The LVDT signal conditioner module offers an open-collector failure warning output signal which is available at J2-2 on the blue terminal block connector. This output is a connection to the collector of an internal transistor switch, whose factory default mode of operation is Normally Open (NO). In NO mode a device connected between the +24 V DC supply and J2-2 will have no current flow when the module is operating without error, producing a logic High (+24 V) at J2-2, but if a failure occurs, current will flow producing a logic Low (0) at J2-2.
For modules with version 2 firmware, the open-collector switch's mode of operation can be changed in the field from Normally Open (NO) to Normally Closed (NC) by using the SET FOP command over the RS-485 bus. In the NC mode, the device connected between the +24 V DC supply and J2-2 will have current flow if the module is operating without error, producing a logic Low (0) output at J2-2, but if a failure occurs, current flow will stop, producing a logic High (+24) output at J2-2. To determine if this change is possible, find the module's firmware version by using the RS-485 bus command VERSION.
It is important to note that there is a built-in time delay between when a failure is detected and when the transistor actually turns on, which is normally the factory default of 200 ms, but can be reprogrammed in the field from 0 to 900 ms in 100 ms increments using the SET FD command over the RS-485 bus. This time delay prevents "nuisance trips" from unusual transients such as nearby lightning strikes, etc., and applies only to the NO or NC Failure Warning Output at J2-2. It does not affect any other fault indicators built into the S2A like the front-panel LEDs flashing or the analog output being driven out of range.
The types of devices that are typically connected between the +24 V DC supply and the open collector failure warning output terminal J2-2 include:
1. A 10 kilohm "pull-up" resistor for generating a TTL digital logic signal between J2-2 and ground in the event of a failure that can be transmitted to the control room and the DCS or operations data recorder.
2. A magnetic DC relay that requires 50 mA or less to operate, which can operate additional devices such as an audible alarm or an annunciator light. The voltage developed across the relay coil can also be used as in the same way as that across a pull-up resistor to provide a TTL digital logic output signal.
3. A solid-state relay that functions in the same manner as (2) above.
4. An LED array (with appropriate series resistors) for local fault indication.
5. A low wattage 24 Volt incandescent lamp for local fault indication.
Note that the open collector can be used with external devices as long as there is a ground connection from the module in addition to the connection to J2-2, but be careful not to produce a "ground loop".