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LJM Feedback Functions


The low-level Feedback functions may be used together or separately to perform Modbus Feedback (MBFB) operations on the LabJack device.

  • LJM_AddressesToMBFB creates a MBFB packet

  • LJM_MBFBComm sends a MBFB packet to the device and receives a MBFB response packet

  • LJM_UpdateValues converts read values in a MBFB response packet

Want Something Simpler?For simpler functions that do the same thing as the low-level Feedback functions, see the Single Value Functions or the Multiple Value Functions.



Write to DAC0, read from AIN0 using LJM_AddressesToMBFB, LJM_MBFBComm, and LJM_UpdateValues

 * Name: stepwise_feedback.c
 * Desc: Shows how to read from a few analog inputs
 *       using a LabJack and the cross platform LabJackM
 *       Library using the C-style API

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <LabJackM.h>

#include "LJM_Utilities.h"

int main()
  int handle, err;

  // Just something that isn't positive so we know
  // errAddress has or hasn't been modified
  int errAddress = -2;

  // Set up the data
  enum { NUM_FRAMES = 2 };
  int numFrames = NUM_FRAMES;

    {"DAC0", "AIN0"};
  const double VALUE_0 = 1.23;
  const double VALUE_1 = 0.0;

  int aAddresses[NUM_FRAMES];
  int     aTypes[NUM_FRAMES];
  int    aWrites[NUM_FRAMES] = {LJM_WRITE, LJM_READ };
  int aNumValues[NUM_FRAMES] = {1,         1        };

  enum { NUM_VALUES = 2 };
  double aValues[NUM_VALUES] = {VALUE_0,   VALUE_1  };


  unsigned char UnitID = LJM_DEFAULT_UNIT_ID;

  // Fill out aAddresses
    aAddresses, aTypes);
  ErrorCheck(err, "LJM_NamesToAddresses");

  // Open first found LabJack
  err = LJM_Open(LJM_dtANY, LJM_ctANY, "LJM_idANY", &handle);
  ErrorCheck(err, "LJM_Open");


  err = LJM_AddressesToMBFB(MaxBytesPerMBFB, aAddresses,
    aTypes, aWrites, aNumValues, aValues, &numFrames,
  ErrorCheck(err, "LJM_AddressesToMBFB");

  printf("\nLJM_MBFBComm will overwrite the Transaction ID and Unit ID of the following command\n");
  PrintFeedbackCommand(aMBFB, "Feedback command");

  // Send the command and receive the response
  err = LJM_MBFBComm(handle, UnitID, aMBFB, &errAddress);
  ErrorCheckWithAddress(err, errAddress, "LJM_MBFBComm");

  PrintFeedbackResponse(aMBFB, "Feedback response");

  // Get the data back in a readable form
  err = LJM_UpdateValues(aMBFB, aTypes, aWrites,
    aNumValues, numFrames, aValues);
  ErrorCheck(err, "LJM_UpdateValues");

  // Print results
  printf("%s: %f\n", ADDRESS_STRINGS[1], aValues[1]);

  // Close
  err = LJM_Close(handle);
  ErrorCheck(err, "LJM_Close");


  return LJME_NOERROR;

Possible output:

deviceType: LJM_dtT7
connectionType: LJM_ctUSB
serialNumber: 470010117
pipe: 0
The maximum number of bytes you can send to or receive from this device in one packet is 64 bytes.

LJM_MBFBComm will overwrite the Transaction ID and Unit ID of the following command
Feedback command:
Header: 0x00 0x00 0x00 0x00 0x00 0x0e 0x01 0x4c
frame 00: 0x01 0x03 0xe8 0x02 0x3f 0x9d 0x70 0xa4
frame 01: 0x00 0x00 0x00 0x02
Feedback response:
Header: 0x01 0x89 0x00 0x00 0x00 0x06 0x01 0x4c
data: 0x40 0x9a 0xfd 0x5f
AIN0: 4.843429

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