We will be explaining the forms and types of multiple electrical signals. Electrical signals are used as a form of communication between two devices, whether it is a PLC and an instrument, a PLC to another PLC, a PLC to a computer, a measuring element to a transmitter, etc. Communication can be a two-way street, meaning data can be sent or received. It is thus important to establish the frame of reference for communication (the PLC is receiving this data, the transmitter is sending that data, etc.). For the purpose of this document we will be referring to a PLC and its interactions with other devices and signals, due to this we will be explaining signals as they would be referenced from the perspective of the PLC.
Analog vs. Digital
Digital signals are signals that convey information as either “true” or as “false”. As far as digital signals are concerned there is no middle ground or gray area, it is either one or the other. An example of this would be a light switch, either the switch is set to on or the switch is set to off. Other examples include running/stopped and open/close and occupied/unoccupied. Computers are also built off of this type of logic, they see everything as either 0’s or 1’s. Digital signals can be generated through AC or DC circuits of varying voltages, resistances, or amperages.
Analog signals on the other hand convey information on a range. While there may still be 0’s and 1’s now there are values in-between as well (e.g. 0.1, 0.32, 0.76, etc.). Examples of this would include the level on a tank, the volume on the radio, and the temperature within a room. Analog signals can take many forms including the following: 4-20mA, 0-20mA, 0-5V, 0-10V, mV, resistance, etc.
Note 1: In comparison with analog signals, digital signals are generally easier to sense and create and thus the hardware cost for these signals is significantly lower.
Note 2: Due to noise, which will undoubtedly affect any significant stretch of wire, it is important to use twisted and shielded pairs for analog signals. Doing so will help reject the noise along the line. Only one end of the shield should be landed.
Miscellaneous Signals
Pulse Signals
Analog signals which are generated through the generation of a discrete signal in relation to another variable (e.g. frequency, length, etc.).
Frequency pulse, in which the number of pulses over time relates to what is being measured. Example: A turbine meter, which gives a pulse every time a revolution is made. The frequency of the pulse indicates the RPM of the meter.
Pulse Width Modulation (PWM) is another form of pulse signal. It is a method of holding signals for varying periods to generate a signal (e.g. generate an average voltage and current).
Temperature Signals
Thermocouples – The generation of a millivolt signal based on the connection of two different metals and how that signal changes with temperature. It is important to note that these signals need to have the same type of metal run across their lines all the way back to the PLC. Therefore, if a terminal block needs to be used it should be a terminal block which has the same material for contacts. The types of thermocouples have been standardized and can be referenced (e.g. type T, J, E, K, N, etc.)
RTDs – Resistive Temperature Devices which vary their resistance relative to the temperature they are exposed to. RTDs are composed of metal and the change in resistance is approximately linear with respect to temperature. Note that there are 2 Wire, 3 Wire, and 4 Wire configurations. The measurement essentially works the same but the ones with additional wires are to minimize error due to resistance in the wire connecting the device to the RTD.
Thermistors – The same thing as an RTD (changes resistance based off of temperature) but is made out of a ceramic semiconductor. The resistance in these devices changes non-linearly with respect to temperature. Note that there are 2 Wire, 3 Wire, and 4 Wire configurations. The measurement essentially works the same for each configuration but the ones with additional wires are to minimize error due to resistance in the wire connecting the device to the RTD.
