LOGICAL DESCRIPTION OF THE MULTICHANNEL BUS

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FIGURE 3-2 Multibus system bus with Multichannel bus extension.

nologies can be taken. The standard interface allows the system designer to take advantage of VLSI interfacing integrated circuits.

A common problem in many system applications is that the I/O devices are physically separated from the processor's system bus by relatively large dis-tances. Normally this requires a specialized bus to be developed to communi-cate with these devices. The Multichannel bus has the added ability to link together I/O devices that are distributed over a distance of 50 ft (15 m) from the system bus.

3.2 LOGICAL DESCRIPTION OF THE MULTICHANNEL BUS

The Multichannel bus is a block-oriented DMA bus which, when used with the Multibus system bus, provides an architectural extension to the Multibus system

bus. Figure 3-2 is a diagram of a typical Multibus system utilizing the Multi-channel bus. The key features of the MultiMulti-channel bus are:

• Standardized controlled interface

• High bandwidth

• Distributed device support over relatively long distances

• Simple data transfer technique

The bus is capable of transferring data at a maximum rate of 8M bytes per second over 50 ft (15 m) of twisted pair flat ribbon cable. The Multichannel bus can support 16 devices with 16M bytes of memory space and 16M bytes of I/O space. Data widths for the devices can be 8- and 16-bit.

The data is transferred via an asynchronous handshake between devices.

Asynchronous transfers were chosen for the bus to allow communication among devices that vary in speed and distance from one another. Figure 3-3 shows an example of the Multichannel bus with several devices attached to it. In the illus-tration, device 1 is writing data to device 2. Device 1 signals to device 2 that data is valid after device 1 places data on the bus. Device 2 ensures that device 1 will hold the data valid until it has read the data. Once device 2 has accepted the data, it signals device 1 that it has done so. The importance of this hand-shake can be seen if device 1 can transfer data at 2M bytes per second and device 2 can accept data only at 1M byte per second. This interlocked hand-shake ensures that device 2 will receive all the data while not constraining device 1 to transfer data at that rate. If device 3 in Fig. 3-3 is capable of

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INTELLIGENT CONTROLLER BASIC TALKER/LISTENER

FIGURE 3-3 Block diagram of bus with supervisor, controller and basic devices attached.

84 THE MUL TlBUS FAMILY OF BUS STRUCTURES

ing data at a 2M bytes per second rate, device 1 could transfer at the higher rate when communicating with device 3.

3.2. 1 Bus States

In order to understand the bus operation, one must first understand the device states: the mode and activity level of a device at any given time during bus operation. The bus is based on a master-slave relationship in that a master ini-tiates the data transfer by some action on the control lines and a slave responds to this action. Referring back to Fig. 3-3, device 1 is the master and device 2 is the slave. In this example device 1 informs device 2 what type of data will be transferred and in which direction. Device 2 looks at these signal lines and decides whether it should receive or send data and when the transfer is to begin.

A master-slave approach was chosen to allow communication between devices that vary in speed and distance from one another. This approach requires a positive acknowledge interlocked transfer between devices. Its draw-back is that a device must synchronize to the acknowledge.

MASTER STATE

A device is in the master state whenever it is controlling the command-action lines on the bus. The master is responsible for addressing devices and determin-ing the length of the transfer. The Multichannel bus allows the bus mastership to be passed among the attached devices. However, only one master can be active at a time. In Fig. 3-3, device 1, the master, is responsible for addressing device 2 or 3 and controlling the data transfer. If device 2 or 3 is capable of bus mastership, device 1 may choose to move the mastership to either of the other devices.

SLAVE STATE

A device is in the slave state whenever it is monitoring the bus command-action lines. The slave is responsible for monitoring the bus for its device address. No action can be performed on the bus by a slave without direction from the mas-ter. A system can contain multiple slaves, each monitoring the bus for its address. However, only one slave can be actively transferring data on the bus at a time. In Fig. 3-3, devices 2 and 3 are the bus slaves. Each device will mon-itor the bus for its address being sent by device 1, the bus master. Once device 2 has been addressed for a data transfer, it will wait for the signal from device 1 to start the transfer.

ACTIVE STATE

A slave device is in the active state whenever it has been addressed for a transfer by a master. Only one slave may be active on the bus at a time. Bus masters are always active on the bus. In Fig. 3-3, device 1 is the master; therefore, it is

active. Device 2, the slave, will be inactive until device 1 addresses it for a trans-fer operation. Once addressed, device 2 will be in the active state.

TALKER ST ATE

A talker is any device which is writing data to the bus and signaling that its data on the bus is valid. Both masters and slaves can be talkers. Referring to Fig. 3-3, the master, device 1, will be the talker during the address cycle, since only the master can write addresses on the bus. However, if the slave, device 2, is addressed to write data to the bus during a data cycle, it will become the talker. The master also can be a talker during data write transfers.

LISTENER ST ATE

A listener is any device that is reading data from the bus and signals that the data has been accepted. Both masters and slaves can be listeners. Referring to Fig. 3-3, the slave, device 2, will be the listener during the address transfer from the master, device 1. When device 2 is addressed to write data to the bus, device 2 becomes the listener. When the slave is writing data to the bus, the master is the listener. When the master is writing data to the bus, the slave is the listener.

3.2.2 Bus Devices

The Multichannel bus supports three classes of devices. Each device has a dif-ferent function or responsibility on the bus. At a minimum the bus requires a supervisor type of device to control the bus and an additional device for the supervisory device to communicate with.

BASIC TALKER-LISTENER

A basic talker-listener device can write or read data to the bus but has no bus control capability. The basic devices in a system can be any combination of talker only, listener only, or talker and listener device. A basic device is a slave;

therefore, its data flow is directed by a bus master. Basic talkers-listeners are addressed by a bus master, and the amount of data is controlled by a bus master.

Typical basic devices are memory cards and simple I/O devices. Device 2 is the basic talker-listener for the implementation of the bus shown in Fig. 3-3.

Device 2 must wait for its address from the master, device 1, and must be told whether to read or write data. In this example device 2 has no bus control capa-bility; therefore, it will only receive the control signals.

BUS CONTROLLER

A bus controller, like the basic talker-listener, can read and write data to the Multichannel bus and is also capable of controlling the transfer signals and pro-gramming other devices on the bus. The bus controller appears as a slave on the bus until it is directed by the bus supervisor to assume mastership of the bus.

86 THE MUL TlBUS FAMILY OF BUS STRUCTURES

Normally, a bus controller is used in a system in which the supervisor cannot keep up with the data transfer rate or the system performance dictates that data be moved only once. Typical bus controllers are disk systems and high-speed I/O devices. Device 3 in Fig. 3-3 is the bus controller. When programmed by the supervisory device, device 3 appears as a slave on the bus; in this example, it is instructed to perform a transfer with the basic talker-listener device 2.

When instructed, it leaves its slave status and assumes mastership of the bus.

The transfer is then between devices 2 and 3.

BUS SUPERVISOR

A bus supervisor has all the properties of the bus controller and basic talker-listener. In addition, it has ultimate control of all data movement over the bus.

A supervisor is always the bus master unless it passes control to a bus controller.

On the Multichannel bus the supervisor is responsible for scheduling all data transfers, resolving and granting bus priority, monitoring bus status, and han-dling all bus interrupts. In a given Multichannel bus system there can be only one supervisor. In Fig. 3-3 the bus supervisor is device 1. In this example device 1 has control of all transfers on the bus. If device 1 requires the bus controller, device 3, to take mastership of the bus, the exchange will be under the control of device 1. Device 1 may regain bus control at any time.

At a minimum level a system would contain a supervisor, which would be the system master, and a basic talker-listener, which would be the system slave.

In Fig. 3-3 the minimum system would contain the supervisor, device 1, and the basic talker-listener, device 2, which is a slave.