Digital controllers - Intelligent Buildings and Building Automation - page 14

Data bus

Address bus

Store

RAM
chip

EPROM
chip

ALU

Control
unit

Input unit Output unit

Control bus

Actuators

Sensors

Figure 2.4 Architecture of an outstation.

components: the microprocessor (CPU), the memory, and input and output
(I/O) units. All the operations (e.g. data, instructions and address signal trans-
mission) are performed via three buses: data bus, address bus and control
bus (bus is an abbreviation of busbar).
Figure 2.4 illustrates how the microprocessor chip of a control station is
connected to the memory unit and the input and output units. In fact, all
units have their own microelectronic chips. Connection between the micro-
processor and the other chips is via the three buses . Each of the buses is a set
of parallel wires, typically printed on the motherboard.
The data bus is for the transfer of data between chips; e.g. transferring the
sensor temperature from the input unit to the memory. The address bus is
for locating where the memory or register of the required data is, or where a
program instruction is located. The address appears like a telephone number
or IP address. Each unit of data stored in the memory unit and all the devices
connected to the buses must have an address.
To send data or instruction from an address (A) to another address (B),
the following steps will take place.

1 Address A is located by the microprocessor on the address bus.
2 The microprocessor control unit sends a signal via the control bus to take
the data from address A to the data bus.