analog input

hello here is marco again

an analog input chart is connected too a temperature sensing device
this is clarely not an I/O signal.
but they are acensing and decensing millivolts.
how the plc translated this reading as degrees celcius.
and show it on the pc monitor at tht mcc room.
when i read this millivolts whit mine multimeter what can i make out of this reading,is it usefull , high -low level what does it say.
please will someone explaine this too me.
thanks a lot

it sound like you want to convert mV to °C

1) you need to know characteristics of "analog"
0° .. 100mV
5° .. 154mV
10° .. 352mV
40° .. 354mV

(for example... it must not be linear)

2) then you need device which will convert analog signal to digital number
3) then you need software which will read this number from device to your PC
4) then you need software to show this number(s) in particular way

of course 3+4 may be joint


- what PLC does, is recalculation of one value-range into another via. particular transformation curve (sometimes linear)...

P.S.: what is "acensing" "decensing" ? ascending descending? did you meant incremental reading?

It appears that you have a thermocouple (T/C) connected to a chart recorder. It also appears that you want to read this temperature on a PLC while keeping the reading at the chart functioning.

If my assumptions are correct, the best way to accomplish this is to use a signal conditioner/transmitter to convert the T/C signal to a 4-20 mA reading. The chart recorder is probably able to be configured to read a 4-20 mA signal, and you can get an analog input for the PLC that can read this as well. The same 4-20 mA signal can operate both devices, unlike a mV signal from a T/C. You will probably want a signal isolator at the PLC to avoid ground loop problems.

The range of the signal is set by the signal conditioner, say 0 to 1,000 °C at 4-20 mA. The signal conditioner linearizes the signal as well as boosting it to a higher level more suitable for transmitting to several devices.

The analog input on the PLC converts the 4-20 mA signal to data, say 0-4095. The PC data display can convert this to engineering units. For example, if the temperature is 500 °C, which is 50% of range, the signal conditioner will send a 12 mA signal to the PLC, which will convert it to a value of 2048 in the analog input register, which will be read by the PC over your network, and the tag configuration will say that 2048 data is 50% which is 500 °C which will be displayed on the PC screen.

hello tom jenkins,
first off all many thanks for the information,
i have been thinking wrong till now,
i thougt that an analog signal was messured by the plc in mV but it realy are mA.but this goes up for an t/c oke,
the instrument in particular that i am talking about got something more than only an t/c.namely it is sensing temperature but it got local a very small display where you can read real time temperature.
this means this device recives first off all 24V DC ,next there are two wires at the output connectors that are going to the plc.
my question in this case is what is transmitting this particular device to the plc ,en iff it is an use full signal to me, too be mesured by a well calibrated multimeter, and iff there is something usefull to read , does it tell me something about the status, mall function off the device, or something else that i am not aware off.
well for so far thanks for your time on reading this.
hoping to get some response from you.

hello marius,
many tanks for your information,
you are right,i meant to write ,ascendig and descending.
can you explain me more about the conversion of mV or mA to degrees celcius.
thank you

Marco, I am going to provide MUCHO information, so be prepared to do some reading.

First I will talk about learning PLCs, this site offers generic material for learning the basics: http://www.plcs.net/contents.shtml

I think you mentioned they used AB PLCs at work, this site offers an inexpensive learning program, with simulation, that is based on AB: http://www.thelearningpit.com/

Just in case they do not have AB software available at work see if you can get them to purchase an AB 10 point Micrologix 1000, cost was around $100 US but the software is free. The free version of software can be had here: http://support.rockwellautomation.com/webupdates/selectfiles.asp?RowID=8322&CatNum=9324RL0050ENE&SN=&SWRel=6.10.00&PR=0&EQ=

RSLinx Lite, free version, is here: http://support.rockwellautomation.com/webupdates/product_list.asp

If they will buy it you can build your own trainer in the shop or at home, maybe both.

Now to the next part concerning analog etc...

You originally stated:

an analog input chart is connected too a temperature sensing device
this is clarely not an I/O signal.
but they are acensing and decensing millivolts.
how the plc translated this reading as degrees celcius.

Click to expand...

i have been thinking wrong till now,
i thougt that an analog signal was messured by the plc in mV but it realy are mA.but this goes up for an t/c oke,
the instrument in particular that i am talking about got something more than only an t/c.namely it is sensing temperature but it got local a very small display where you can read real time temperature.
this means this device recives first off all 24V DC ,next there are two wires at the output connectors that are going to the plc.

Click to expand...

A PLC can use mV input if has modules for that purpose, an AB SLC has modules for that purpose.

I/O: I is input, that means any device that can send a signal TO a plc. A pushbutton, limit switch, thermocouple, analog devices etc may be an input. The input may be digital or analog.


There are numerous devices to measure temperature, the most common are;
THERMOCOUPLE:
English; http://en.wikipedia.org/wiki/Thermocouple
Spanish; http://es.wikipedia.org/wiki/Termopar
http://www.omega.com/prodinfo/thermocouplesensor.html

THERMISTOR: http://en.wikipedia.org/wiki/Thermistor

RTD: http://en.wikipedia.org/wiki/Resistance_temperature_detector
Spanish: http://es.wikipedia.org/wiki/RTD

I am sure there is more I can offer but see if any of this may help.