus. The use of three-state logic to handle signals with multiple drivers was introduced in Chapter 7. 

Another upgrade of a signalling system. Milton Keynes is on a 200 km/hr railway with an intensive train service. On the first morning after the modification work was completed, a signal wrongly showed green when the track ahead was occupied. Fortunately a train driver noticed the anomaly. An accident with almost certain multiple fatalities was only just averted. 

Conventional mirrors are very limited in their flexibility and are used primarily for the display of a single direction view. The trend in the automotive industry however is moving to modem display and driver information systems, based exclusively on digital displays because their unlimited flexibility allows the display of all types of information, previously unthinkable for drivers and passengers. The flexibility of such systems can be further enhanced if the display is coupled with multiple camera signals. 

To provide multiple graphic display of radiation levels at different locations in the reactor hall, a signal driver circuit was developed. The driver, which can be used with CGA, EGA and Hercules (monochrome) adapter cards, allows two or more monitors for IBM PC and compatibles to be driven by a single video signal. The multiple display circuit consisted of several octal, tristate, non-inverting buffers. The buffers had low power dissipation and were permanently enabled by an on-board 5V dc power supply. The TTL levels from the computer video signal were applied to the inputs of the buffers and the outputs were fed to the video monitors. 

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