Information Technology Reference
In-Depth Information
Data/addressing
line
Computer bus type 1
Bridge
Bridge
Computer bus type 2
Bridge
Processor
Bridge
Processor
Computer bus type 3
Bridge
Bridge
Handshaking
line
Direct-connect
bus
Figure 2.4
Computer bus connections
Figure 2.5 shows a simple form of handshaking of data, from Device 1 to Device 2. The
sending status is identified by READY? and the receiving status by STATUS. Normally an
event is identified by a signal line moving from one state to another, this is described as
edge-triggered (rather than level-triggered where the actual level of the signal identifies its
state). In the example in Figure 2.5, initially Device 1 puts data on the data bus, and identi-
fies that it is ready to send data by changing the READY? line from a LOW to a HIGH level.
Device 2 then identifies that it is reading the data by changing its STATUS line from a LOW
to a HIGH. Next it identifies that it has read the data by changing the STATUS line from a
HIGH to a LOW. Device 1 can then put new data on the data bus and start the cycle again by
changing the READY? line from a LOW to a HIGH.
This type of communication only allows communication in one direction (from Device 1
to Device 2) and is know as simplex communications. The main types of communication are:
•
Simplex communication
. Only one device can communicate with the other, and thus
only requires handshaking lines for one direction.
•
Half-duplex communication
. This allows communications from one device to the
other, in any direction, and thus requires handshaking lines for either direction.
•
Full-duplex communications
. This allows communication from one device to another,
in either direction, at the same time. A good example of this is in a telephone system,
where a caller can send and receive at the same time. This requires separate transmit and
receive data lines, and separate handshaking lines for either direction.
