Concept

A consecutively changing amount such as voltage, current, temperature, velocity, pressure and flow rate is called analog amount. Temperature, for an example, is consecutively changing with time as shown in Fig 1.1 However, as this changing temperature is directly inputted into PLC, its equivalent analog amount of DC voltage, 0 ~+10V or DC current 4~20 ㎃ is inputted into PLC via transducer.

Non-consecutively changing numbers such as 0, 1, 2 or 3 is called digital amount. On and Off signal is expressed by digital amount of 0 and 1. BCD value and binary value is also digital amount. CPU module is unable to directly input analogue amount to calculate it as digital amount. Therefore, the system converts analog amount into digital amount and then input the digital amount into CPU as shown in 1.4. In addition, it’s necessary to convert digital amount of CPU into the amount of analog in order to output analog amount to external side.