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Why do I get +943 "Remote module topology change" and +935 "Overcurrent detected" errors?

Avoiding Overcurrent Conditions

During operation, exceeding the 34945A, L4445A, or L4490A/L4491A internal switch drive power supply current specification of:

100 mA continuous + 200 mA (15 ms pulse, 25% duty cycle)

...can cause overcurrent conditions within the instrument’s power supply, thus shutting down the supply and generating errors such as:

  • +943,"Remote module topology change; Slot 1: Slot must be reset"
  • +308,"Channel not able to perform requested operation; Chan 1101: Remote module channel drive disabled"
  • +935,”Overcurrent detected”

In this situation, the overcurrent condition must be removed and you must cycle power on the 34945A, L4445A, or L4490A/L4491A to re-enable the supply.

Overcurrent conditions are dependent upon the amount of actuation (switching) current the switch requires, and the duty cycle of the actuation pulse.

For the 34945A, L4445A, or L4490A/L4491A, the duty cycle is set through presets of the distribution boards, or can be set programmatically by combination of the commands:


The figure below shows the relationship between actuation pulse width and power supply recovery time.
The 25% duty cycle specified for the instrument allows the power supply to recover before the next actuation pulse is sent. An example of pulse width and recovery time values that achieve a 25% duty cycle is:

ROUT:CHANnel:DRIVe:PULSe:Width 0.015, (@1101)
ROUTE:CHANnel:DRIVe:TIME:RECovery 0.045, (@1101)

Because switching speeds vary between switches, it may be necessary to vary the pulse width and recovery time settings to achieve values that prevent overcurrent conditions, yet still achieve the desired system performance. Using switches with current interrupt options can also increase performance as less power is consumed by the switch.

Overcurrent Conditions with External Supplies

Overcurrent conditions may occur using an external supply if the 2A limit of the 34945EXT within the 34945A, L4445A, or L4490A/L4491A is momentarily exceeded. The duty cycle of the externally supplied actuation pulse is regulated using the same commands as used with the internal supply.