Digital controllers - Intelligent Buildings and Building Automation - page 20

Output Band

R =

21

(2-2)

DA / n

−

The output signals from the BAS control stations or D/A converters are con-
trol signals only. If large voltage or power, AC and DC, is needed to drive a
device, a voltage or power amplifier is needed. Opto-isolation (transferring a
signal via a short optical path, thus keeping the elements of the circuit electri-
cally isolated) is often included in the output unit to protect the low-voltage
microprocessor signal circuitry from any larger voltages. This prevents dam-
age caused by the powerful driving circuits.

2.5 Processor operation and software

2.5.1 Machine and assembler languages

Different levels of language can be used to communicate instructions to a
computing system. Typical examples are as follows:

￿ machine language (expressed in machine code);
￿ assembler language (expressed in mnemonics);
￿ high-level languages (expressed in codes closely related to English
words).

An instruction consisting of sequential executions for the processor is
represented by a unique set of binary characters (e.g. 010011000), which
the machine can recognise as code for that instruction. A program might
be prepared directly by the programmer writing instructions in this form,
which is called machine language . But it is very difficult for people to use
such language and therefore rare for programming to be carried out at this
level today, although the processor can only recognize the instructions at
this level.
Compared to machine language, people find assembler language more
understandable, with each instruction being typed as a set of alphabetic
characters known as a mnemonics. This enables the programmer to type the
program and to understand the instructions typed. The mnemonics have to
be translated into machine language which the processor can understand.
The following assembler program contains the instructions to take two
units of data in the memory (address: 2240H, 2241H), add them and store
the result to the memory (address: 2242H) for the processor Z-80 (a proces-
sor popularly used in the control industry during the 1980s):

LD A, (2240H)

LD B, A