Frequently Asked Questions

Programming an Alarm on a Status Input

Setting an alarm on a channel that reads only status values is confusing for many users. The source of the confusion is that status channels do not read a numeric value but alarms require numeric values. With a little background data and a couple of rules to follow, this is really not difficult to do.

When a telemetry input is programmed as a status channel it assumes a default scale. The scale goes from 0 to 2040 (just like the factory default setting) with the trip-point at mid-scale, 1020. Below the trip-point the system says "status off" and at or above the trip-point it says "status on".

This information, and some knowledge of the state of the input signal, is all that is needed to apply some numbers to the alarm. With a status input, one state is normal and the other is the alarm condition.

Alarm on "Status off"

Assume that we have a transmitter that is either on or off and the RFC-1/B reports the status accordingly. Suppose we want an alarm if the transmitter turns off.

In this example the upper limit is meaningless because if the transmitter is on nothing needs to happen. Set the upper limit of the alarm to "9999" so that it has no effect.

Use the lower limit to sense when the transmitter turns off. The trip-point is 1020 so select a value below this. Any value between 0000 and 1020 will work. We often use 0500 since it is about half way between the trip-point and the bottom of scale, 0000.

Alarm on "Status on"

Perhaps the transmitter from the example above has an inverted logic sample. When the transmitter fails the sample is high and the telemetry reading is "status on". The logic is inverted so the reading is backward but we can compensate and make the alarm work correctly.

In this case the lower limit is meaningless because the reading will be off, or zero, when the transmitter is working normally. Set the lower limit to 0000 so that it has no effect.

Use the upper limit to sense when the transmitter fails. The trip-point is still 1020 and we need a value above this. Any value between 1020 and 2040 will work. We often use 1500 since it is about half way between the trip-point and the top of scale, 2040.