PLC communication

Research the ability of PLC's to communicate via 4-20mA and 0-10volts between the laboratory devices and the PLC

This would include how to communicate between these devices using an analog device that has 0-10 volts and converts to 4-20mA.

Perhaps for YOUR project, you could use GOOGLE as suggested by your instructor.

Literally 60 seconds of effort turned up a great starting point: The Basics of 4-20mA Current Loop Transmitters.

TI (Texas Instruments) makes a number of current loop transmitters and receivers, so you should be able to find some references there.

Visitor said:

Perhaps for YOUR project, you could use GOOGLE as suggested by your instructor.

Literally 60 seconds of effort turned up a great starting point: The Basics of 4-20mA Current Loop Transmitters.

TI (Texas Instruments) makes a number of current loop transmitters and receivers, so you should be able to find some references there.

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Believe it or not, I am very familiar with GOOGLE, which is how I stumbled upon this forum to begin with. So instead of only using what I'm learning from the internet through my own research, I figured I could also use the knowledge that others may already possess, which is technically another form of research and the whole point of forums in the first place. Work smarter not harder when you can right? And by the looks of it, it seems like I'm yielding positive results. So thank you for YOUR response, as it was actually very helpful. I'll think of you when I'm handed my diploma.

Realistically... You need to look at the XTR115 and XTR116 datasheets, These devices are used a lot with interfacing PLC's I have used them extensively... They are dedicated 4~20mA transmitters..

I think you'll have more success here if, rather than copy/pasting your exact assignment, you actually do some research and learn the basics, and then ask more specific questions. People here tend to be way more willing to help than to do everything for you.

Absolutely, working smarter, learning from the successes and the mistakes of others, is a good thing. But many of us have worked with the "working lazier" guy who doesn't know even the basics, can't think through a problem and dumps his projects on other people.

Here's another good reference:

Fundamentals, System Design, and Setup for the 4 to 20 mA Current Loop

Current loops have been around for more than 50 years and are in use for many industrial applications, so there's a ton of information available.

Plc's such as siemens step 7 usually have analoge built in, regarding 0-10v.
For 4-20ma current loop you would usually get a plug in module that plugs in the end of the Plc.
Then you would use a function bloxk within the Plc, after setting the module up it would return a value to you which would be a representation of the input.
One you get one to work using an example it doesnt seem all that hard to do, so long as you have a grasp of binary, hex & basic programming.

I honestly don't like the question, probbaby because I know too much.

Analog deices, Maxim and TI have lots of industrial sensor interfacing IC's. See https://www.maximintegrated.com/en/products/interface/sensor-interface.html

Lately, the trend has been towards I2C. RS485/Modbus is slow far-reaching communication,

The PLC has it's own language to communicate to the hardware.

Ian Rogers said:

Realistically... You need to look at the XTR115 and XTR116 datasheets, These devices are used a lot with interfacing PLC's I have used them extensively... They are dedicated 4~20mA transmitters..

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From what I understand, most PLCs have analog inputs that only measure voltage. So if i have a transmitter that is outputing 4-20 mA, i need to covert that to a voltage so the PLC can measure it. I have been reading about using shunt/drop resistors in 2/3/4-wire analog inputs to do this conversion. Someone pointed out to me that the FC-33 has this feature. The XTR115 you mentioned seems like it converts a voltage signal to a current signal. What would be the purpose of this since the communication between input devices and the PLC needs to be a voltage? Or am i completely wrong about that

taylerjay97 said:

From what I understand, most PLCs have analog inputs that only measure voltage.

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No, that's not correct. PLCs have numerous input options, and most of the ones that I've worked with could do either one, with the preference towards 4-20mA since that has been the principal industry standard for decades.

BobW said:

No, that's not correct. PLCs have numerous input options, and most of the ones that I've worked with could do either one, with the preference towards 4-20mA since that has been the principal industry standard for decades.

Click to expand...

So what would be the specific application of the XTR115 and 116 that you mentioned. The whole concept of the current loop is confusing me

The 4-20mA current input originates back when remote instruments were very low tech. Sometimes they were nothing more than a variable resistance that was a function of a process variable. Having the value of 4mA representing zero, allowed a number of advantages. For example:
1. That 4mA could power some electronics in the remote instrument, so that it didn't need a separate power supply
2. It allows detection of a broken circuit. If the current goes to zero, there's obviously something wrong.
3. It's more immune to noise than voltage signals.

Current loop allows one to transmit a single value and power a transmitter, So, the 4mA powers the transducer and the 4-20 mA is 0 to100% of the sensor value. Usually 0-20 mA is available too. By placing a 250 ohm resistor at the end of the loop, one can convert to voltage. e.g. 0-5V or 1-5V. 1-5V avoids quantization errors.

There is active and passive current loop devices too.

In a plant, the signals are usually far apart distance wise.

In a laboratory setting, pseudo-differential voltage measurement is available. The external device can be grounded to it's own local ground and measured pseudo-differentially. Low to ground and Hi to ground measurements are done. Your not connecting the grounds together, which would result in a ground loop. This measures the output of a device,

Now suppose it needs a setpoint. You can use current loop to provide the setpoint 0-20 mA gets converted to 0-5V without a ground loop.

For one instrument, it doesn't work very well and that's a linear power supply. The archetecture makes the setppoint and and the control voltage relative to the plus output. let's suppose this is a grounded power supply.

Now suppose you want to measure current. There is a resistor in the + output. You have 7 power supplies that need to be controlled/
the references are all over the map. You need isolated voltage input for current (voltage across a low value resistor) and Isolated current output for the setpoint. Convert 0-20 mA to 0-5V. let's say the output voltage is measured using a resistive divider, taking 0-40V orr 0-120V DC to 0-5V.

You can use an isolated digital interface IEEE-488, USB or RS-485 as well. I had exactly that problem to solve.+

Take a look here: https://www.maximintegrated.com/en/design/technical-documents/tutorials/4/4699.html

If a PLC has a voltage-input ADC channel, you can also use that as a 4-20mA receiver just by adding a load resistor.

eg. 220 Ohms would translate the current to a voltage range of 0.88V - 4.4V which can be used in your program.
Some PLCs have a load resistor on one of the ADC terminals, or alternate inputs with load resistors included.

You can use a higher value resistor to give a larger voltage range and higher resolution, providing that 20mA does not exceed the ADC input limit and the overall supply voltage is adequate, eg. no more than 500 Ohms with a 10V ADC input.