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Entry type: FAQ Entry ID: 18475312, Entry date: 09/30/2010
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How do you connect a 2-wire measuring transducer to analog module 6ES7 331-1KF0.-0AB0, a compact CPU or a C7 device?

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Connecting 2-wire measuring transducers to analog module 6ES7 331-1KF0.-0AB0, a compact CPU or a C7 device
For analog modules that can be configured for 2-wire measuring transducers, please refer to the descriptions in the relevant manuals.

This description covers the following configurations:

  • Analog modules that can only be configured for 4-wire measuring transducers,
    like analog module 6ES7 331-1KF0.-0AB0.
  • Mixed operation of 2-wire and 4-wire measuring transducers on one channel group.
  • Compact CPUs or C7 devices.

Definition of the different transducers:

  • 4-wire measuring transducers have a separate power supply and two measuring cables that are connected to M+ and M- of the analog module. For this reason they are also called active measuring transducers.
  • 2-wire measuring transducers are also called passive measuring transducers, because they are normally supplied via the analog module.

To connect a 2-wire measuring transducer to an analog module (e.g. 6ES7 331-1KF0.-0AB0), a compact CPU or a C7 device, you need an external 24V power supply (Fig. 1).
This supply to the 2-wire measuring transducer must be short-circuit-proof. Provide a fuse to protect the power supply unit (Fig. 1). Please note that the analog input is not protected against destruction in case of a measuring transducer defect (short-circuit). For this case, make sure that you take the appropriate protection measures.


Fig. 1: Connecting 2-wire measuring transducers (block diagram)

M+: Positive measuring circuit
M-:  Negative measuring circuit
MANA: Reference potential of the analog measuring circuit
M :  Ground connection
L+:  DC 24V voltage supply connection

Notes:

  • The marking of the inputs depends on the module type; e.g. M+, M- .
    Please refer to the circuit diagrams for the individual modules.
  • The analog module must be configured in the HW Config as a 4-wire measuring transducer.
    This also applies when operating a mixture of 2-wire and 4-wire measuring transducers on one channel group.
  • Please refer to the description of the relevant module:
    S7-300 Programmable Logic Controller Module Data; Entry ID: 8859629
    CPU 31xC and CPU 31x, Technical Data Device Manual; Entry ID: 12996906
    In the case of C7 devices, separate descriptions have been assigned for each device.
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