No-Load Threshold

The no-load threshold template is used for output modules and uses the following syntax:

The template sets the current per slice (in milliamps) below which an output signals state 3 (no load), and each of the parameters sets the threshold for an entire power group of eight digital outputs. Without this template, the defaults are 10mA. For example, to recognize the load requires at least 30mA.

Outputs do not share current evenly across the three TMR slices and there may be a significant difference between slices. As an example, the 8451 output module has a minimum on state load current of 50mA per channel. With a load of 51mA, the load may be spread across the slices as follows:

While the channel total is greater than the nominal minimum, slice C is below the default no-load threshold (10mA) and signals a no-load state while the other two slices record an acceptable load. This causes a discrepancy alarm. In earlier firmware versions this may lead to a module shutdown.

It is recommended to group comparatively high and low power loads on different power groups, preventing potential noise from a higher powered output affecting the lower powered signals. The following no-load threshold settings are recommended for each group:

As an example, if groups 1 and 2 (channels 1 to 16) are approximately 100mA per channel and the rest are 50mA per channel, the no-load thresholds should be set as:

Output modules such as the 8471 (120Vdc) module only have four output groups and the template is written, for example, as:

While some output modules have a minimum on state load current of 20 or 25 mA per channel, is it still recommended to use the nlthresh template.

For the 8473 module, you are recommended to set nlthresh = 20, 20 because the module is unable to accurately detect the no-load condition below this value.