PLC communication


I'm going to offer this. A PLC can't directly understand speed or pressure or temperature or position. It can only understand bits and bytes converted from information that is either a variable voltage or current signal. The variable voltage or current has to be proportional to speed or pressure or temperature or position so the PLC can understand the magnitude or rate of change of the signal.

4-20 mA output signals from sensors have to be scaled properly to provide a signal proportional to the parameter being measured, AND the PLC has to have it's input scaled properly to understand the signal from the sensor.

Let's say a PLC needs to know the pressure at some point in a process. A sensor with an output of 4-20 mA or 0-10 VDC can be used to measure the pressure and send a signal to the PLC that the PLC can use to control or monitor or protect the process or the equipment. If the process pressure being measured never exceeds 150 psig, then a sensor with an input range of 0-150 psig could be selected and installed and connected to provide a pressure signal to the PLC. Is a 4-20 mA transmitter is selected the PLC must have an available 4-20 mA input the transmitter output can be connected to. The transmitter output would probably be 4-20 mA for 0-150 psig. And the PLC input would have to be scaled for a 4-20 mA input signal corresponding to 0-150 psig. That way, the transmitter and the PLC are ”speaking the same language” and the PLC can understand what the pressure the sensor is measuring.

The input circuitry of the PLC converts the transmitter signal to bits and bytes the program in the PLC can use for monitoring and control and protection.

If the PLC didn't have an available 4-20 mA input but it did have a 0-10 VDC input, then a suitably ranged 0-10 VDC transmitter could be used to provide the indication to the PLC. So, if the transmitter output was 0-10 VDC for an input of 0-150 psig the PLC 0-10 VDC input would have to be scaled for 0-10 VDC equals 0-150 psig. The PLC input circuitry would then convert the voltage input to bits and bytes for the PLC program to use.

The same scenario can be used to control a flow-control valve--a 4-20 mA output or a 0-10 VDC output of the PLC can be used to send a signal to the control valve's actuator to open or close the valve to control the flow downstream of the valve. Appropriate scaling has be used in the PLC as well as the control valve actuator.

It gets much more complicated, especially when the PLC inputs can be used for several different types of inputs, usually meaning that particular screw terminals on the input terminal board gave to be used for particular types of inputs and/or jumpers on the input terminal board have to be moved to particular positions for different input types and/or ranges. Same is true of outputs.

As for drawing format, you are going to have to look at different PLC manufacturer examples for that, and some control system integrators use their own drawing formats and styles, some just tables of wires/conductors and screw terminal information. Very often the drawings which depict PLC inputs and outputs (I/O) are called ”liip sheets.” A loop refers to the circuit which has to be continuous (a ”loop”) for current to flow or voltage to be present and measured.

This should get you started on your searching and on understanding the drawings you will be seeing. And, apparently have to produce.

One final hint: Most input circuits can really only measure voltage, so in the case of 4-20 mA inputs there is usually a ”dropping resistor” to convert the current to a voltage (to produce a voltage ”drop” to use old-school terminology). The dropping resistor values typically used are 100 ohms, or 250 ohms or 500 ohms, which makes it easy to think about the voltage ranges from 4-20 mA (0.4-2.0 VDC: or, 1.0-5.0 VDC; or, 2.0-10.0 VDC).

Hope this helps. If you have more questions after you do more searching, we might be able to help. But, it would have been (would be) nice to know what you have been studying prior in this particular class and what the textbook you have been using is. I can't tell if you are in a PLC programming course or a printed circuit design course or ??? If you haven't really been learning about PLCs and controls and you've been thrown in to the deep end of the pool with this assignment, the instructor needs to give better direction or some examples or references--unless he/she is trying to expose you to the next step in your curriculum and wants to see how far you can get with an assignment like this. OR, it's a grad student/teaching assistant making this assignment up and doesn't really know much about the topic to begin with and is hoping to benefit from the fruits of others' labours and maybe spark a few students to comment, ”This was a great assignment!”

In any case. It could be very complicated and difficult without some more guidelines and limits and an example or two. And if the expectations are that with all this time on your hands you're going to become very familiar (but certainly not an expert) with PLC design and application just by using a World Wide Web search engine, well, that doesn't seem reasonable to me, anyway.

Back to our regularly scheduled programming....