Basic function principles


The functionality of the KL3681 is similar to that of a commercial digital multimeter. The terminal offers the following features:

  • Single-channel measurement
  • AC/DC voltage measurement, automatic range selection: “Autorange” or specified via the controller
    Measurement ranges: 300 mV, 3 V, 30 V, 300 V
  • AC/DC current measurement in the 1 A path (internal fuse: 1.25 A) or 10 A path (no internal fuse)
    Measurement ranges: 100 mA, 1 A, 10 A
  • Formation of measured values:
    Alternating current and alternating voltage are calculated as true RMS values, no DC component
    Direct current and direct voltage are calculated as arithmetic average value
  • Electrical isolation from the fieldbus
  • Very good interference immunity through dual-slope conversion technique
  • Display of measurement type (current/voltage) and overload through LED
  • Typical update rate approx. two seconds, after measuring range change approx. one second

Quick start

The factory setting for the multimeter terminal enables voltages up to 300 VDC to be measured directly without additional settings. The Autorange function is active and selects the measuring range automatically.

The measured value is displayed with 1 bit per microvolt, i.e. no adjustment is required.

Functional description

Measuring ranges and output

The terminal starts measuring automatically. New measurement readings will be put out as soon as the calculation is complete. If the last used and the requested measuring range do not match, the bit “Data invalid” is set. In the event of an error (generally measuring range exceeded) the respective error bits are set. New measurement values are provided approx. every 0.5 seconds. The filter time is reset if the measuring range is changed via Autorange or by the user during the filter period. If the value fluctuates too much in Autorange mode, it may not be possible to determine a new measured value. As a remedy the filter can be deactivated.

The terminal has an extended measuring range, if a larger measuring range is available (Example: 30 V range, extended measuring range is 300 V). In Autorange mode the extended measuring range is used as a hysteresis range. In the extended measuring range overflow and underflow bits and the error bit are set at 115 % of the normal measuring range.

Please note

  • In AC mode only the RMS value of AC voltage is shown (no DC component).
  • Error bits are under-range, over-range and data invalid. The error bit is set together with these bits. In some cases several bits may be set simultaneously in the event of an error (e.g.: Overrange and Data invalid).
  • Since the terminal integrates the input value over an interval of 20 ms (16.66 ms in 60 Hz mode), superimposed interferences of the input values from the supply network can largely be compensated.

Measuring range selection

The measuring range is selected in process data mode via the control byte.

The Autorange function selects the best measuring range for the selected measurement type. In Autorange mode the 10 A measuring ranges are distinct types of measurement, i.e. it is not possible to switch from 1 A and 10 A or back (Causes: different input contacts as like fuse protection of the1 A range with 1.25 A).

The limits for changing the measuring ranges range are 10 % above the full scale value (extended measuring range, larger range) and 10 % of the full scale value (smaller range). If "Left aligned" view is selected, the limits are 100 % and 5 % of the full scale value.


The built-in filter averages the values for the last half second. If the measuring range is changed the filter is reset. The filter is active by default. If the filter is switched off the terminal supplies approx. 16 measured values per second.

Mains frequency

The integration times are adapted to the mains frequency (50, 60 Hz). The default is 50 Hz.


The presentation of the values in the process data can be changed.

left aligned (0)
Left-justified presentation, full-scale value corresponds to 0x7FFFE000 (0x7FFFFFFF) or 0x80000000.

right aligned (1)
Right-aligned presentation: 18 bit resolution of the measuring range, end value corresponds to 0x0003FFFF (262143dec) or 0xFFFC0000 (-262144dec).
In the extended measuring range these limits are exceeded!

scaled (2)
Scaled presentation: this presentation corresponds to 1 bit per µV / µA. Examples:
- Mode 0, Autorange, 2.5 V: 2500000dec
- Mode 2, 800 mA: 800000dec

float (5)
Scaled presentation in floating-point format

Scaled presentation (2) is set as default.

Please note

  • Only in scaled presentation the factors 0.1/1/10/100 are taken into account between the measuring ranges.
  • In left aligned presentation the extended measuring range is not available.

Zero Compenzation Interval

The compensation function compensates the internal offset drift of the ADC. The interval length is selectable.

If setting 4 "Triggered by PDO" is used, the time for reading the internal offset can be selected manually via the "Start Calibration" process data bit.

50 Hz mode

In 50 Hz mode this function should not be used (0: off).


The terminal features manufacturer and user calibration options, which can be switched on and off independently. The calibration always applies to all measuring ranges. Adaptation in 60 Hz mode and compensation of the AC system offset cannot be switched off.