The PLC operates by receiving information that contains the status of system components, levels, numerical counts, and sensor outputs. The PLC processes the information and returns output signals that are appropriate to control the output devices. The input signals, coming from a variety of different sources, may be in the form of analogue or digital information. Sensors such as pressure, temperature, and flow are usually analogue devices which output an analogue signal that is proportional to the parameter being measured. The standard output of 0-10 VDC can be used, though 0-20 ma has gained wider acceptance due to its immunity to loss over long cable runs. Because a PLC is a digital device, these analogue signals must be converted to digital in an analogue to digital (A/D) converter. However, simple digital information from on/off switches and limit switches may be input directly to the PLC.
When incorporating analogue sensors, the degree to which the system is able to detect a change and react to that change is known as resolution. Theoretically, an analogue sensor has infinite resolution. However, the resolution of digital information is limited by the number of bits available to express the analogue data. A 1 bit system has the possibility of being on or off, or 2 discreet states. The addition of a second bit would double the resolution to 4 discreet states. Each successive bit added to the digital input will double the resolution. The number of bits used to transmit the digital data is referred to as word length and is often based on the 8 bit byte. However, the need for greater control has required longer word length to gain greater resolution. An 8 bit word length has 256 discreet levels or approximately .4% resolution. A 9 bit word doubles that resolution and is able to discern a .2% change. Each bit added doubles the resolution resulting in a 10 bit system of .1% and a 12 bit system able to resolve a .025% change.
Digital data and variable word length has required the engineering community to adopt standard conventions to express and communicate the meaning of the data. BCD was an early entry using 4 bits to express the decimal digits 0 through 9. Octal is a base 8 system that uses 3 bit bytes and expresses each word as a number between 0 and 7. The addition of another bit doubles the amount of data to a base 16 system known as hexadecimal. No matter what system is being used, they all convey a digital pattern as a number or letter that is associated with a distinct numerical value. Hexadecimal uses the number 0-9 for numbers 0-9, and also the letters A-F to designate the numbers 10-15. Thus, 0000 would be referred to as "0" and 1111 would be referred to as "F". Long strings of data can be tagged, such as 0110 0010 1110, and be called simply "62E". This has allowed designers to manage complex systems and large amounts of digital data.
Once the digital data is available for the PLC to communicate with the system, a protocol needs to be implemented for the devices to share a common understanding of the meaning of the data. RS232 was an early adoption standard that is used primarily in low speed serial applications. An enhanced version, RS422, allows longer cable runs for serial data. Where the maximum cable