S7-200 detects a broken 4-20 mA current loop

Hi to all I was told by SIEMENS that the analog input module doesn't have this facility. It needs to be done by the application software.
Isn't that strange? They told me the thermocouple modules DO detect open circuit... Thanks in advance ! Eduardo.
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Not at all. Most analog modules measure 0-10V (or 0-5 volts) across the input resistor, and therefore measure 0-20 mA. There is no way for the module to determine what is or is not legitimate. It is a trivial matter in the ladder logic to use a reading below 4 mA as an "open loop" indication.
Thermocouples put out a mV signal, and an open will result in no voltage, making it simple to detect the open circuit.
Ben Miller
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Ah... not quite. An 'open loop' in a thermocouple reads no voltage, that is true. But that is the same as when the sensor temperature equals the reference junction temperature. Unless the reference junction temperature is well outside the expected range of the sensor, there is no easy way to detect an 'open' in a thermocouple instrument.
daestrom
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Good point, yet some thermocouple modules have a "t'cpl burnout" sensing feature. I know Automation Direct does on their module. I haven't looked at how they do it.
Ben Miller
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Ben Miller wrote:

By performing a periodic continuity test? I'm not familiar with the A.D. module, but that's how others do it.
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