Aalog inputs

I was wondering what is better a signal coming into the plc with 1 - 10 milli volts or one that sends 0 - 20 milli amps
what are the good and bad of both.

Generally most instrument engineers prefer a "4-to-20 miliampere" signal, because it has a defined non-zero minimum input of 4 miliamps. If the signal is lost due to a disconnected wire or other failure, then you can detect that with a simple miiiammeter test. For the other voltage methods, a "0" input might mean an actual 0 signal, or it could be a disconnected wire. A disconnected current signal will normally show an open circuit reading, which can also be confusing.

I try to go for 4-20mA when I can.
In addition to the reasons given by Lancie1, I have also have the following reasons:
*4-20mA seems to be the best for noise immunity
*It is very common, and most instrument manufacturers have instruments in this range
*Lines often get milivolt range noise in them, so using a milivolt range gets a low signal to noise ratio

In addition, devices are now more commonly fitted with Bus systems.
For analog devices, you may find it better to use Profibus, Devicenet or ASi 2. This also gets you better diagnostics.

Hope this helps,

Doug

Also with 4-20 you do not have to calculate the voltage drop for the length of wire your using.


Ken

"The older I get,
the better I was"