Independent Request Control

preset method decides the order in which the devices can use the computer’s bus. Refer to figure 7-16 as you read. An I/O controller of some type will correspond with the external devices. When an external device requests control of the bus, it signals the controller by activating the common bus request line. The devices on the line have ORed connections. The controller acknowledges the use of the bus on a separate line. The I/O controller will scan the chain with an acknowledge signal until it reaches the external device that requested the bus. The external device stop further propagates the acknowledge signal and accesses the bus. When two or more devices request control of the bus, the external devices closest to the I/O controller will be granted access to the bus first. Thus the order of connection on the daisy establishes the priority of which external devices are given access to the computer’s bus. Independent Request Control Independent request control (fig. 7-17) offers a faster and more flexible way to the control bus requests. In this method, separate lines are used for the request and acknowledge lines. The I/O controller assigns priority to each external device, which can be fixed or programmable. A combination of the two methods produces greater flexibility when dealing with simultaneous requests, particularly when dealing with interrupt driven I/O. When signaled on a common interrpt request line, the CPU can poll all external devices in a predetermined order to find which external device needs to be serviced. This method is entirely software. Generally speaking, computers that use a request and acknowledge system, prioritize the functions and the channels. Some of the functions, in descending order, include the following: External Interrupt External Function Output Data Input Data The channels/ports are also prioritized. Equipments are assigned a channel/port and usually the channel with the highest number will be serviced first by the computer. Figures 7-14 and 7-15 apply. I/O INTERFACING STANDARDS There are two major types of computer/extemal equipment communication formats: serial and parallel. The communication formats are governed by the standard that is identified by the interface. The interfacing standards provide valuable information. As a general rule the standards can be divided into four categories: mechanical, electrical, functional, and procedural. The standards can provide other standards that must be adhered to but do not fall into anyone of these four categories. Mechanical —The mechanical portion takes into account such things as the type of connectors to be used, the number of pin connections in the connectors, and the maximum cable lengths allowed. Electrical —The electrical characteristics include the allowable line voltages and the representations for the various voltage levels. Functional —The functional interface specifies such things as which signals—timing, control, data, or ground leads—are to be carried by each pin in the connector. Figure 7-16.—Connecting external devices in a daisy chain. 7-18