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Why does the TM3 Analog module channel configured for 4..20mA return a value of 4000, even though there is no wire connected?

Published date: 31 July 2019

The TM3 Analog Input module configured for 4..20mA input connected to my Modicon M221, M241, or M251 PLC shows a value of 4000 even though there is no equipment connected to it or there is 0mA on the channel input. Is this expected?

Product Line
TM3 Analog Input Modules

Modicon M221, M241, or M251 PLCs

When the TM3AI8 is set to 4..20mA, even if no equipment is connected, showing a value of 4000 (the default minimum value) is the expected behaviour.

If no equipment is physically connected, the actual current would be 0mA, which would set the Status BYTE for the channel to be equal to 6.
A Status Value of 6 means: Wiring error detected (input voltage/current low limit exceeded). Use this value to detect wire errors and out-of-range current.
A Status value of 0 means: Normal (input/current is within normal range of the configuration of the input channel)

See page 70 of the "Modicon TM3 - Expansion Modules, Programming Guide" for the possible status BYTE values returned:

This is different from the 0..20mA setting. If you set the range on a channel to be 0..20mA, and there is no current on the line, the Value returned will be 0 (by default).
And the Status Byte will return 0 (Normal) and not 6 (as seen in the 4..20mA case), since 0mA is not outside of the range of 0..20mA.

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