At Beyond Logic and Embedded Processor Control you will find interesting information on interfacing.
Real World variables are analog values, like the temperature of the Human Body. There may be Yes-No or digital events too, like the Water Level of a Tank or Door Close-Open. The former is measured by a Thermocouple and the Latter with a Proximity Switch.
These values from the sensors have to be Quickly converted to a digital form and fed to the the uC by an interrupt or cyclic polling of ports. The "How Quickly" determines How "Real Time" you system response is.
An analog value in instrumentation has to be at least 12 bit wide resolution, which is like 3-1/2, and the
The ICL7135 4-1/2 gives +/- 20,000 counts at full scale, nearly 14bit wide data. This can be interfaced both in serial and parallel data links. The serial method is used when optical isolation is required from the sensor to control circuits. If the sensor is measuring the temperature of a Transformer, an optical isolation is absolutely required.
delabs Technologies 06:48 20-Apr-07