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What is principle of P Interface and the DPI/IP method with reference to Balluff BTL having P interface?

P Interface for Balluff transducer

The P interface is a universal impulse interface and unifies the functions of the falling and rising edges. The position measuring system control is done via Init and start/stop signals. Here, the “start pulse” is the reference point for the travel time measurement.

Reliable signal transmission, even with cable lengths of up to 500 m between the evaluation unit and BTL is ensured by the particularly fail-safe RS485 differential driver and receiver. Interfering signals are effectively suppressed.

DPI/IP is a protocol for direct data exchange between the controller and BTL. Here, the signal lines transmit additional information, such as manufacturer, sensor type, measuring length, and waveguide velocity. This makes it possible to start up or exchange a BTL without needing to manually change the control parameters.

The interface enables bi-directional communication and includes integrated diagnostic functions. Downtimes are reduced thanks to Plug & Play and automatic parameterization.


The DPI/IP method includes two operating modes, DPI measuring operation and operation with the IP data protocol.

  • DPI = digital pulse interface
  • IP = integrated protocol.


DPI measuring operation:

The Init pulse is sent to the BTL via the Init line at regular intervals, its rising edge triggers a measurement.


Principle of data transfer in DPI measuring operation:

  • TInit       1 μs to 5 μs
  • TStart    3 μs to 5 μs (typ. 4 μs)
  • TStop    3 μs to 5 μs (typ. 4 μs)


Operation with IP data protocol:

If the length of the Init pulse TIP is extended to 10 μs to 50 μs, the BTL switches from DPI measuring mode to operation with the IP data protocol (see Fig. below). Here, a character string (command) is transferred to the BTL after the Init pulse. While the start pulse is still sent by the BTL as a response on the start/stop line, a character string (response) is transferred to the controller instead of the stop pulses, which contains the requested response dependent on the command.


Each character in the transfer protocol has the following bit structure:

  • Start bit               Start-of-frame bit
  • Bit 0 to bit 7       8 data bits
  • PBit                       Parity bit (even parity)
  • Stop                      Stop-of-frame bit
  • TBit                       4 μs (bit length at a data rate of 250 kbit/s)


Data security during transfer of the string is achieved with the parity and CRC16 checks with polynomial X16+X12+X5+1 (corresponds to 0x1021). If there is a transfer or protocol error, the BTL sends an appropriate error message as the response.



Principle of data transfer with the IP data protocol:

  • TIP                        10 μs to 50 μs
  • Operation with IP data protocol
  • Command           Command to request BTL data
  • (information that is stored in the BTL)
  • TStart                   3 μs to 5 μs (typ. 4 μs)
  • TD1                       > 50 μs
  • Response            Response in line with the request
  • Alternative: error message.


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