Bi-level Switch Monitor (BSM) interface
The bi-level switch monitor (BSM) interface is used by the core element to determine the status of a switch in the peripheral element. The peripheral element of this interface is entirely passive, consisting only of a single pole switch electrically isolated from all other components of the peripheral element. This interface is used by the core element to determine the status of switches and relays in the peripheral element and has the advantage that it can be operated even when the peripheral element is powered down. The switch status interface is entirely driven and operated by the core element. The core element provides a continuous reference voltage signal, and periodically samples the input signal and compares it to the reference. The result is encoded into a binary bit to indicate whether the switch was closed or open. There are no timing constraints related to this interface since the switch status being monitored is normally static or changing infrequently. However, if the input signal is sampled while the switch status is changing, the result can be invalid.
The signal interface receiver is similar to a bi-level discrete (BDM) input with the following exceptions:
- Ground is referred to receiver (instead of source) ground.
- It is biased to a high level, when it is not being driven, by the connection of the input to a reference voltage through a resistance. When the switch contact is closed, the input signal is forced to a low level by presented low impedance.
The interface receiver converts such input signal in one of two digital states, high (logical ‘1’) for switch source open status, or low (logical ‘0’) for switch source closed status. In case of specific needs, opto-couplers can be used. In that case the specific interfaces are defined on a system basis, thus they are not covered by this Standard.
Last update: 21 March 2012