2．Centralized or Decentralized Control
The control of a general-purpose bus can be either centralized or decentralized．The basic requirement is to grant or not grant a device access to the bus．With centralized or decentralized control，all devices are treated equally except for their priority of access．Thus，if one of the devices is the processor，it may be given the highest priority for bus access．However，in some systems an I/O device may have the highest priority because of the loss in system performance；for example，if a disk is unable to read and misses a complete revolution，many milliseconds will be lost．[2]
Centralized Control．A single hardware control unit will recognize a request and grant access to the bus to a requesting device．It is the responsibility of the controller to resolve simultaneous requests and assign priority to the requests．At least three designs are used for centralized controllers：daisy chain，polling with a global counter，and polling with local counters．
Distributed Control．Distributed control，also known as decentralized control，distributes the control function between all the devices on the bus．The major advantage of decentralized control is that the system is easily expandable by the addition of modules．As with centralized control，there are three basic designs：daisy chain，polling，and independent requests．
3．Synchronous or Asynchronous Communication
The transmission of addresses，control information，and data between two devices may be synchronous with a clock，or asynchronous without a clock and self-timed．
Synchronous Communication．A simplified diagram of a synchronous bus（the data and clock portion）connected between two devices is shown on the left of Fig. 1-19．Data are to be transmitted between the card in the right-hand slot to the card in the left-hand slot．The transmitter and the receiver are clocked from a common source on the left-hand card．
Asynchronous Communication．Asynchronous communication for buses was developed to overcom the worst-case clock rate limitations of synchronous systems．With asynchronous communications，data transfers occur at the fastest rate and smallest delay possible under the circumstances of the physical status of the bus．As cards are added to the bus，the timing automatically adjusts for each card．There are a number of asynchronous protocols；however，only one of the simpler ones is discussed here．
The timing diagram for an asynchronous exchange is shown in Fig. 1-20．This figure illustrates the action when a source sends data to a destination．
2．集中式或分散式控制
通用总线的控制既可以是集中式，也可以是分散式。其基本要求是授予还是不授予设备对总线的访问权。除了访问总线的优先级以外，集中式和分散式控制对待所有的设备都是一视同仁的。因此，如果设备是处理器，则应给它以总线访问的最高优先级。但是在某些系统中，为避免对系统性能的损害，让某种I/O设备具有最高优先级。例如，磁盘若转动一圈内还读不出数据，则损失很多时间。
集中式控制。用一个控制器硬件去识别总线请求并允许请求设备去访问总线。该控制器的责任是处理同时来的多个请求并对这些请求安排优先级。集中式控制器至少有3种方式：菊花链式、带全局计数器的轮询和带局部计数器的轮询。
分布式控制。分布式控制又称分散式控制，它把控制功能分布在总线上的所有设备中。分散式控制的主要优点是通过增加模块使系统容易扩充。与集中式控制一样，分散式控制也有3种基本方式：菊花链、轮询和独立请求。
3．同步或异步通信
地址、控制信息和数据在两个设备之间可以用一个时钟同步传送，或不用时钟，而用自身的定时器实现异步传送。
同步通信。连接两个设备的同步总线（数据和时钟部分）简化框图如图1-19（左）所示。数据从右边槽中的卡传向左边槽中的卡。发送器和接收器用左边卡上的公共时钟源同步。
异步通信。总线的异步通信是为了克服同步系统中极差的时钟频率限制而开发的。异步通信中，数据在总线的物理状态环境下以最快的速率和最小的时延传送。当一些卡加到总线上时，会自动为每一卡调整时钟。异步协议有很多种，这里只讨论比较简单的一种。
图1-20为异步交换时序图，该图说明了当发信方向接收方发送数据时的工作过程。