Digital controllers - Intelligent Buildings and Building Automation - page 15

3 Address B is located by the microprocessor on the address bus.
4 The microprocessor control unit sends a signal via the control bus to write
the data to address B.

(Note that these steps take place at extremely high speed within the
computer.)
As the unit of information or data is a bit which corresponds to one-eighth
of a letter, a large amount of binary data is required to represent quite a small
amount of what we perceive as information. Data can be 'packed' into data
bytes or words .
Processors transmit information between different memory registers
(or component parts) by manipulating the bits of either a byte or a word
simultaneously, which is called parallel processing . Data bytes or words are
transferred within the computer in parallel between memory registers or
devices along multiple cables of the buses as the data highway.
The units in which the data transfer speed is usually specified are:

Baud = 1 bit/second OR kBaud = 1000 bits/second

2.2.3 Memory

Much of the data and the program instructions are stored in the memory
chips. The microprocessor has only a small temporary store, which oper-
ates extremely quickly. Therefore larger and consequently slower operating
storage, in the form of separate memory chips, is used. There are two types
of memory chips: read only memory (ROM) and random access memory
(RAM). For microprocessors the 16-bit address bus allows 2 16 (65,536 =
64K; 1K = 1,024 = 2 10 ) address locations to be handled. These days, address
buses of computers have a greater number of bits, allowing much larger
memory capacity.
The ROM chip can send only data or instructions. It cannot receive and
store them from other chips or I/O ports in the outstation; this memory unit
can only be 'read from', not 'written to'. The ROM chip therefore contains
the manufacturer's program and data which the user cannot alter. The
program and data are 'burnt' into the ROM during manufacturing. As micro-
processors can deal only with binary signals, i.e., 1 s and 0 s corresponding
to high and low voltages, such as 5 V and 0.5 V respectively, the memories
simply have to store 1 s and 0 s.
The data in ROM is permanent after the chips are 'burnt'. Therefore, the
data will not be lost even if the chips lose power. But the data in RAM will
be lost in this case. Therefore, a battery is often used in BAS outstations to
prevent power loss to memory chips in the event of power failure. The essen-
tial functions of the outstation are written and stored by the manufacturer in
ROM chips. Some manufacturers also store the 'standard control functions'