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