PLC input cards

[permitivity 0]

Hello, I am new to this forum. Do input cards like <AB 1394 discrete input> carry current in the input signal wire? For example, if you have a limit switch, at one end is the 24V DC supply, and the other end is connected to the input card. When the swtich is closed, does current (and how much?) actually flow through the wire, and the input card sees "ah, 24V, I see signal"? I'm not sure if the input cards act like volt meter - they just sense voltage? Is there actually any current flow? I tried the data sheets, but I couldn't find much. Thanks, Eric

[rdrast 3]

Well, yes. Current does flow. If it didn't, then the module wouldn't do anything. Typical current's to input modules are around 2 to 10 ma, which usually drives the opto-isolators on each input channel. The specifications for input current and impedance for all AB modules are listed in the manual for the I/O module. For the record, even when using a volt-meter, current flows, and the meter loads down the signal source. Typical digital volt meters are approximately 1 to 10 megohms input impedance, while analog meters are usually rated in kilo-ohms per volt.