Concept 2.6

Hi there:

I have a problem from Schneider Modicon PLC, there is an output from output module but no LED display this output on the module, and also I found this output has logic "0" in the program when you go online. Anybody can tell me if it's because somebody forced this output and forgot remove the forcing before disconnecting from PLC? And how can I find out and remove it? Thanks!

best regards,

Junsheng Liu

The LED refelects the processor side of the opto-isolator for the output circuitry. If the processor is telling the output to be on, whether by logic or through forcing, the LED will illuminated on a working module. If the output triac(?) is conducting without a processor signal then you probably have a bad module.

Have you replaced the output module with a known working one?

A triac output will always read high with a meter or even light a test light if there is no load connected. Make sure a load is connected and then measure the voltage available at the output terminal.

Solid state output will usually read as "on" if the meter is electronic or if a neon or LED test lamp is used. In fact the more expensive the meter the more inaccurate it will normally be for use in determining if solid state outputs are conducting. This is not usually the situation if you use a cheap analog meter or an incandescent lamp. The problem is caused by the leakage current of the output and the input impedance of the meter, it is basic E=I*R.