I am looking for a way to read and manipulate some 4-20ma signals from my PC. ( Basically I am building a Engine Dyno and looking for a way to read and trend the signals from it). I presume there is a way to connect 4-20ma signals and convert them to RS232 which I can then analyse through my PC. Is anybody aware of any such device or can advise the best way to do it? Have though about bringing the signals to a PLC and then using a Scada package connected to the PC through the COM port to read the data but the cost is quite prohibitive for a DIY project.  Any thoughts?

Like this?
http://www.bb-elec.com/product.asp?sku=232CLDR

Well done ,This is the type of unit I am looking for . May require 3-4 inputs and one output but should be on the right track now. Have you any experience with displaying these signals graphically on the PC eg through a scada type package?

Are you sure about this device? Looking at the schematic it seems to only be converting the Current levels to a varying voltage drive on the 232 lines. Its hard to see how the digital interface on a PC would be able to interpret this, unless its not a true 4-20mA but rather a PWM Signal. (I suspect this may be the case)

If its true 4-20mA, then if the PC has a A/D I/O card, you could use that.

To use a standard RS232 port on a PC

I would take the approach of using an A/D converter to generate 4-20 decimal values, then using a Processor to embed this into a suitable protocol, and transmit that.
SCADA devices usually conform to a protocol.

  

Bert

Bert, you are absolutely correct!  I didn't look closely enough at it before posting.

Here's something more applicable:
http://www.bb-elec.com/product.asp?sku=232SPDACL

Is there some particular reason for using RS-232?  RS-232 ports may not be as available on newer computers.  USB or Ethernet might be better for something new.

Oh, those CL converters are for DIGITAL current loops, so if your current loop output is digital, then the original suggested converter will work.  That's why the current loop can be opto-isolated.

TTFN

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I think I will go down the route of a plc. I have a Seimens S5 PLC which I can aquire a Scada package for , Then I would have a few analouge inputs and outputs available and the system would be expandable. I can then use the graphics editing software to create my own graphics for displaying any values.Thanks again for your help ,